# work

## horsepower

Sometimes power is measured in a unit called the horsepower. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. The horsepower was introduced by James Watt, who invented the first powerful steam engine in the 1770s. Watts steam engine led to a revolution in industry and agriculture because of its power. Watt wanted to impress people with the power of his steam engine, so he compared it with something familiar to people of his time: the power of workhorses, like those pictured in Figure 16.6. Watt said his steam engine could produce the power of 20 horses, or 20 horsepower. The most powerful engines today may produce more than 100,000 horsepower! How many watts of power is that?

## calculating power or work

Power can be calculated using the formula above, if the amount of work and time are known. For example, assume that a small engine does 3000 joules of work in 2 seconds. Then the power of the motor is: Power = 3000 J = 1500 J/s, or 1500 W 2s You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power Time For example, if you use a 2000-watt hair dryer for 30 seconds, how much work is done? First express 2000 watts in J/s and then substitute this value for power in the work equation: Work = 2000 J/s 30 s = 60, 000 J For a video presentation on calculating power and work, go to this link: Problem Solving Problem: An electric mixer does 2500 joules of work in 5 seconds. What is its power? Solution: Use the equation: Power = Work Time . Power = 2500 J = 500 J/s, or 500 W 5s You Try It! Problem: How much work can be done in 30 seconds by a 1000-watt microwave?

## what is power

Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules and time is measured in seconds, so power is expressed in joules per second (J/s). This is the SI unit for work, also known as the watt (W). A watt equals 1 joule of work per second. The watt is named for James Watt, a Scottish inventor you will read about below. You may already be familiar with watts. Thats because light bulbs and small appliances such as hair dryers are labeled with the watts of power they provide. For example, the hair dryer in Figure 16.5 is labeled "2000 watts." This amount of power could also be expressed kilowatts. A kilowatt equals 1000 watts, so the 2000-watt hair dryer produces 2 kilowatts of power. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave, a high-power microwave can cook more food in the same time or the same amount of food in less time.

## work and power

Did you ever rake leaves, like the woman in Figure 16.4? It can take a long time to do all that work. But if you use an electric leaf blower, like the man in the figure, the job gets done much sooner. Both the leaf blower and the rake do the work of removing leaves from the yard, but the leaf blower has more power. Thats why it can do the same amount of work in less time.

## calculating work

The equation for work given above can be used to calculate the amount of work that is done if force and distance are known. For example, assume that one of the weight lifters in Figure 16.2 lifts a weight of 400 newtons over his head to a height of 2.2 meters off the ground. The amount of work he does is: Work = 400 N 2.2 m = 880 N m Notice that the unit for work is the newton meter. This is the SI unit for work, also called the joule (J). One joule equals the amount of work that is done when 1 newton of force moves an object over a distance of 1 meter. Problem Solving Problem: Todd pushed a 500 N box 4 meters across the floor. How much work did he do? Solution: Use the equation Work = Force Distance. Work = 500 N 4 m = 2000 N m, or 2000 J You Try It! Problem: Lara lifted a 100 N box 1.5 meters above the floor. How much work did she do?

## defining and calculating work

Work is defined differently in physics than in everyday language. In physics, work means the use of force to move an object. The teen who is playing tennis in Figure 16.1 is using force to move her tennis racket, so she is doing work. The teen who is studying isnt moving anything, so she is not doing work. Not all force that is used to move an object does work. For work to be done, the force must be applied in the same direction that the object moves. If a force is applied in a different direction than the object moves, no work is done. Figure 16.2 illustrates this point. The stick person applies an upward force on the box when raising it from the ground to chest height. Work is done because the force is applied in the same direction as the box is moving. However, as the stick person walks from left to right while holding the box at chest height, no more work is done by the persons arms holding the box up. Thats because the force supporting the box acts in a different direction than the box is moving. A small amount of work in the horizontal direction is performed when the person is accelerating during the first step of the walk across the room. But other than that, there is no work, because there is no net force acting on the box horizontally.

## work force and distance

Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation: Work = Force Distance This equation shows that the greater the force that is used to move an object or the farther the object is moved, the more work that is done. You can see a short video introduction to work as the product of force and distance at this link: . To see the effects of force and distance on work, compare the weight lifters in Figure 16.3. The two weight lifters on the left are lifting the same amount of weight, but the bottom weight lifter is lifting the weight a longer distance. Therefore, this weight lifter is doing more work. The two weight lifters on the bottom right are both lifting the weight the same distance, but the weight lifter on the left is lifting a heavier weight. Therefore, this weight lifter is doing more work.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

unit for power that equals 745 watts

``````a. joule

-->  b. horsepower

c. power

d. force  distance

e. watt

f. work  time

g. work
``````

Assume that a friend hands you a 15-newton box to hold for her. If you hold the box without moving it at a height of 1.5 meters above the ground, how much work do you do?

``````a. 22.5 J

b. 15 J

c. 10 J

-->  d. none of the above
``````

SI unit for work

``````-->  a. joule

b. horsepower

c. power

d. force  distance

e. watt

f. work  time

g. work
``````

Which weight lifter described below does the most work?

``````a. Tom lifts 195 N a distance of 2.0 m.

b. Ted lifts 190 N a distance of 2.1 m.

c. Tad lifts 185 N a distance of 2.2 m.

-->  d. Tim lifts 180 N a distance of 2.3 m.
``````

how to calculate work

``````a. joule

b. horsepower

c. power

-->  d. force  distance

e. watt

f. work  time

g. work
``````

Another way of writing 1 joule is

``````-->  a. 1 N  m.

b. 1 N/m.

c. 1 N  m2 .

d. 1 N/m2 .
``````

One horsepower is about equal to

``````a. 1 watt.

b. 75 watts.

-->  c. 745 watts.

d. 1 kilowatt.
``````

use of force to move an object

``````a. joule

b. horsepower

c. power

d. force  distance

e. watt

f. work  time

-->  g. work
``````

how to calculate power

``````a. joule

b. horsepower

c. power

d. force  distance

e. watt

-->  f. work  time

g. work
``````

How much work is done by a 1000-watt hairdryer in 40 seconds?

``````a. 0.4 J

b. 25 J

c. 960 J

-->  d. 40,000 J
``````

SI unit for power

``````a. joule

b. horsepower

c. power

d. force  distance

-->  e. watt

f. work  time

g. work
``````

measure of the amount of work that can be done in a given amount of time

``````a. joule

b. horsepower

-->  c. power

d. force  distance

e. watt

f. work  time

g. work
``````

Every time you apply a force you do work

``````a. true

-->  b. false
``````

A more powerful device can do more work in the same amount of time than a less powerful device.

``````-->  a. true

b. false
``````

The power of a machine equals the work it does multiplied by the time it takes to do that work.

``````a. true

-->  b. false
``````

The steam engine invented by James Watt had the power of one horse.

``````a. true

-->  b. false
``````

The most powerful engines today can produce more than 100,000 horsepowers.

``````-->  a. true

b. false
``````

Whenever you move your body you are doing work.

``````-->  a. true

b. false
``````

You do work when you push a heavy object even if the object does not move.

``````a. true

-->  b. false
``````

Work can be expressed in the unit N m.

``````-->  a. true

b. false
``````

A more powerful device can do the same work in less time than a less powerful device.

``````-->  a. true

b. false
``````

If you move an object that weighs 10 newtons a distance of 2 meters, you do 5 joules of work.

``````a. true

-->  b. false
``````

If you move the object in question 5 a distance of 5 meters, you do 2 joules of work.

``````a. true

-->  b. false
``````

A device that does 100 joules of work in 3 seconds has 300 watts of power.

``````a. true

-->  b. false
``````

The unit called the horsepower was introduced by James Watt.

``````-->  a. true

b. false
``````

A 2-horsepower device has almost 1500 watts of power.

``````-->  a. true

b. false
``````

The more force you apply to move an object, the more work you do.

``````-->  a. true

b. false
``````

For work to be done on an object, force must be applied

``````a. in an upward direction.

b. against the force of gravity.

c. in the same direction as gravity.

-->  d. in the same direction that the object moves.
``````

Work is directly related to the force applied to an object and to the

``````a. mass of the object.

-->  b. distance the object moves.

c. direction of the applied force.

d. amount of time the force is applie
``````

If a mover pushes a box weighing 100 newtons a distance of 3 meters, how much work does she do?

``````a. 3J

b. 33 J

-->  c. 300 J

d. 3000 J
``````

The power of a device can be expressed in

``````a. joules.

b. joules per meter.

-->  c. joules per second.

d. none of the above
``````

Work can be calculated as

``````a. force  time.

b. force  power.

-->  c. power  time.

d. power  distance.
``````

A device does 2000 joules of work in 10 seconds. What is the power of the device?

``````a. 20,000 W

b. 2000 W

-->  c. 200 W

d. 20 W
``````

One horsepower is the amount of work a horse can do in one

``````a. second.

-->  b. minute.

c. hour.

d. day.
``````

## diagram questions

No diagram questions associated with this lesson