physics at work and play

8/3/2019 Physics at Work and Play

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Physics at Work and Play

Biman Basu

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1. A matter of inertia

Many of you commute to school by bus. Sometimes, when all the seats are fullyou have no alternative but to keep standing, and in a moving bus you thinkit's safe to hold on to the overhead rod. Otherwise it's quite difficult to keepyour balance. You're quite right. By holding on to the rod you can saveyourself from toppling over when the bus turns and stops. But if the driversuddenly slams the brakes, the sudden jerk could make you lose yourhandhold and fall forward over your friends, some of whom may havetumbled to the floor as a result of the jerk.

Why do you fall forward when a running bus stops suddenly? The simpleanswer is: Newton's first law of motion, which states that "an object at rest ortravelling in uniform motion will remain at rest or travelling in uniformmotion unless acted upon by a net force." This law is also called the law ofinertia. When the bus moves, both the bus and its passengers including youalso move together with the same velocity in the same direction. When thedriver applies the brakes smoothly, the bus slows down along with all itspassengers, ultimately coming to a stop. You don't feel any sudden jerk.

But when the driver applies the brakes suddenly, the bus along with yourfeet, which are in contact with the floor, come to rest instantly. But the inertiaof the upper part of your body keeps it moving forward and as a result youfall. The same principle applies when you get down from a running bus.

Unless you run forward a few steps to slow down your motion you'd fallforward because your feet comes to rest instantly on touching the ground, butthe rest of your body keeps moving forward.

2. Air pressure of football

Football is a game most of us must have played in our childhood. It is acommon observation that if football bladder is not adequately filled with airthe ball does not go far even when kicked hard. A ball pumped stiff to its fullcapacity goes much farther even with a moderate kick. Why does the distance

a football goes when kicked depend on the air pressure?

The ball used in football games is an air-filled sphere with a circumference of6870 cm, a weight of 410450 g, inflated to a pressure of 60110 kPa (or 8.515.6 psi). The design of footballs has changed over the years. The ball used inthe 2006 World Cup was of a new 14-panel design that replaced thetraditional 32-panel hand-stitched balls. The new design has fewer seams, sothe ball is rounder and performs more uniformly, regardless of where it is hit.


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The more perfect the sphere is, the more balanced it will be, offering greateraccuracy due to the predictability of its flight. But more than the shape it is theair pressure inside that is important.

How far a football goes when kicked depends on its bounce, which in turndepends on the ball and the surface it hits. Balls with air inside, such asfootballs, bounce very poorly if they are not pumped up no matter howhard the surface they hit. This is because a low-pressure ball gets deformed alot when kicked with the boot sinking into the leather surface. This wastes alot of energy, which is converted into heat and lost to the bounce. A properlyinflated ball doesn't deform much when hit, so little energy is lost bending itsskin and the bounce is more.

With a pumped-up ball, the kinetic energy of the boot is changed to potentialenergy stored in the air molecules inside the ball at the instant the boot hits it.Quick as a bounce, the potential energy is released on rebound and turns into

the kinetic energy of a rebounding ball, which goes much farther than apoorly inflated ball.

3. Balancing a bicycle

Bicycle, or the bike, is a common mode of transport in cities and villages alike.In both biological and mechanical terms, the bike is extraordinarily efficient.In terms of the amount of energy a person must expend to travel a givendistance, it is the most efficient self-powered means of transportation. From amechanical viewpoint, up to 99% of the energy delivered by the rider into the

pedals is transmitted to the wheels. But you can't balance on a bike standingstill; if you try you'd fall down. To keep the bike balanced you have to keep itmoving. Why is it so?

Different principles of physics are involved here. If you were on a stationarybike it'd be stable as long as the vertical line from its centre of gravity fallswithin its base. But in this case the base is extremely narrow only a fewcentimetres wide. So even a slight tilt would bring the line dropped from thecentre of gravity out of the base, making the bike unstable. Now, suppose youfind the standing bike leaning to the left; your natural tendency would be tolean to the right to counterbalance the lean. But in moving the top of your

body to the right, you'd be actually pushing the bike to lean more to the left,according to Newton's 3rd law. So it'd be almost impossible to stop theleaning bike from falling when it is standing still.

On a moving bike, however, rotational momentum makes the bike easier tobalance. By slightly turning the handlebars right or left, you impart some ofthe rotational momentum of the front wheel to rotate the bike around its longaxis, the direction in which it moves. That way you can counteract any


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tendency of the bike to topple to one side or the other and keep it balanced.The faster it moves, the harder it is to make the body of the bike changedirection and you have much more time to make slight adjustments in bodyposition to prevent the bike from falling over. That's why a moving bike isstable.

4. The clinical thermometer

Whenever anyone has a fever a clinical thermometer is used to measure thebody temperature. But before the thermometer is used why is it necessary toshake it? Well it's to bring down the column of mercury and make it rejointhe mercury in the bulb and reset it for making a new measurement

Like any other mercury thermometer a clinical thermometer also has amercury-filled glass bulb and a graduated glass capillary into which the

mercury rises to show the temperature. But in a clinical thermometer there isconstriction at the point where the capillary joins the glass bulb. Thisconstriction is meant to cut off the mercury column from the bulb when themercury in the bulb shrinks.

When the bulb of the thermometer is kept in contact with the skin under thearmpit (or under the tongue in the mouth), the mercury in the bulb expandsdue to heating by body heat. The expanding mercury rises in the capillary.The height up to which the mercury column rises depends on the temperatureof the body. After the thermometer is removed from the armpit it isimmediately exposed to room temperature, which may be lower than the

body temperature. If there were no constriction in the capillary the mercurycolumn would start falling immediately and would not show the actualtemperature of the body. But the constriction, which is narrower than thecapillary, breaks off the mercury in the bulb from the mercury column in thecapillary. This happens because, during expansion, there is enough force topush the mercury up through the constriction. However, during contraction,the forces pushing the mercury back down through the capillary are too weakto force it through the constriction. As a result the mercury column breaks.Since the top portion of the mercury column is left almost undisturbed whenthe column breaks at the constriction, it's easy to read the highest temperaturereached by the thermometer. Shaking the thermometer gets the mercury

down due to inertia and ultimately drives it through the constriction so that itrejoins into a single column.

5. Colours without dyes

The colours of nature are all around us and are produced by different aspectsof the interaction of light with matter. The most common is light interacting


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with coloured pigments. The reflection and absorption of light on a red flowerproduces a red colour due to the pigments present in the petals. Some coloursin nature are produced by the break-up and interplay of white light. The skyappears blue because molecules of air scatter blue colour more than the othercolours. Sunsets appear red because the light from the Sun passes through athicker layer of the atmosphere which scatters away most of the blue colourleaving only the red/orange colours that reach our eyes. The colours ofrainbow are produced by break-up and total internal reflection of sunlight byraindrops in the atmosphere.

The colour in oil films are produced by an entirely different process called"interference of light" which is due to waves of light interacting with eachother. If the crest (peak) of one wave meets the trough (low) of another theycancel each other a process called "destructive cancellation". When the crestof one wave meets the crest of another they reinforce each other and becomestronger a process called "constructive reinforcement."

When diffused white light strikes an oil film on water it is reflected from boththe top surface as well as the bottom surface of the film. The film being verythin and of non-uniform thickness, light reflected from the two surfacesundergo constructive or destructive interference when seen from differentangles. But since white light is made up of several wavelengths, only light of aparticular wavelength or colour undergoes destructive interference whenreflected from certain regions of the film; the rest of the colours reach the eye.As a result we see bands of colour in the film. The same principle applies forcolours seen on soap bubbles.

6. Ink dropper

Most of us have used a dropper to fill ink in a pen, pour a few drops ofreagent in chemistry lab, or put medicine drops in the eye. A typical dropperconsists of small glass tube with a narrow tip at one end and a rubber bulb atthe other. To fill ink or medicine in the dropper we squeeze the rubber bulband dip the narrow tip in the liquid. The liquid fills the tube when the bulb isreleased. The dropper comes in handy when we need to measure only a fewdrops of a liquid. How does it work? How does the liquid fill the tube?

Well, it's the atmospheric pressure that does the trick. The rubber bulb ismade of elastic material and so if the bulb is squeezed and then released itregains its shape because of its elasticity. But after squeezing the bulb if weclose the narrow tip with a finger, and release the bulb it doesn't regain itsshape. What actually happens is that when we squeeze the bulb air inside it isdriven out, and when we release the bulb after closing the tip with a finger aircannot come back in. Atmospheric pressure acting from outside does notallow the bulb to regain its shape.


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If, instead of closing the tip after squeezing, we dip the tip in a liquid and thenrelease the bulb, atmospheric pressure acting on the liquid forces it into thetube and also allows the bulb to regain its shape. Once in the tube the liquidwon't come out by itself because atmospheric pressure holds it back. But wecan bring the liquid out of the tube in controlled drops by gently squeezingthe bulb.

7. Golf ball dimples

The shape and size of a ball depends on the game it is played with. But inmost games the ball used has a more or less smooth surface, except a golf ball,which has dimples on it. Most golf balls have between 300 and 500 dimples,which have an average depth of about 0.254 mm. Why is the golf balldimpled? Let's find out.

Air exerts a force on any object moving through it. A ball moving through airhas a high-pressure area on its front side. Air flows smoothly over thecontours of the front side and eventually separates from the ball toward thebackside. A moving ball also leaves behind a turbulent wake region resultingin lower pressure behind it. The size of the wake affects the amount of drag;that is, the slowing action on the ball. The dimples on a golf ball helps reducethe size of the turbulent wake region behind the ball by creating a thinturbulent boundary layer of air that clings to the balls surface, which allowsthe smoothly flowing air to follow the ball's surface a little farther around theback side of the ball. A dimpled ball thus has about half the drag of a smooth

ball. Thus the dimples help the ball travel much farther when hit by the club.A smooth golf ball hit by a professional golfer would travel only about half asfar as a golf ball with dimples does.

Dimples also help the golf ball convert spin into lift. A smooth ball withbackspin creates lift by warping the airflow such that the ball behaves like anaircraft wing, making the air pressure on the bottom of the ball higher thanthe air pressure on the top; this imbalance creates an upward force on the ball,producing the lift. In case of a dimpled golf ball the pressure difference ishigher due to creation of a thin turbulent boundary layer, which increases thelift and makes the ball go much farther than a smooth ball would go.

8. Ironing clothes

Cotton clothes are more comfortable than clothes made of synthetic fabrics.But cotton fabrics have one disadvantage -- they crumple easily. Especiallyafter washing and drying cotton clothes become so wrinkled that you can'twear them without ironing. But you can't iron a cotton shirt or cotton kameez


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without wetting it first, using a dry iron. Why do you need to wet cottonfabric before ironing?

Polyester and nylon are synthetic fabrics that become soft below 100C andcan be ironed smooth at a low temperature. Cotton fibre is made of cellulose,which cannot be softened by heat. In cotton fibre the cellulose chains are heldtogether by weak attractive forces called "van der Waals" forces. Whencellulose absorbs water this attractive force becomes weak and the fibrebecomes soft; now it can be reshaped into any form, which it retains afterdrying. In fact that is what happens when wet cotton fabric dries -- the fabricretains its wrinkled shape. And that is why we need to wet cotton clothesbefore ironing. When water is sprayed on the dry garment the fibres becomesoft and can be stretched smooth. When the hot iron is moved over the wettedcloth with pressure the wet fibres in contact with the smooth bottom of theiron dry and set with a smooth surface, free of any wrinkles. In modern steamirons steam rather than water is applied on the cloth, and produces the same

result.

9. Javelin throw

Javelin throw is a popular athletic event in which a metal or metal-tippedspear is thrown for distance. The men's javelin is about 2.6 metres in lengthand weighs 800 grams; the women's is about 2.2 metres in length and weighs600 grams. Modern day javelins are made out of aluminium or graphitecomposite.

Two major aerodynamic forces -- lift and drag -- act on a javelin in flight. Liftis the force that keeps the javelin in the air, and drag is the force that opposesthe javelin's flight. Drag works against the javelin at any angle of flight but itis the greatest as the angle of attack increases and more of the javelins surfacearea is exposed. These two forces act on the javelin in a spot know as the'centre of pressure', which is not fixed but can shift in relation to the centre ofgravity. When the centre of pressure is in front of the centre of gravity thejavelin remains tip up. When the centre of pressure moves behind the centreof gravity the javelin tips down. So to reach the greatest distance the throwerhas to strike a balance between the two.

The key objective in javelin throwing is to throw as far as possible withoutcrossing the foul line. There has been much debate over what is the idealangle to throw the javelin at. Although no there is no consensus any anglebetween 34-36 degrees is considered appropriate in calm conditions, but theappropriate angle can shift anywhere from around 30 degrees to around 40degrees depending on wind conditions. Throwing at a lower angle, byexposing less of the javelin to air pressure, can reduce the drag, while stillenjoying an increase in lift.


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Although the throwing angle is important, it is not the only factor thatdetermines the distance of throw; there are three variables that determine it.These three variables are the height of the throw, velocity at the release, andangle of release. Velocity at release is the perhaps the most important factor injavelin throwing.

10. Karate blow

In recent years, the ancient art of karate (from Karate-Do, a Japanese word,literally translated as the way of the empty hand) has become quite popularin India. Lakhs of children and young people are going for karate training as aself-defence tactic. One of the common feats of a karateka (practitioner ofkarate) is to break a pile of boards or bricks with nothing but a fist blow. Howdo they achieve it?

The basic principle behind a karateka's performance is a rapid transfer ofmomentum to the object being hit. The momentum of moving hand is masstimes the velocity. If the velocity is sufficiently high the momentum will alsobe large. It therefore follows that the karateka should move his or her hand asfast as possible in order to hit as hard as possible. An experienced karatekacan attain a velocity of 15 m/sec and a momentum of 45 kg m/s.

To be most effective, however, the momentum has to be transferred to theobject being hit in the shortest time possible, to maximise the impulse. Thatmeans, the moving hand should hit the target without rebound that would

lead to loss of kinetic energy. If all the momentum of the moving hand stopsat one point, the momentum is transferred to the object being hit and is notlost as kinetic energy. If a momentum of 45 kg m/s is transferred to the objectbeing hit in just 4 milliseconds (0.004s), the resulting force would be equal to10,000 Newtons, which is much more than the structure of the board or brickcan handle.

To make a hit more effective the karateka also minimizes the area of thestriking surface to maximize the amount of force and energy transferred perunit area. So he/she uses the side of the palm and not the whole palm tostrike, which increases the energy transferred per unit area almost nine times.

But it needs a lot of training and deep concentration for a karateka to break apile of boards or bricks with a fist blow.

11. Load and comfort


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If you have ever seen a pucca house under construction you'd have noticedthat the foundation of buildings up to four storeys are usually made muchbroader than the wall thickness. Do you know why is it so?

Well, the foundation is made broader to make the load-bearing area as largeas possible. As a result the weight of the building is distributed evenly over awide area and the building does not 'sink' into the ground. All this has to dowith how much force is transferred per unit area to the load-bearing surface --the more the surface area over which the force is distributed the less is thepressure experienced.

We have many such examples -- the camel's feet, fibreglass moulded seats,etc., where a larger area of contact reduces the pressure experienced. If welook at the footpad of the camel we'll find it is quite broad, whichconsiderably reduces the force acting on the ground per unit area. As a resultthe camel's feet do not sink in the soft sand.

Although hard, a moulded fibreglass seat feels comfortable because itscontours almost match our body contours and thus greatly increase the areaof contact and reduce the pressure points. The same is true of the track onwhich a heavy battle tank moves; it also distributes the heavy weight of thetank over a large area and the force per unit area experienced by the ground isreduced considerably. As a result the heavy tank can move over soft groundwithout sinking. The story of sadhus lying on beds of nail without feelingpain can also be explained by the same principle. Although a single nailwould easily pierce through the skin because the entire weight of the sadhu'sbody would act on a single point, when several dozen nails are used the

pressure felt at each point of contact would be much less because of the largenumber of contact points. Similarly, if you prick a balloon with a sharp pin itwill burst. But if you make a 'bed' of several dozen pins and press the balloonagainst it the balloon won't burst. Here, too, when several dozen pins areused, the applied force is distributed over a large number of pin tips each ofwhich is insufficient to pierce the rubber membrane of the balloon.

12. Long jump

Many of you may have seen long jumpers in action at an athletic meet. After

running a short distance before taking off the athletes land into a pit filledwith fine sand. Each participant attempts to land as far from the take-off pointas possible. But if you observe closely you'll find that the long jumpers don'tjust land a distance away; they appear to 'run' several steps through air aftertaking off and before landing. Why do they do that?

If you thought it increases the jumper's speed through air you'd be mistaken.The running action is done purely to maintain balance. The 'hitch-kick', as the


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running motion in air is called, stops the forward rotation of the jumper'sbody that he gets when he springs into the air. Just before taking off, as the jumper plants his foot on the jumping board, the motion of his lower bodystops for the fraction of a second when his foot is in contact with the board.But his upper body continues to move forward, which makes him start torotate forward around his centre of gravity. If no corrective action were taken,this rotation could send him toppling over and fall facedown into the sand.This is prevented by the hitch-kick.

During the hitch-kick, jumpers hold each leg straight as it moves backwardand bent at the knees as it comes forward. This difference in leg positioncauses the jumper's lower body to move forward. Similarly, the jumper's armmovements during the hitch-kick push the jumper's upper body backward.These body motions, which appear as running in air, neutralize the takeoffrotation and allow the jumper to maintain an upright posture and get into abetter position for landing.

13. Mirage on a road

Deserts are extremely hot places where temperatures in summer can reach50Celsius. There are any numbers of stories of thirsty travellers in search ofwater who keep moving attracted towards what looks like a distant lake orpool of water, only never to find it. In city roads in summer you can see asimilar phenomenon; distant buildings and cars appear reflected on a puddleof water on the road where there is none. What produces these reflectionswithout water?

What appears to be a lake or pool of water in a desert or a puddle of water ona city road is actually an optical phenomenon called a 'mirage' that creates theillusion of water; it is produced by layers of hot and cool air. Cold air beingdenser than warm air bends light more. So as light passes from colder air towarmer air it bends less, moving away from the direction of boundarybetween the two layers. When light passes from hotter to colder air, it bendsmore, towards the direction of the boundary.

Ordinarily hot air rises and the density of air decreases with altitude. But ifthe ground surface is very hot, as in the desert or a city road in summer, the

air immediately in contact with the ground remains very hot while air aboveit remains relatively cooler. When light from the sky or distant objects entersthe layer of the extremely hot air close to the ground at a shallow angle afterpassing through the cooler layers of air above, it curves upwards, reaching theeye of the observer from below. This produces the illusion of a shimmeringreflecting expanse resembling the surface of a body of water that does notexist. In deserts, the mirages are actually images of the sky being refractedback up from the hot air in contact with the hot sand.


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14. Physics of swings

Every child and even adults love swinging. In playgrounds swings are rarely

empty, with children vying with each other to get on to the swings. Littlechildren need a push to keep them swinging, but grown up children can keepthe swing moving and going higher and higher by a simple process of'pumping'. How do they do it? How do the keep the swing moving withoutanyone giving it a regular push? Let's see.

Swings are really a form of pendulum and so use the same physics concepts.When you use your legs to make yourself stand up or squat on the swing youare doing so by raising and lowering your centre of gravity, which generatesthe extra movement. Pulling backward on the ropes raises your body,decreasing the radius with respect to the support point, and thus increasing

your velocity. The same principle applies when you pump in a sitting positionby stretching and folding your legs. But the pumping would not work if notdone precisely at the right time in each cycle to synchronise with the naturalfrequency of the swing.

Since a swing is basically a pendulum it's possible to calculate its resonant ornatural frequency using pendulum equations as follows:

f = 1/2p (g/L)0.5 where: g = gravity constant = 9.8 m/s/s for Earth, andL = Length.

Note that the natural frequency of the swing is not influenced by the mass ofthe person in it. In other words' it makes no difference whether a swing has alarge adult or a small child in it. If the swing is pushed, or pumping is appliedat the natural frequency of the swing it would resonate and its amplitudewould increase during each back and forth cycle.

15. Reducing friction

Whenever two surface rub against each other we encounter friction. Friction isa force that resists the relative motion or tendency to such motion of two

bodies in contact and always acts in a direction opposite that of the motion.Friction poses a real problem in smooth running of machinery bicycles, cars,fans, sewing machines everything that have moving parts. Unlesssomething is done to reduce friction the moving parts become hot and wearout fast. Fortunately, there are substances called lubricants, like grease,lubricating oil, and graphite powder that, when applied as a surface coatingto moving parts, can reduce friction substantially. How do they do it?


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Usually, surfaces of machine parts that appear smooth and polished, haveirregularities -- little bumps and scrapes that can be so small that they showup only under a powerful microscope. When two such surfaces in closecontact move in opposite directions these minute irregularities get caught oneach other and act to oppose the movement. That is friction. The job of alubricant is to fill up those tiny irregularities and allow the two surfaces toslide over each other smoothly.

A wide range of substances solids, liquids, and greases are used aslubricants, depending upon the purpose for which they are used. Lubricatingoils are easy to apply but cannot be used in places where they can flow out.For applications such as fan bearings, and pump bearings and moving carparts, grease is used. Grease lubricants have several advantages over oillubricants because they require less maintenance and do not need stringentsealing of the lubricated parts. Solid lubricants include substance such asgraphite, molybdenum disulphide, Teflon, and boron nitride. They are useful

for conditions where conventional lubricants are inadequate such as inapplications where a sliding or reciprocating motion is involved; at hightemperatures where liquid lubricants typically would not survive; and underextreme contact pressures.

16. Remotes

In cities many of us use remote controls for switching on and changingchannels on the TV, to operate a VCD or DVD player, to control an airconditioner, or to lock or open cars. But all remote devices do not work in the

same way. For instance, remotes used for TV, VCR, DVD players or airconditioners have to be pointed at the device being controlled. If the remoteis pointed away it doesn't work. But a car remote control works even if it isnot pointed at the car. Why this difference?

Remotes are primarily wireless devices, which use some kind ofelectromagnetic waves to control a gadget kept at a distance. TV, VCR, DVDplayer remotes and remotes used for air conditioners use a narrow beam ofinvisible infrared waves for operation. The remote has an electrical circuitthat produces pulses of infrared waves from a light emitting diode (LED)fixed at the front end of the remote device. The LED has a reflector or a lens to

produce a narrow beam that can be directed at the gadget being controlled.The remote also has several buttons for different channels and otheroperations.

When you press a button on the remote, a specific connection in the circuit iscompleted. The chip in the remote senses that connection and knows whatbutton was pressed. It produces a Morse-code-like pulsed signal specific tothat button. The transistors amplify the signal and send them to the LED,


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which translates the signal into infrared light. The sensor in a TV, VCR, DVDplayer, or air conditioner can see the infrared light and react appropriately.Since the infrared beam is highly directional the gadget can respond onlywhen the beam is directed at its sensor. That is why it is necessary to point theremote at the gadget being controlled.

Car remotes used for locking and unlocking car doors from a distance usehigh-frequency (300 or 400 MHz) radio waves. The small unit attached to thekey chain is actually a small radio transmitter. When you push a button onyour remote, you turn on the transmitter and it sends a digitally coded radiosignal to the receiver fitted in the car, which is tuned to the frequency that thetransmitter is using. Once the car receiver senses the correct digital code itprovides power to the actuator that unlocks or locks the doors. Since radiowaves travel in all directions, a car remote need not be pointed at the car.

17. Sagging wires

If you have ever watched high-tension transmission lines that criss-cross thecountryside, you must have noticed that the wire cables strung across thetransmission towers are not stretched taut; they sag between the towers. Butwhen you see the overhead lines of electrified railway tracks they lookperfectly horizontal. Of course, they need to be perfectly horizontal;otherwise it won't be possible for the pantograph of the electric locomotive tobe constantly in contact with the overhead wire. But why do transmissionlines sag but overhead electric traction lines do not?

When a flexible cable of uniform density and cross section is hung freely fromtwo fixed points it has a natural tendency to take the shape of a curve called a"catenary" due to the effect of gravity. The cable sags because it has weight. Asagging cable is stable; that is, it does not exert any sidelong force on thesuspension points. If the cable is not allowed to sag the towers could collapse.Although a sagging cable presents no problem in power transmission, it iscertainly unacceptable for railway electrification. But making the wire tautcould make the structure unstable. Railway engineers found a simple solutionto the problem. They devised a structure consisting of an upper structuralwire in the form of a shallow catenary, to which a lower conductive contactwire is attached with short suspender wires of different lengths. In this

arrangement, since the upper structural wire is allowed to sag, it exerts noextra force on the pillars. By adjusting the tension in various elements theconductive wire is kept horizontal -- parallel to the centreline of the track,allowing uninterrupted contact with the overhead pantograph of the train.

18. The starting block


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If you have ever seen athletes at the start of a sprint event you may havewondered why they crouch low with their hands touching the ground andfeet firmly set against what look like inclined foot pads. The inclined footpadsare 'starting blocks' which are used by the athletes to get off to a good start ina race. By starting from a crouching position, and pushing against startingblocks the sprinters are able to accelerate better.

Early sprinters used to dig holes in the track in which to place their feet whenstarting, to get an extra push at start. Track coaches have been striving foryears to develop some kind of technique to improve their sprinters'performance. Research has produced some staggering advances, but none hashad a more significant effect than the starting block, especially after theadvent of synthetic track surfaces, where digging holes was out of question.Blocks were introduced in the late 1920s and were first used at the 1948Olympic games in London.

Starting blocks are usually made of aluminium and have a centre rail andslotted angles to firmly grip the blocks. They are made adjustable to fourdifferent angles and are fitted with special thick rubber to take in spikes of theathletes boots.

But merely using starting blocks cannot improve the performance of asprinter unless the blocks are set properly and the sprinter takes up the rightposture. Technically, the distance between the front block and the starting lineshould be two foot-lengths of the athlete. The rear block is to be placedanother foot length behind the front block. Spacing can be adjusted based oncomfort, existing strength levels, etc. For best start, the front knee angle

should be between 90 and 110 degrees, while the rear leg angle should bebetween 120 and 135 degrees.

19. Swinging the ball

Swing is one of the most important weapons in the arsenal of a fast bowler.After leaving the bowler's hand the ball at first appears to come straight at thebatsman but then swerves towards or away from him, often forcing him to'nick' the ball and get caught behind. How does a bowler swing the ball?

Swing is easily explained by physics. When the ball moves through air it cutsthrough the air which moves around it, but the velocity at which the airmoves depends on the nature of the surface. If the surface is smooth andshiny the velocity of air moving in contact with it would be fast, but airmoving in contact with a rough surface would not move as fast. The key tomaking a cricket ball swing is to cause a pressure difference between the twosides of the ball. Since the air pressure depends on the flow of air over eachside of the ball, if one side of the ball is made rough then air flow on that side


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is reduced but air flow on the other, shiny side would be fast. As a result,according to Bernoulli's principle, the air pressure on the shiny side isreduced and the ball would swerve towards the shiny side. By shining theball on one side, and carefully positioning the seam, which runs around theball, bowlers can make it curve through the air as it approaches the batsman.

Bowlers are allowed to polish the ball by rubbing it with cloth (usually ontheir trouser legs) and applying saliva or sweat to it. Any other substance isillegal, as is rubbing the ball on the ground. It is also illegal to roughen theball by any means, including scraping it with the fingernails or lifting theseam.

20. The cutting edge

It is common knowledge that cutting tools should be sharp. The carpenter

always keeps his tools sharp, as does the cobbler, barber, or the man in themeat shop. A sharp knife cuts better than a dull knife. It is difficult to cutsomething with a dull knife; you need to apply more pressure than youwould need if you use sharp knife. Why is it so? Why doesn't a dull knife cutwell?

In simple words, all this has to do with how much force is transferred per unitarea to the surface being cut. If the knife-edge is sharp the total area of thecutting edge (area of contact) would be much smaller than the total area of thecutting edge of a dull knife. So, if equal force is applied on both the knives,the force per unit area experienced by the surface being cut with the sharp

knife will be much more than the force experienced by the surface being cutby the dull knife. Obviously, the former will cut better than the latter.

Dull knives not only lead to excessive use of force to cut materials, they alsoincrease the chance that the blade may slip and the force transferred to anunintended destination such as the user or another person and cause injury.The same logic applies to other sharp objects like needles, nails and pins thesharper the better.

21. The fictitious force

If you are travelling in a car and the car swerves around a corner, you'd findyourself pushed against the outer edge of the car. It appears that some unseenforce is acting to push you against the side of the car. Commonly this unseenforce is termed as 'centrifugal force', a force that tends to move objects awayfrom the centre in a system undergoing circular motion. It also keeps thewater in a whirling bucket from spilling or keeps roller coaster riders fromfalling out when coaster 'loops the loop'. However, it is not a real force but an


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apparent force, equal and opposite to the centripetal force, that draws arotating body away from the centre of rotation; it is caused by the inertia ofthe body.

Centrifugal force can be explained in terms of Newton's laws of motion. Asthe car changes direction the passenger's inertia resists acceleration andchange in direction, keeping the passenger moving with constant speed in thesame direction. But since the car turns it appears that the passenger is beingpushed against the side whereas actually the passenger does not movetoward the side of the car; instead, the car curves in to meet the passenger.

Although considered a fictitious force, centrifugal force has manyapplications. Centrifuges and ultracentrifuges are used in science andindustry to separate substances by their relative masses. Centrifugalgovernors use spinning masses to regulate the speed of an engine bycontrolling the throttle. Centrifugal force can be used to generate artificial

gravity in space stations. The oblate shapes of the planets Jupiter and Saturnare explained as due to centrifugal force created by their rapid spins.

22. A question of steering

When you ride a bicycle, or drive a motorbike, or drive a car you need to steerit to turn it in any desired direction. In bicycles, motorbikes and two-wheelersin general the steering is done with a handle bar, but four-wheelers use asteering wheel for the purpose. Why are they different?

To find an answer this question we've to first understand how a vehicle issteered. A two-wheeler such as a bicycle or a motorbike is steered primarilyby turning the front wheel (along with a slight tilting, called banking, in thedirection of the turning). Since the front wheel is fixed directly under thehandle bar that can be turned around a vertical axis fixed to its centre, thefront wheel can be turned simply by turning the handle. When a torque isapplied around the middle of the handle bar the front wheel turns.

In a four-wheeler not one but both front wheels need to be turned for steeringand here a system of rack and pinion is used. For a car to turn smoothly, eachwheel must follow a different circle. Since the inside wheel follows a circle

with a smaller radius, it actually makes a tighter turn than the outside wheel.And to do this a rack and pinion mechanism is used with a steering wheel.The pinion gear is attached to the steering shaft. When the steering wheel isturned, the gear spins, moving the rack. The tie rod at each end of the rackconnects to the steering arm on the spindle, which turns each of the wheels.On most cars, it takes three to four complete revolutions of the steering wheelto make the wheels turn from lock to lock (from far left to far right).


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23. Tractor wheels

We see many types of four-wheelers on the road cars, buses, trucks, tankers,tractors, and many others. Among these there is something peculiar about

tractors their huge rear wheels, sometimes as large as 1.6 metres across. Allthe other four-wheelers have all the four (sometimes more) wheels of thesame size. Why do tractors have large rear wheels?

To answer this question we have to look at the purpose for which tractors areused. Tractors are mostly used for farming and that means they have bedriven over soft, often muddy, soil. The large rear wheels with large surfacearea distribute the weight of the heavy tractor over a large area and therebyprevent the tractor from getting bogged down in wet field. The deep andwide treads also provides a firm grip in mud preventing the wheels spinningfreely as would happen with ordinary car wheels.

Also the large rear wheels means larger area of contact with ground than asmall wheel. So a large wheel provides more traction power, which isnecessary for the tractor to pull, farming implements such as ploughs andtillers through the dry or wet ground. Larger diameter means greater pull. InIndia, tractors are also used for hauling a variety of goods and here also thelarge rear wheels provide better hauling power.

The front wheels of tractors are small because they are used mainly forsteering, and large wheels are harder to steer. Small front wheels also give thetractor operator a clearer view of the rows through which it is working and

possibly better turning capability. Of course, many of today's tractors comewith all wheels of the same size, but they are used for different purposes. Inmany foreign countries four-wheel-drive tractors are also available withpower-assisted steering.

24. Tyres with tread

Millions of vehicles ply on the roads every day and all use wheels with air-filled rubber tyres. Air-filled tyres make the ride comfortable by cushioningthe jerks caused due to bumps on the road surface. They also provide a high-

friction bond between the vehicle and the road surface to improveacceleration and handling.

Although vehicle tyres come in a wide variety of shapes and sizes they haveone thing in common all come with treads cut around their circumference.A tyre is considered safe as long as the treads remain visible; tyres withcompletely worn-out treads are considered unsafe. How do the treads on arubber tyre make it safe?


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Rubber tyres offer good grip on the on the road because of friction that arisesdue to adhesion between surfaces. Increase in contact area between twosurfaces increases the frictional forces. If an elastic material like rubber is usedthe real contact area further depends on the load pressing the two surfacestogether. If the surface is grooved, as in a tyre with treads, the increase incontact area with load is much more compared to a surface without grooves.The design of the grooves and ridges of the tread affect the amount of thedeformation, and hence, the friction or grip on surfaces.

Besides increasing road grip, treads also cool the tyre while running at highspeeds, and provide a safe margin of rubber before the complete tyre wearsout. In wet conditions they provide ducts through which the water issqueezed out. This in turn helps the tyre have better grip on wet roads.

25. Voltage vs current

We use all kinds of electrical appliances. Some work on batteries, others runon mains power. We know that mains power is dangerous because thevoltage is high -- 220 volts for domestic supplies. If we accidentally touch alive wire we get a shock. But we never get a shock if we touch the terminalsof a 6-volt battery although if we connect its two terminal with a wire the wiregets hot. Similarly, transformers used for welding works at only 12 volts andwon't give a shock, but it can melt steel for welding. Why is it so?

To understand why a low voltage doesn't give a shock but can produce

enough heat to melt metal it is necessary to distinguish between voltage andcurrent. Voltage is a term used to designate electrical pressure or force thatcauses current to flow. It is like the height of a waterfall; the differencebetween the top and bottom levels represents the voltage. If the water dropsfrom a low height it would be analogous to a low voltage. When the waterfalls from a higher level the force of the fall is much greater than the force offall from a lower height. In an electrical circuit it is this force that we feel as ashock. If the voltage is low we don't feel any shock.

Current is like the volume of water flowing in the waterfall. Even if the heightis large the water flow may be just a trickle, which won't produce any useful

work. If the volume of water flow is large, even with a lower head it canproduce useful work.

Heating of an electrical conductor is proportional to the square of currentflowing through it (Heat produced = I2R), so the wire connecting two terminalof a battery get hot because more current flows through it. The transformerused for welding is a step-down transformer, which reduces the outputvoltage to 12 volts from 220 volts of the mains. But, since the product of


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voltage and current remains constant, stepping down the voltage increasesthe current flowing through the circuit several folds, which is essential forwelding.

26. Of friction and walking

If you have ever walked on ice, you might know how easy it is to slip and fall.Ice is very slippery because it offers little friction. Similarly, if you happen tostep on a banana peel you'll meet the same fate, because the banana peel alsoreduces friction and makes the road below your feet slippery. But why can'twe walk on a slippery surface? Let's find out.

Usually, even surfaces that appear smooth have irregularities -- little bumpsand scrapes that can be so small you cant see them. Similarly, the sole of ourfeet or shoes also have little bumps and scrapes. Friction is caused by these

irregularities getting caught on each other as two surfaces rub together.Friction is a force that always acts in the opposite direction of the objectsmotion. Friction is necessary for walking.

When you walk, with each step you plant your foot firmly on ground andpush against it, thereby pivoting your body around that foot to moveforward. As you plant the other foot on the ground in front of you, theground exerts a force back up your leg and you rise up on that foot and moveanother step forward. Every time you put your foot on ground it is held inplace by friction, which makes pushing easier. But if there is no friction yourfoot cannot hold on to the ground and cannot push against it. The result? You

slip and fall.

Some things like a highly polished floor and ice dont have manyirregularities to get caught on, so there is little friction. Without the friction,you slip. On a concrete or paved road surface there are many rough spots foryour shoes to get caught on so you dont slip. But a banana peel acts as ablanket, smoothening out the rough spots, and makes you slip when you stepon.

In the language of physics, the 'required friction coefficient' represents theminimum friction needed to support walking, while the 'available friction

coefficient' represents the maximum friction that could be supported atinterface between the shoe and floor without a slip. When the requiredfriction for an activity exceeds the available friction at the interface, you aremore likely to slip.

27. Water as fire extinguisher


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It is rightly said that things that burn never return. Fire is a great destroyer. Abadly managed fire can raze buildings and burn down hundreds of hectaresof forestland in no time. While prevention is the best course, once a fire startsthe best option is to extinguish it as fast as possible, and there are fewsubstances as efficient in killing fire, except electrical or oil fire, as water.Although water is not directly used to extinguish oil fires the foam used toextinguish such fires is mostly water-based. What makes water such anefficient extinguisher of fire?

For a fire to occur, there must be available oxygen, a supply of fuel, andenough heat to kindle the fuel. Therefore, the three basic ways ofextinguishing fire are to smother it, to cut off the fuel supply, or to cool itbelow the flammability temperature.

Fires involving solid materials, such as wood, paper, straw, textiles, coal, etc.,are the most common types of fire and the best way to control such fires is to

bring down the temperature of the burning material quickly, which waterdoes most efficiently. The property that makes water an efficient fireextinguisher is its capacity to absorb large amounts of heat. In fact, water hasthe highest specific heat capacity of any known chemical compound, as wellas a high heat of vaporization. That means, water absorbs the largest amountof heat per gram for every degree rise in temperature and also for reachingthe boiling point and thus cools fast and puts out a fire.

28. Laser printer

Ever since the German inventor Johannes Gutenberg invented movable typesfor printing in the mid-1450s that revolutionised written communication, thetechnology has come a long way. Movable types have given way to linotype,offset, intaglio, and flexi printing. Today personal computers allow users toprint a document at home or in office. Home printers even allow one to getcolour prints of one's favourite photos taken with a digital camera.

Home and office printers come in three basic types: dot matrix, inkjet andlaser printers. The terms dot matrix and inkjet printer are very descriptive ofthe processes at work. The first prints the alphabets as a combination of dotsimprinted by a matrix of pins impacting against an inked ribbon. Inkjet

printers put an image on paper using tiny jets of ink. But the term laserprinter is a bit more mysterious how can a laser beam write letters and drawpictures on paper?

Actually the laser beam does not do the writing. The primary principle atwork in a laser printer is static electricity, which is used to charge a polishedsurface of an insulated drum coated with a photoconductive material. Thedrum is first given a total positive charge by an electrically charged corona


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wire. The unique property of the drum coating is that the charge can bereversed by exposure to light. When the "print" command is given to theprinter, a tiny laser beam "draws" the letters and images to be printed as apattern of electrical charges an electrostatic image on the charged drum.At the regions where the laser beam strikes the positive charge on the drum isneutralised and the characters imprinted on the drum gets a negative chargewith respect to the rest of the drum. When a positively charged toner isapplied on the drum it sticks only to the negatively charged areas; that is, thecharacters imprinted by the laser beam on it. The toner pattern is thetransferred to paper, which is then heated to fix the toner permanently. Thusin a laser printer the laser does not actually do the printing, but only "draws"an electrostatic image which is turned into visible characters on paper by thetoner.

29. Is it vertical?

In any building construction work the walls have to be perfectly vertical. Thisis necessary to ensure that the load falls vertically on the ground. A wall tiltedfrom the vertical would collapse under load if not supported on the tilted sideby buttresses. But how does one ensure the verticality of a wall? Masons do itusing a simple device called the "plumb line". The plumb line employs thelaw of gravity to establish what is "plumb"; that is, what is exactly vertical, ortrue.

The plumb line is basically a conical metal weight attached to a string whichhangs vertically in Earths gravitational field if let free. When freely

suspended, the hanging string is directed exactly toward the Earth's centre ofgravity and can be used as a vertical reference line. The line has in every pointthe same direction as that of the force of gravity of the Earth; thus, an objectdropped on the surface of the Earth tends to follow this line. To use the tool,the string is fixed at the point to be plumbed. The weight, or bob, is thenallowed to swing freely; when it stops, the point of the bob is precisely belowthe point at which the string is fixed above. When the plumb line issuspended from a wall under construction the bob should just touch thelower end of the wall if the wall is perfectly vertical.

The plumb line is also used to transfer points from a height on to ground. Of

course, the transfer can be done without using a plumb line, by mere eyesightif the observer looks vertically straight down, but that is not always possible.Any shift in the eye position from the vertical would introduce a parallaxerror and the point marked on the ground would not be directly below thepoint at a height but away from it.

30. The spade


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The spade is a common gardening implement used for a variety of purposes.The spade consists of two parts; the blade, of plate-iron, and the handle, madeof tough wood. The wooden handle ends in a crosspiece, usually forming akind of loop for the hand. The blade consists of two parts; the plate, by whichthe soil is cut and carried, and the tread, which is a piece of strong iron fixedon the upper edge of the blade, to receive the impulse of the foot of theoperator. Spades are manufactured of different sizes, and usually with a flatblade. In gardening, a spade is used to dig or loosen ground, or to break upclumps in the soil. It is sometimes considered a type of shovel.

A spade is used as a lever of the first class and also as a lever of the third class.When it is used as a digging tool, the load acts at the end of the flat blade, themiddle of the blade acts as fulcrum and the force of the hands acts on thehandle at the other end. Depending on the size of the blade and the handle aspade can give considerable mechanical advantage, which can be used for

extracting dead or cut tree stumps from the ground. When a spade is used liftsoil or rubble, the spade acts as a lever of the third class -- the load acts on theblade, the handle acts as fulcrum and the hand holding the middle exerts theforce to lift the load. Although the mechanical advantage of a third class leveris always less than 1, for least effort the lifting force should be applied as faraway as from the fulcrum; that is, the lifting hand should be placed as nearthe blade as possible.

31. Carrying a load

Throughout history men and women of all ages have carried goods, food,supplies and arms for the purpose of survival. Today, despite manytechnological advances, this basic form of human-powered transportationremains an indispensable resource for many occupational tasks and activitiesof daily life.

A person can carry a load in many different ways -- on the back, on one hand,on both hands, on shoulder, or on the head. In studies on the energetics ofdifferent modes of carrying load it has been found that the metabolic cost ofcarrying large back-supported loads was almost twice that of carrying largehead-supported loads for the same load at the same walking speed. The

rucksack method of carrying load on the back was found to be the moreeconomical than carrying load in hand, in terms of energy expenditure.

It is easy to understand why carrying a load on the head needs the least effortcompared to any other method. From physiological point of view, the bestway to carry a load would be one that does not put undue strain on the bodystructure to sustain balance. Obviously carrying a load on the head would bethe most comfortable because here the load would act vertically and would


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not upset body balance, although it would raise the centre of gravity of thebody as whole, making it prone to fall. Carrying too much weight on the headalso may be dangerous; it could cause cervical spinal cord injuries.

On the other hand, carrying load on the shoulder or in hand puts strain on thebody structure in the form of a bending force that the body has to counterconstantly to maintain balance. This has also been proved in studies, whichshowed that hand carriage caused marked side bending of the trunk and poorposture. Carrying the load with the hands by the side proved to be the worstin terms of physiological efficiency. Carrying a bag on one shoulder, byputting constant stress, was found to lead to posture that might predispose toback pain.

32. Reinforced concrete

Reinforced cement concrete (RCC) is a standard building material used inbuilding construction of all types. Brick buildings may have walls made ofbricks, but the roof is always cast in RCC. The material is called "reinforced"cement concrete because it contains steel reinforcements in the shape of thinbars embedded in the concrete.

Typical concrete mixes have extremely high resistance to downwardcompressive stresses (about 21 million pascals); however, any appreciablestretching or bending (tension) will break the microscopic rigid latticeresulting in cracking and separation of the concrete. For example, afoundation of cement concrete without steel reinforcement would be able to

take the load of a large building without failing, but if a roof slab or a beam iscast without reinforcement it would easily give way under heavy load. Thesteel reinforcement embedded in RCC helps the concrete withstand hightensile loads and thus prevents the concrete structure from breaking up underheavy load.

When a load is placed on a slab or a beam supported on two ends, the loadinduces compression on the upper part and tension in the lower part of themember. So the reinforcement is also placed in the lower part; that is, close tothe lower surface of the slab or beam. However, if the structural member issupported only on one end, then loading produces tension in the upper part

of the member. So the reinforcement is placed near the upper surface of themember.

33. Why do we need earthing?

Domestic electricity supply in India is 220 volts AC. Any accidental contactwith a current carrying conductor, due to a faulty electrical gadget or a short


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circuit at 220 V AC can be fatal. Hence a safeguard is provided in AC circuitsin the form of a third conductor called the "earth".

The main objective of earthing is to provide an alternative path for anyleaking current to flow to the ground so that it would not endanger the user.The earthing of an electrical installation not only provides protection forpersons against the danger of electric shock, but also maintains the properfunction of the electrical system. The Earth always maintains a zero potential -- it is neither positively nor negatively charged. So, when a faulty electricalcircuit is connected to earth the leaking current can safely flow to the groundthrough the earth wire because it offers much less resistance compared to thebody of the user, thus causing no harm.

The 'earth' terminal of an electrical circuit is made up of a conducting wireconnected to a metal conductor buried in the ground. All exposed metal partsof an electrical installation or electrical appliance need to be connected to this

wire. This is done by using 3-wire conductors and a three-pin plug andsocket. All electrical outlets with a 3-pin socket have an earth connection. In a3-pin plug the longest prong, which is connected to the green wire, is theearth connection. The other end of the green wire is usually connected to themetal body of the electrical appliance. The earth prong is made longer so thatin case of any current leakage the earth connection is established before thefaulty gadget is connected to the mains and an accident can be prevented.

34. Pumping water

Every day, millions of water pumps deliver water from wells to homes, farmsand businesses. Conventional hand pumps or centrifugal pumps can liftwater from a well only if the water level is less than 10 metres below thesurface. This is because these pumps depend on the pressure of theatmosphere to lift water. The low pressure created inside the pump by thepiston or impeller makes the normal pressure of the atmosphere push thewater up through the pipe. You can think of it as a long straw you use to sucksoft drinks. As you suck on the straw, a low pressure is created in the strawabove the liquid and normal atmospheric pressure pushes the liquid up thestraw. Consequently, the height that you can lift water with a hand pump orcentrifugal pump relates to the height of a water column the atmospheric

pressure can support. This height is about 10 metres because the atmosphericpressure is able to support a water column about that high. So a hand pumpcannot lift water from a depth greater than 10 metres. Then how do you liftwater to the top of multi-storeyed buildings?

The answer is simple. While there is a limit to the depth from whichconventional pumps can lift water, there is no limit to the height to which apump can push water up. So, if the depth is greater than 10 metres, water can


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be lifted if the pump is placed near the bottom of the well, submerged inwater. Such a pump, called a submersible pump, can lift water from depthsof more than 10 metres because they actually push the water up from thebottom of the well. A typical submersible pump is characterized by a longcylindrical shape that fits inside the well casing. The bottom half is made upof a sealed pump motor that is connected to the aboveground power sourceand controlled by wires. The pump itself is made up of a stacked arrangementof impellers that drives the water up the pipe to the plumbing system. Here,depending on the power of the pump, water can be lifted to almost anyheight.

35. Sticking them together

In our daily life we use a variety of substances to stick things together. Glue,Fevicol, Cellotape, and many others are used as adhesives. But how do they

work; how do they keep two surfaces from coming apart once stuck together?Well, it all has to do with some kind of bonding.

One thing that adhesives have in common is that they're made of polymers,which are chain-like molecules. A good adhesive has excellent properties ofadhesion (the ability to stick to the surfaces to which it's applied) andcohesion (the ability to stick to itself). The bonding can be of three kinds --mechanical bonding, physical interaction, and chemical interaction.Mechanical bonding involves some kind of "anchoring". The adhesive flowsinto microscopic pores in the two surfaces and hardens after drying such thatit keys into the surfaces and forms a strong surface bond to hold them

together. Starch glue, and Fevicol are typical examples of this kind ofadhesive.

Bonding by physical interaction involves weak intermolecular attractioncalled van der Waals force between the materials being bonded and theadhesive. Here, the adhesive has a low surface tension and easily "wets" thesurfaces being bonded, which stick together due to van der Waals forces.Adhesives used to bond smooth surfaces like steel and glass belong to thisclass.

Some adhesives come in two parts -- a resin and a hardener or catalyst -- that

are mixed just before use. After application the mixed adhesive hardens bychemical reaction. Epoxy-based adhesives such Araldite belong to thiscategory.

Pressure-sensitive adhesives stick to a surface when pressure is applied. Mostadhesive tapes use a pressure-sensitive adhesive. The tape looks smooth, butit's not. Its adhesive coated side has tiny pits in which air bubbles get trappedwhen the tape is applied to a surface. When pressure is applied air escapes


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from the tiny bubbles, which then act like minute suction cups holding thetape strongly to the surface.

36. Engines without spark plugs

Motor vehicles that use fuel are primarily of two types those which run onpetrol or CNG and those that run on diesel. In both the expansion of hotgases provides power to the piston that drives the engine, but there is basicdifference between the two. In petrol and CNG driven vehicles spark plugsare used to ignite the fuel mixture in the engine cylinder to provide thepower. But in a diesel vehicle no spark plugs are used. How is the fuel burntin a diesel engine?

In a four-stroke petrol or CNG engine the fuel and air is first drawn into thecylinder, which is then rapidly squeezed by the piston into a small volume

(about 1/10th the original volume) that heats up the mixture. At the end ofthe compression stroke the heated mixture is ignited by a spark and theexpanding hot gases push the piston down, providing the driving force.Here, the fuel and air mixture is expected to wait until it's ignited at theproper instant by the spark plug. That's why gasoline is formulated to resistignition below a certain temperature. The higher the "octane number" of thegasoline, the higher its certified ignition temperature.

A diesel engine doesn't use spark ignition. Instead, it uses the hightemperature produced by extreme compression of air to burn the fuel. Whenpure air is rapidly squeezed up to 1/20th the original volume, it becomes so

hot that it can ignite the fuel. Thus when diesel is injected into the cylinder atthe end of the compression stroke it bursts into flames and burns quickly inthe superheated compressed air. The rapidly expanding hot gases push thepiston to drive the engine.

37. Potholes

Every year during monsoon the rains play havoc with city roads. After a fewdays of rain most city roads show up numerous potholes that keep growingin size as the days pass, making driving a nightmare for motorists. How do

these potholes appear?

The appearance of pot holes have something to do with the material roadsurfaces are built of, and simple physics. They appear only on asphalt roadsbut never on concrete surfaced roads, for the simple reason that in concretethe aggregates (stone chips) are firmly bound in a cement matrix, which isimpervious to water. And concrete can withstand large compressive loadwithout fracturing. But in an asphalt surface the aggregates are weakly


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bonded by asphalt, which deteriorates in contact with water, making thesurface vulnerable to break up.

A pothole usually begins as a tiny crack on the road surface. After rain, ifwater accumulates on the surface, water seeps through the crack, and theasphalt below the surface starts losing hold of the stone chips, which come offgradually. The process is hastened when vehicles pass over the damagedsurface. Water forced out of the treads of the tyres moving vehicles act ashigh-pressure jets, dislodging more chips out of the weakened surface and thepot hole becomes bigger and bigger. However, if rainwater is not allowed toaccumulate on the road development of potholes may be prevented, as it isthe combined effect of accumulated rainwater and action of moving vehiclesthat produce potholes on roads.

38. Screwdriver

Screwdriver is an essential ingredient of any toolbox. As the name implies, ascrewdriver is used to drive a screw into a surface or take it out. Sometimesscrews are inserted and fixed into threads cut in a surface, or used with a nutto fix things. But in all these actions the screw has to be turned to fix it orunfix it.

Screwdrivers come in many sizes and the size of the screwdriver used woulddepend on the job to be done. A watchmaker can do with a very smallscrewdriver to fix tiny watch screws, but a carpenter would need a longscrewdriver with a large handle for joining wooden pieces or fixing objects to

wooden frames. A watchmaker's screwdriver would be useless for thecarpenter and so would be a carpenter's screwdriver for a watchmaker. Whythis difference?

A screwdriver makes use of the lever principle for turning and fixing screws.The tip of the screwdriver fits snugly on the head of the screw to be driven.When the screwdriver is to be used, equal and opposing parallel forces, whichform a couple, are applied to turn it. Turning the tiny screws used in watchesneeds very small force that can be applied by twirling the thumb and theforefinger. Here, the diameter of the handle of the screwdriver is very smalland so is the lever advantage. But driving larger screws into wood needs

much stronger force, which cannot be provided by a small screwdriver. Herea larger screwdriver with a thicker handle is required. A handle with a largediameter not only gives a much higher lever advantage than a watchmaker'sscrewdriver but also provides a stronger grip, making it possible to applymuch stronger opposing forces to turn the screw.

39. Propeller vs jet engine


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Since the American brothers Orville and Wilber Wright flew their firstheavier-than-air machine in 1903, air travel has come a long way. In its firstpowered flight, Wright brothers' "Flyer", which flew on propeller power,remained in air only for 12 seconds, and covered a distance of about 40metres. Today's jet airliners can fly much faster than any propeller-drivenaircraft and can remain in air for more than a dozen hours, covering morethan 12,000 kilometres non-stop. Jet aircraft can also fly at much higheraltitudes than propeller-driven planes can. How do they work at altitudeswhere air pressure is less than one-fifth at sea level? Let's find out how.

A propeller-driven aircraft makes use of Bernoulli's principle both for lift andforward motion. The aerofoil shape of the wings makes the air over the topmove faster than the air under. Slower air has more pressure, so there is a netupward thrust on the aerofoil, which produces lift. The blades of a propelleract as rotating wings, and produce force through application of both

Bernoulli's principle, generating a difference in pressure between the forwardand rear surfaces of the airfoil-shaped blades. But a propeller's performancesuffers as the blade speed exceeds the speed of sound. That is why aircraftwith conventional propellers do not usually fly faster than Mach 0.6; that is,faster than 60 percent of the speed of sound.

Unlike propellers, jet engines work well at high speeds and jet aircraft can flyat speeds greater than the speed of sound. A turbojet engine is a type ofinternal combustion engine. It works by first compressing incoming air with aseries of fan-like blades. Fuel is then mixed with the compressed air and themixture ignited. Finally, the high-energy gases and hot air is ejected at high

speed out of the rear of the engine, which pushes the aircraft forwardaccording to Newton's third law of motion. Most modern jet engines areactually turbofans in which a large fan attached to the front end of a turbojetengine is used to supply supercharged air to not only the engine core, but to abypass duct, which increases the efficiency.

40. Cutting glass

Glass is a unique material. It has the stiffness and brittleness of crystals butlacks their large-scale regularity of structure. Glass is an amorphous

substance; with no regularity in the way their molecules are arranged in thesolid. Strictly speaking, glass is not a solid but a highly viscous liquid, but atthe same time it is very brittle. If you take a regular piece of windowpanebetween your hands and try to break it in half by bending, it appears quitestiff. If you apply enough force you can break the pane into two or moreirregular pieces. But, then, how do carpenters and photo framers cut glasssheets with such ease -- just by making a thin scratch and applying a littlepressure?


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Actually, glass isn't really 'cut' in the normal sense of the word, but is onlysubjected to a controlled break. Since glass has no crystalline structure it hasno cleavage planes (like gemstones, for instance). Also, since glass is equallystrong in any direction, you normally won't have to worry about direction ofgrain like carpenters do. To cut a sheet of glass you first create a fine crack onthe surface by scoring with a diamond tip and then applying pressure in theopposite direction. When you bend a glass sheet, you stretch one surfacewhile compressing the other and create a tensile stress on the stretchedsurface. If there is a crack on the stretched surface, glass breaks along thecrack. By scoring the surface of the glass with a diamond stylus, you cancreate that crack and control exactly where that lapse in tensile strength willoccur. Scoring disrupts the surface integrity along a thin line along which thebreak occurs. This happens because glass is brittle and cracks can travelthrough them easily. Once the crack starts to grow things go from bad toworse. The crack becomes sharper and the stress increase at the tip becomes

larger and larger. The crack tip propagates through glass at roughly the speedof sound and results in a clean break.

41. Badminton shuttlecock

Badminton is a popular game played with a racquet, which consists of ahandle and an oval frame with a tightly interlaced network of strings. Butunlike other games played with racquets like tennis and squash, badminton isplayed with shuttlecocks -- a lightweight, open conical-shaped object made bysticking 16 goose feathers into a hemispherical piece of cork (called the

bumper). Also known as a 'birdie', 'cock', or 'shuttle', shuttlecocks are high-drag projectiles unlike the spherical balls used in other games; they encounterhigh air resistance in flight. Yet, badminton is the fastest racquet sport in theworld with shuttles known to reach speeds of up to 332 km/h. How does theodd-shaped shuttle fly so fast and why does it always turn around and hit theracquet bumper first?

A spherical ball, being symmetrical in shape does not experience any torquewhile moving through air. But a shuttlecock is symmetrical only around itslong axis and experiences least air resistance and torque only if it flies bumperfirst through the air. In any other orientation, it experiences significant air

resistance and torque while moving through air. This is because theshuttlecock's centre of pressure -- the point at which the overall pressure forceeffectively acts -- isn't located at its centre of mass, which lies in the middle ofthe cork bumper. It is this torque that quickly turns the shuttlecock around inthe air immediately after being hit by the racquet and makes it fly through airbumper first.


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Being lightweight, a shuttlecock can be quickly accelerated to very highvelocities by applying the same force that would accelerate a heavier tennisball to a much lower velocity (F = m x a). With its bumper flying ahead of itsfeathers, the shuttlecock has dynamic stability. If it turns in any direction, airpressure immediately turns it in the opposite direction, thus maintaining itsorientation and stabilising it during flight. This aerodynamic stabilising effectalso flips the shuttlecock quickly after each hit and then keeps it flyingbumper forward until the next hit.

42. Xeroxing

The Xerox machine is standard equipment found in almost all offices. It isused for making copies of documents including text and images. To makecopies you have only to place the document to be copied face down on theglass sheet on the top of the machine and press a button. The machine does

the rest and a copy of the document comes out from the side of the machine.How are the copies made?

Xeroxing is a photocopying process, which makes use of a combination ofphotoconductivity a electrostatic charges. Light is used to discharge regions ofa charged surface to produce a latent image, which is transferred to paperusing a toner. At the heart of the photocopier is a drum made out ofphotoconductive material that is first charged positively using a corona wire.When an intense beam of light is moved across the paper placed on thecopier's glass surface, the image of the document is focussed on the chargeddrum. Light reflected from white areas of the paper and falling on the

charged drum neutralises the charge in those areas. Dark areas on theoriginal (such as pictures or text) do not reflect light onto the drum, leavingregions of positive charges on the drum's surface intact. When negativelycharged, dry, black pigment called toner is spread over the surface of thedrum, the toner particles adhere only to the positive charges that remain,creating a temporary image of the original, which is transferred to a positivelycharged sheet of paper. The paper is then heated and pressed to fuse theimage formed by the toner to the paper's surface. That is why the paper feelswhen it comes out the photocopier.

Since the working of photocopiers depends mostly on static charges, high

humidity affects the quality of photocopies produced.

43. Car transmission gears

Whether you ride a bus or a car you must have noticed that after starting, thespeed of the vehicle increases in steps as the driver changes gears. And hecan't change gears randomly; he has to do it in a certain order -- first, second,


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third, and fourth. Some modern cars even go up to the fifth gear. What dothese 'gears' mean and why changing gears is necessary?

The car or bus engine can produce a certain amount maximum powerdepending upon the engine capacity, design and other factors. But a runningvehicle does not require the same level of power at all times. It needs themaximum power when starting from rest or while climbing a gradient. Onlevel a road, as the speed increases, less and less power is needed to maintainthe speed. Changing gears allow the driver to transmit the required amountof power to the wheels.

Internal combustion engines used in cars and other vehicles have narrow rpmranges where power and torque are at their maximum. For example, anengine might produce its maximum power at 5,500 rpm. The transmissionallows the gear ratio between the engine and the drive wheels to change asthe car speeds up and slows down, while maintaining the engine speed at

5,500 rpm. To de-link the driving shaft from the engine during gear changethe transmission is connected to the engine through the clutch.

The transmission allows the 'gear ratio' between the engine and the drivewheels to change as the car speeds up and slows down, and at the same timeto maintain an optimum power level. Gear ratio refers to the ratio betweennumber of teeth of two meshing gears, which also decides the ratio of thespeed or rotation of the two gears. In first gear the driving shaft rotates muchslower than the engine shaft, which produces more torque at the drive wheelrequired for starting the vehicle from rest. As the vehicle speed increases, lessand less torque is required to maintain the motion and change to higher gears

helps increase the speed, as the gear ratio allows the drive shaft to spin almostat the rpm of the engine. In cars with automatic transmission the gear ratio ischanged continuously as required.

44. Physics of hammers

We all use a hammer to fix nails or break stone or bricks, or shape metalsheets. A hammer is basically a tool meant to deliver blows to a target,causing it to move or deform. Scientifically, a hammer can be looked upon asa force amplifier that converts mechanical work into kinetic energy and back.

Early humans used lumps of stone to break stone or animal bones to maketools. The amount of energy delivered to the target by a hand-held stone isequivalent to one half the mass of the stone times the square of the stone'sspeed at the time of impact. When we use a hammer, the handle, byincreasing the radius of the swing, allows us to maximize the speed of thehammerhead on each blow. Here what really matters is the length of the


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handle and mass of the head. Longer the handle or heavier the head, more isthe energy delivered to the target.

In the swing that precedes each blow, a certain amount of kinetic energy getsstored in the hammer's head, depending on its mass and the speed of itsmotion. When the hammer strikes, the head transfers its momentum to thetarget. To be most effective a hammer has to transfer its momentum fastest,which is why most hammers have hardened steel heads. Steel headedhammers are suitable for driving nails into wood or brick wall, or forbreaking a piece of stone, or bend a sheet of metal. For really big projects suchas driving wedges into wood and posts into the ground, a sledgehammer,with massive head and a long handle, is usually used.

45. Corrugated boards

We buy many things that come packed in boxes, be it a TV, VCR, computer,fridge, even fruits. And the packing box is invariably made of corrugatedboard. If you look carefully, you'll find that the board is actually made oflayers of paper in which the middle layer is fluted or corrugated. How doessimple corrugation make paper so stiff and strong to be used as a rigidpackaging material?

If you take a piece of corrugated board apart you'll find that it is a compositestructure made up of three or more layers of paper with differentcharacteristics. It has two main components: the liner and the medium. Bothare made of a special kind of brown paper. The liner is the flat layer that

adheres to the medium, which in turn is the wavy, fluted paper in betweenthe liners.

The strength of corrugated board comes from the wavy, fluted middle layer,which provides reinforcement. If you've made a paper fan you'd know thatfolding makes paper more rigid. In corrugated board the folds in the flutedlayer make a series of parallel arches. Architects have known for thousands ofyears that an arch with the proper curve is the strongest way to span a givenspace. The inventors of corrugated paperboard applied these same principlesto paper when they put arches in the corrugated medium. When anchored toliners on both sides with an adhesive, they resist bending and pressure from

all directions.

Corrugated board is not only rigid but also has superior cushioning qualities,which resists crushing under compression and gives cushioning protection tothe box's contents. Containers, boxes and pallets can hold products in anoptimally protective environment, so even heavy or fragile contents can betransported undamaged.


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46. The violin

The violin is a stringed musical instrument, but unlike many other stringedinstruments like the sitar, sarod, and guitar, which are played by plucking,

the violin is played by bowing. The bow is made of horsehair, pulled taut by awooden stick. It is easy to understand how sound is produced in a sitar,sarod, or guitar; the plucking sets the strings into vibration, which leads to theproduction of sound. But how does drawing the bow across the strings makethe violin produce sound?

A bowed string works in an entirely different way. Horsehair of which thebow is made has a rough surface made of scales. When rosin is applied on thebow it makes the surface of the hairs sticky, but unevenly so. As the bow hairsrub across the violin string the sticky areas grab the string and push itforward a little bit till the string's restoring force overpowers the friction and

string starts sliding backwards in a jerk till grabbed by the next sticky area onthe hairs, and the process goes on as long as the bow is drawn across thestring. As a result, the string vibrates as a harmonic oscillator, with afrequency depending on the position of the finger of the player on the string.

Apart from difference in the mode of sound production, the violin also differsfrom the plucked string instruments in the quality of sound produced. In fact,every musical instrument has a very separate, distinct sound, called "timbre"in music lingo, which helps us distinguish between them. Each instrumenthas a specific pattern of harmonics, which create the unique sound. A givennote on a violin usually has several frequencies vibrating at once. This distinct

combination creates the uniquely beautiful timbre of the violin.

49. Pole vaulting

Pole-vaulting is an exciting athletic event in which the vaulter, running with along pole in hand, jumps over a crossbar placed several metres above ground.It is a wonderful illustration of how one type of energy is converted toanother type of energy. Through a proper use of the pole, the energy ofmotion of the athlete is converted into the energy needed to overcome gravityand reach a certain height. How does it work?

The crucial component of pole vaulting is the vaulting pole, which is a veryadvanced piece of equipment. It is constructed from carbon fibre andfibreglass composite materials in several layers and is usually 5.00-5.20 mlong. An ideal pole should absorb all of the vaulter's energy while bending,and then return all of that energy as it straightens out. When the vaulterreaches the end of his/her run-up and engages the pole in the take-off box,the pole begins to bend under the effect of the momentum of the vaulter, and


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the vaulter and pole system rotates about the take-off box. At this point theinitial kinetic energy of the run-up is transformed into potential energy of thevaulter above the ground. As the pole bends and recoils, the vaulter rotatesabout the shoulders, and then pulls up on the pole so as to pass over thecrossbar feet-first.

What a pole-vaulter would ideally want to achieve is to convert all of his/herkinetic energy into gravitational potential energy. In the real world, however,a 100 percent conversion is never possible because some of the kinetic energygets converted into other kinds of energy, such as heat, friction, sound,

physics at work and play

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