FrictionScience for Grade 7

Chapter 3

Module Objectives

• To define the term Friction and its types• To compare the magnitudes of different types of friction• To distinguish between advantages and disadvantages of

friction• To differentiate the methods of increasing and reducing

friction

What is Friction?• Friction is the force resisting the relative motion of solid

surfaces, fluid layers, and material elements sliding against each other.

• Friction is what happens when any two things rub against each other.

Examples of Friction1. Rubbing: Rubbing your hands together

to create warmth

2. Tire Traction: The friction between the tires and the surface of the road that allows your car to accelerate, slow down, and negotiate turns and corners

3. A hammer hitting a nail

Frictional Force: Activity

When does Friction occur?

• Friction occurs when two surfaces come in contact of each other

• Friction can also occur when a liquid or a gas is pushed out of the way of a moving object

Why does Friction occur?• Friction occurs because the hills and valleys of a surface are in

contact with the hills and valleys of other surfaces. This produces friction.

• Friction depends on the roughness of surfaces and force with which objects are being pressed one against the other, or their weights.

Measuring Frictional Force

• There are different methods of measuring friction Weight Ratio Spring Balance Tilted Plane Clamping Pedulum Motorized Tribometer

Measuring Frictional Force: Weight Ratio

• The sketch shown originates from Leonardo da Vinci (ca 1500). He studied friction by measuring the load hanging on a cord, at which the block begins to slide. The coefficient of friction is found by the quotient of the dead weight of the mass hanging on the cord and the mass of the block, i.e. µ = Ff / N = mdead weight / mblock

Measuring Frictional Force: Spring Balance

• Pull a spring balance connected to the block and slowly increase the force until the block begins to slide. Make sure the spring balance is parallel to the surface. The reading on the spring balance scale when the load begins to slide is a measure for the static friction, while the reading when the block continues to slide is a measure of dynamic friction. The coefficient of friction is simply µ = Fspring /Fnormal = Fspring /(mblock ·g ), g=9.81 m/s²

Measuring Frictional Force: Tilted Plane

• Place a block on a tilted plane and increase the angle of tilt until the block begins to slide. The tangent of the tilting angle just found is the so called "friction angle". This angle is related to the coefficient of friction µ, i.e. µ = tanø = Ff /F

•

Measuring Frictional Force: Clamping

• To measure the static coefficient of friction under conditions of high contact pressure the object may be clamped between two surfaces. The force necessary to put the object in motion must be halved to obtain the friction force because of the two contacting surfaces.

Measuring Frictional Force: Pendulum

• The pendulum is suitable to analyze the static and dynamic friction under reciprocal motion by monitoring the bearing torque. This however requires a torque sensor.

• The energy loss of combined static and dynamic friction can be analyzed by considering the reduction of the amplitude of motion in time. This only requires a simple rotary potentiometer or pulse rotation sensors to visualize the amplitude reduction in time.

Measuring Frictional Force: Motorized Tribometers

• In the measuring methods discussed above the friction coefficient is measured in fresh contacts, not after running in. The coefficient of friction may change significantly during first half hour of sliding.

• The time necessary to obtain a stable value of the coefficient of friction can be observed in a motorized tribometer by monitoring the friction over time.

• This method is common for measuring the specific wear rate and the contact temperature during operation.

Frictional Force: Activity 3.2

Factors affecting Frictional Force

• Frictional force is caused due to inter molecular interactions between the bodies

• Frictional force always opposes relative motion• Frictional force varies in strength according to how hard the

two surfaces are pressed against one another and how slippery or how rough the surfaces are

• The harder you press the two surfaces together, the larger the frictional force they experience

• Frictional force is more for rough surfaces and less for smooth surfaces

Factors affecting Frictional Force: Activity 3.3

What are the different types of Friction?

• Following are the different types of friction:1. Static Friction2. Dynamic Friction

a. Sliding Frcitionb. Rolling Friction

3. Fluid Friction

Types of Friction with Examples

Static Friction• Static friction is a force that resists the movement of two objects

against one another when the objects are initially at rest. • Static friction comes into play when a body is forced to move along

a surface but movement does not start. • The magnitude of static friction remains equal to the applied

external force and the direction is always opposite to the direction of motion. The magnitude of static friction depends upon μs (coefficient of static friction) and N (net normal reaction of the body).

• An example of static friction is pushing a wall• More examples and formula can be found here:

http://easycalculation.com/physics/classical-physics/learn-static-friction.php

Example of Static Friction

Static Friction: Activity 3.4

• Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction for the same materials.

• New models are beginning to show how kinetic friction can be greater than static friction.

• Kinetic friction is now understood, in many cases, to be primarily caused by chemical bonding between the surfaces, rather than interlocking asperities; however, in many other cases roughness effects are dominant, for example in rubber to road friction.

• Surface roughness and contact area, however, do affect kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.

• An example for kinetic friction is slipping on a wet floor• More examples and the formula for Kinetic friction can be found here:http://easycalculation.com/physics/classical-physics/learn-kinetic-

friction.php

Dynamic Friction

Example of Dynamic (Sliding) Friction

Sliding Friction: Activity 3.5

• Rolling frictional force is a force that slows down the motion of a rolling object.

• Basically it is a combination of various types of frictional forces at point of contact of wheel and ground or surface.

• When a hard object moves along a hard surface then static and molecular friction force retards its motion.

• When soft object moves over a hard surface then its distortion makes it slow down.

• A car will eventually come to a stop if just allowed to roll as the friction between the road surface and the wheels causes friction that causes the vehicle to stop.

• More examples can be found here: http://examples.yourdictionary.com/examples/examples-of-rolling-friction.html

Rolling Friction

Examples of Rolling Friction

• When a body moves in a fluid or in air then there exists a resistive force which slows down the motion of the body, known as fluid frictional force.

• A freely falling skydiver feels a drag force due to air which acts in the upward direction or in a direction opposite to skydiver’s motion.

• The magnitude of this drag force increases with increment in the downward velocity of skydiver. At a particular point of time the value of this drag force becomes equal to the driving force and skydiver falls with a constant velocity

Fluid Friction

• Suppose there is a wet surface between two thin glass plates, you may find the plates get stuck and the bottom plate does not fall when you hold only the top plate.

• This is due the fluid friction that over comes the tensile stress (the weight) acting on the bottom plate.

• The quantum of fluid friction or the viscosity of blood is related to the biomedical condition of a person.

• When an object is dropped into a fluid, the extent of splash due to the buoyant force of the fluid depends on the fluid friction of that particular fluid.

• You find lighter dust articles move fast on the surface of a flowing river. This is due to the high velocity gradient at the top layer of water due to lower dynamic fluid friction at that layer.

• A dynamic fluid friction is much lower in magnitude compared to the sliding friction of solids. For this reason some fluids are used as lubricants in between surfaces of sliding or rotating objects. The contact surfaces then become two sets of surfaces. Solid to fluid contact and fluid to solid contact. Because the fluid friction is less in each set, the over all sliding friction gets reduced.

Examples of Fluid Friction

Examples of Fluid Friction

Effects of Friction• Friction opposes motion• When force is applied on a body to move over the other

body, irregularities of the surfaces interlock. • Frictional force created due to this cause always acts in

the opposite direction of the applied force.• Therefore Friction always opposes the motion of the

body and tries to stop it• Example: Bicycle braking system works on the basis of

this effect of friction

Effects of Friction• Friction produces heat: Activity 3.6

Effects of Friction• Friction causes wear and tear• Moving parts of the machine under friction are frequently

replaced due to wear and tear

Advantages of Friction• Friction is a necessary evil. It enables us to walk with out slipping. To

enhance friction, it is advisable to use shoes with rough sole while walking on slippery floors. For a similar reason, crutches are provided with rubber tips at their bottom to provide sufficient friction.

• The breakers and tiers of our cars and bicycles depend on friction to function properly.

• The ridges in the skin of our fingers and palms enable us to grasp and hold objects due to friction.

• To prevent patients being uncomfortable in bed rubber sheets with spongy under surfaces are placed over mattresses. The friction between the spongy under surfaces and the mattress prevents the rubber sheet from slipping and wrinkling.

• Nails and screws are held in wood by friction.

Advantages of Friction

Disadvantages of Friction• Friction causes unnecessary wear and tear of the machinery.• Friction reduces the speed of machines • Due to friction between the moving parts of a machine heat is produced,

which in a what affects the working as well as the life of a machine. • A part of useful energy is dissipated in overcoming the friction.

Methods of Increasing Friction• Friction is an important force of nature that you deal with

everyday.• Friction affects the way you walk, your ability to pick up things

and even your ability to drive down the street in your car. • Friction is simply the resistant force between two objects

moving while in contact with one another. But many people do not understand friction completely and are unaware that there are ways to increase the frictional force between two objects.

• Some ways to increase the friction are listed in the next slides.

Methods of Increasing Friction• Smoothness

• Generally, the more surface contact two objects have, the less the friction between them will be. • To increase frictional force between two objects, you can make one or both of the object surfaces

rougher. For example, it will take more energy to push a block of wood across a wooden tabletop if the block has been cut roughly on the side where it contacts the table.

• But if you sand down the block to make it relatively smooth, then the block will slide easier. This is because the roughness allowed for less surface contact between the two, which increased frictional force.

• This is not a universal rule, because two extremely smooth surfaces actually have increased friction because of electrostatic forces between them. Two plates of smooth glass, for example, do not glide across one another easily.

Methods of Increasing Friction• Weight

• Adding weight to one object can significantly increase the frictional force between two moving objects.

• The more gravity presses the two surfaces together, the more friction exists and the harder it is to move them. The force of gravity will cause more pressure to press the objects together because of the weight of the object pressing down.

• The change in weight has to be to the object on top however. Consider the block of wood on a table idea again. If the block weighs one pound you could easily slide it across the table with one hand.

• Now imagine you added a 20-pound weight on top of the block. It would obviously take more energy to move it. This is because the force of friction between the two has increased.

Methods of Increasing Friction• Stop Movement

• One way to increase frictional force between to two objects is to take them out of motion. • Static friction is a different type of friction that indicates the amount of energy required to

set an object in motion instead of the energy needed to keep it moving along, which relates to kinetic friction.

• Static friction always requires more energy to start the object moving than it takes to keep it moving. Imagine pushing a car in neutral on a flat surface.

• Getting it started can be tough, but once it gets rolling, the momentum makes it much simpler to keep it rolling. This is similar to the theory of static friction.

Methods of Increasing Friction

• Remove Lubrication• One way to reduce friction is to lubricate the surface so the objects slide against one another easily. • With that in mind, it is reasonable to assume that taking away lubrication will increase the frictional

force. It's true. • Think about the wooden block on the table one more time. If the table were covered in motor oil,

lubricating the entire surface, the block would slide very easily across the table. It would slide with minimal effort by the pusher.

• But spray off the table, cleaning up the mess entirely and try the same thing. You would see that it takes much more energy to push the block because the oil is no longer lubricating the surface.

•

Methods of Increasing Friction

• Do you know why kabaddi players and wrestlers rub their hands with soil? Weight lifters, gymnasts use some coarse material to increase friction for better grip.

Methods of Reducing Friction• Polishing

• Polishing the surface, reduces the irregularities and hence, reduces friction

• Lubrication• It provides a layer of the lubricant on which the bodies can

easily slide.

• Use of ball-bearing or roller bearing• Roller bearing help to convert sliding friction into a milder form

- rolling friction. Rolling friction is lesser than sliding friction.

• Streamlining• Fast cars, boats, planes etc., have a streamlined body. This is to

allow air (or water in the case of boats) to easily flow by, without offering much resistance.

• Use of correct combination of surfaces in contact• Use of alloys on moving and sliding parts reduces friction because

alloys have a low coefficient of friction.

Methods of Reducing Friction• Have you played carrom? A fine talcum powder is sprinkled on the carrom board. Do you know why? It is to reduce friction

between the board and the striker so that the striker and coins’ movement become easier.

Reducing Friction: Activity 3.7

Points to Remember

• Friction is a force which tends to slow down or stop the moving bodies.• Friction opposes the relative motion between the two surfaces in contact.• Friction is caused due to interlocked irregularities of the surfaces in contact.• Friction depends on how hard the two surfaces pressed together, and how

rough they are.• Without friction, no bodies will move. Moving bodies will not stop either.• Static friction does not allow the surfaces in contact to start slide upon one

another when they are at rest.

Points to Remember

• Sliding friction does not allow the surfaces in contact to start slide with uniform speed

• Rolling friction does not allow the surfaces in contact to roll over one another.

• Things moving under friction produce heat, noise, wear and tear resulting in loss of energy.

• Friction can be reduced by using lubricants, polishing surfaces, streamlining and by using ball-bearings.

Tips• Apply oil or grease on the moveable parts of the

machines to reduce friction.• Carry heavy things on rollers with ball bearings

to save energy and to reduce friction.• Do not use highly polished flooring tiles in toilets

and bathrooms.• Replace worn out tyres of the vehicles to avoid

accidents due to skid.• Use foot wears and seat belts with proper grip to

gain necessary friction.

Exercises

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Exercises

Thank You