Chapter 15

Work, Power and Simple MachinesWork, Power and Simple Machines

15-1 What does it mean to do work? In science, to do work is probably different In science, to do work is probably different

than what you consider work to be.than what you consider work to be. In order to do work on an object, a force In order to do work on an object, a force

must be applied and the object must move must be applied and the object must move in the direction of the force.in the direction of the force.

Work is equal to force (N) times the Work is equal to force (N) times the distance the object travels (m).distance the object travels (m).

The unit for work is the Joule (J).The unit for work is the Joule (J).

Let’s practice

How much work would be done on an How much work would be done on an elephant that moves 3.0 meters when elephant that moves 3.0 meters when pushed with a force of 10 000 N?pushed with a force of 10 000 N?

The answer to this really depends on The answer to this really depends on whether the elephant moves in the same whether the elephant moves in the same direction of the force that is applied. Let’s direction of the force that is applied. Let’s say it did rather than turning around and say it did rather than turning around and stomping on you. Then we use the formula:stomping on you. Then we use the formula:

Work = force x distanceWork = force x distance 10 000 N x 3.0 meters = 30 000 J10 000 N x 3.0 meters = 30 000 J Remember, joules measures energy.Remember, joules measures energy.

Now let’s say that a student is carrying a 3 Now let’s say that a student is carrying a 3 kg book and crosses the school campus to kg book and crosses the school campus to the main building (15 meters). The student the main building (15 meters). The student goes up the stairs (5 meters) and down the goes up the stairs (5 meters) and down the hall to their 1hall to their 1stst hour class (10 meters) and hour class (10 meters) and sets the book on a desk. How much work sets the book on a desk. How much work was done on the book by the student?was done on the book by the student?

Let’s look at this.

First you have to identify the force that is being First you have to identify the force that is being applied to the book. applied to the book.

Next, you need to identify the direction of the Next, you need to identify the direction of the force on the book.force on the book.

Then you must identify the direction of the motion Then you must identify the direction of the motion of the book. of the book.

The only distance that counts is the distance the The only distance that counts is the distance the book travels in the direction of the motion.book travels in the direction of the motion.

What’s your answer?What’s your answer?

If you counted the distance that the book If you counted the distance that the book traveled going up the stairs, you are correct!traveled going up the stairs, you are correct!

Now, let’s look at the force that is being Now, let’s look at the force that is being applied to the book. Is the book moving in applied to the book. Is the book moving in the person’s hand? If not, the book is an the person’s hand? If not, the book is an example of a balanced force. What force is example of a balanced force. What force is acting upon the book?acting upon the book?

15-2 POWER

In science, power is the rate at which work In science, power is the rate at which work is done. It relates work with time. The is done. It relates work with time. The formula for power is:formula for power is:

Power = work Power = work ÷ time÷ time Power is measured in watts or Joules per Power is measured in watts or Joules per

second. W or J/s are the units for power.second. W or J/s are the units for power. Usually power is measured in kW. The k is Usually power is measured in kW. The k is

for kilo. How many watts is in a kW?for kilo. How many watts is in a kW?

1 kW = 1000 watts or 1000 J/s1 kW = 1000 watts or 1000 J/s To understand power, consider a man with To understand power, consider a man with

a shovel and a bulldozer. Over a period of a shovel and a bulldozer. Over a period of an hour both the man and the bulldozer an hour both the man and the bulldozer work. If the man produces 100 joules of work. If the man produces 100 joules of work and the bulldozer produces 1000 work and the bulldozer produces 1000 joules of work, which is the more powerful, joules of work, which is the more powerful, the man or the bulldozer?the man or the bulldozer?

15-3 Machines

You are familiar with machines because you use You are familiar with machines because you use them everyday. In science, a machine is anything them everyday. In science, a machine is anything that makes work easier. that makes work easier.

Machines make work easier by converting the Machines make work easier by converting the energy that goes into the machine into energy that energy that goes into the machine into energy that comes out of the machine. comes out of the machine.

The work that goes into a machine (remember The work that goes into a machine (remember work = force x distance) is called work input.work = force x distance) is called work input.

Work input

The formula for work input is:The formula for work input is: Work input = effort force x distance of the Work input = effort force x distance of the

effort.effort. If you had a 2 meter shovel and exerted a If you had a 2 meter shovel and exerted a

force of 20 N to move some dirt what force of 20 N to move some dirt what would your work input be?would your work input be?

It would be 2m x 20N = 40 Joules.It would be 2m x 20N = 40 Joules.

Work output

Work output is the amount of work that you use to Work output is the amount of work that you use to overcome the resistance on the machine. The overcome the resistance on the machine. The formula is:formula is:

Work output = resistance force x distance of the Work output = resistance force x distance of the resistance.resistance.

Let’s say that the person with the shovel put in 40 Let’s say that the person with the shovel put in 40 J of work is met with a resistance force of 10 N. J of work is met with a resistance force of 10 N. Since the distance does not change with the Since the distance does not change with the shovel, how much work is actually done by the shovel, how much work is actually done by the shovel?shovel?

Work output= 10 N x 2 m = 20JWork output= 10 N x 2 m = 20J Machines do not increase the amount of energy Machines do not increase the amount of energy

that goes into them. All they do is change either that goes into them. All they do is change either the size or direction of the force put into the the size or direction of the force put into the machine. machine.

All the shovel did was change the direction of the All the shovel did was change the direction of the force, it did not increase the amount of energy. force, it did not increase the amount of energy. Machines can change force or direction, but they Machines can change force or direction, but they cannot do both.cannot do both.

How Helpful is a Machine?

Some machines are more helpful than Some machines are more helpful than others. Because work output can never be others. Because work output can never be more than work input due to friction, no more than work input due to friction, no machine is 100% efficient. Too much machine is 100% efficient. Too much energy is lost usually in the form of heat.energy is lost usually in the form of heat.

The formula for work efficiency is:The formula for work efficiency is: Work output Work output ÷ work input x 100÷ work input x 100

Practice

You have a can opener that is 10 cm long You have a can opener that is 10 cm long and use 10 N of force to open a can of peas. and use 10 N of force to open a can of peas. The metal of the can is particularly The metal of the can is particularly stubborn and resists with a force of 3 N. stubborn and resists with a force of 3 N. How efficient is your can opener?How efficient is your can opener?

First of all, what is the can opener First of all, what is the can opener changing, the size or the direction of the changing, the size or the direction of the force?force?

Next, identify the factors involved.Next, identify the factors involved. Work input = .10m x 10N = 1 JWork input = .10m x 10N = 1 J Work output= .10m x 3 N = .3 JWork output= .10m x 3 N = .3 J .3J .3J ÷ 1 J = .3 J x 100 = 30% efficient.÷ 1 J = .3 J x 100 = 30% efficient.

Mechanical advantage

Mechanical advantage is the number of times a Mechanical advantage is the number of times a machine multiplies the effort force to overcome machine multiplies the effort force to overcome the resistance force. the resistance force.

M.A. = Resistant force M.A. = Resistant force ÷ Effort force÷ Effort force There is no unit for mechanical advantage.There is no unit for mechanical advantage. When the resistance force is greater than the effort When the resistance force is greater than the effort

force, the M.A. is greater than 1. What if the force, the M.A. is greater than 1. What if the resistance force is less than the effort force?resistance force is less than the effort force?

15-4 Simple and Compound Machines There are six types of simple machines:There are six types of simple machines:

Inclined planeInclined plane The wedgeThe wedge The screwThe screw The leverThe lever The pulleyThe pulley Wheel and axleWheel and axle

Inclined plane

An inclined plane An inclined plane decreases the amount of decreases the amount of force you need to use by force you need to use by increasing the distance. increasing the distance. An example is a ramp. It An example is a ramp. It would take a lot more would take a lot more effort to lift that bike into effort to lift that bike into the pickup than drive it up the pickup than drive it up a ramp even though you a ramp even though you have to move it farther.have to move it farther.

Wedge

A wedge is an inclined A wedge is an inclined plane that moves. Unlike a plane that moves. Unlike a ramp, where the object ramp, where the object moves on up, a wedge moves on up, a wedge itself moves. Most wedges itself moves. Most wedges are made of two inclined are made of two inclined planes and usually a piece planes and usually a piece of wood or metal that is of wood or metal that is thinner on one end. thinner on one end. Knives and axes are Knives and axes are examples of wedges. A examples of wedges. A key is a series of wedges key is a series of wedges that lift the pins of a lock.that lift the pins of a lock.

Screw

A screw is an inclined A screw is an inclined plane wrapped around plane wrapped around a cylinder to form a a cylinder to form a spiral. Because it spiral. Because it increases the distance increases the distance by a great deal, the by a great deal, the closer the threads are, closer the threads are, the greater the the greater the mechanical advantage.mechanical advantage.

Levers

There are three types of levers:There are three types of levers: 11stst class levers class levers 22ndnd class levers class levers 33rdrd class levers class levers

All levers are a rigid bar that is free to All levers are a rigid bar that is free to pivot around a fixed point called a pivot around a fixed point called a fulcrum.fulcrum.

1st Class Lever

A first class lever has A first class lever has the fulcrum in the the fulcrum in the middle between the middle between the resistant force and the resistant force and the effort force. In this effort force. In this picture, which is the picture, which is the resistance force, the resistance force, the girl or the bear?girl or the bear?

2nd Class Lever

Second class levers Second class levers have the resistance have the resistance force in between the force in between the effort force and the effort force and the fulcrum. Where is the fulcrum. Where is the fulcrum on the fulcrum on the wheelbarrow?wheelbarrow?

Third Class Levers

Third class levers have the Third class levers have the fulcrum on one end, with fulcrum on one end, with the effort force between the effort force between the fulcrum and the the fulcrum and the resistance force. Fishing resistance force. Fishing poles and brooms are poles and brooms are examples of 3examples of 3rdrd class class levers. Identify where the levers. Identify where the fulcrum, resistant and fulcrum, resistant and effort forces are.effort forces are.

Jack Cavelle fishing

Pulleys

A pulley is a rope, belt or A pulley is a rope, belt or chain wrapped around a chain wrapped around a grooved wheel. Pulleys grooved wheel. Pulleys can either change the can either change the direction or the amount of direction or the amount of a force. Fixed pulleys a force. Fixed pulleys only change the direction only change the direction of a force while moveable of a force while moveable pulleys change the amount pulleys change the amount of force. Pulley systems of force. Pulley systems increase the mechanical increase the mechanical advantage.advantage.

Wheel and Axle

A wheel and axle consists A wheel and axle consists of two different sizes of of two different sizes of wheels that turn around wheels that turn around each other. The wheel is each other. The wheel is always the larger object always the larger object and the axle the smaller. and the axle the smaller. The mechanical advantage The mechanical advantage is determined by the radius is determined by the radius of the wheel as compared of the wheel as compared to the radius of the axle. If to the radius of the axle. If the radius of the wheel is 4 the radius of the wheel is 4 times that of the axle, the times that of the axle, the MA is 4.MA is 4.

Compound Machines

Compound machines Compound machines are a combination of are a combination of two or more simple two or more simple machines. A car is an machines. A car is an example of a example of a compound machine. It compound machine. It has wheels and axles, has wheels and axles, gearshift lever, gears, gearshift lever, gears, brake lever, steering brake lever, steering wheel, etc.wheel, etc.