14.1 Work and Power The weight lifter applies a large force to hold the barbell over his head. Because the barbell is motionless, no work is done on the barbell.
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14.1 Work and Power
The weight lifter applies a large force to hold the barbell over his head.
Because the barbell is motionless, no work is done on the barbell.
14.1 Work and Power
When does a force do work?
In science, work is the product of force and distance.
What Is Work?
14.1 Work and Power
For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done.
What Is Work?
Any part of a force that does not act in the direction of motion does no work on an object.
14.1 Work and Power
Work is done when a force acts on an object in the direction the object moves. Work is done when the weightlifter exerts an upward force to raise the barbell.
What Is Work?
14.1 Work and Power
Work Requires Motion
The weight lifter does no work on the barbell as he holds it over his head. The force applied to the barbell does not cause it to move.
What Is Work?
14.1 Work and Power
Work Depends on Direction• If all of the force acts in the same
direction as the motion, all of the force does work.
• If part of the applied force acts in the direction of motion, that part of the force does work.
• If none of the force is applied in the direction of the motion, the force does no work.
What Is Work?
14.1 Work and Power
A. All of the force does work on the suitcase.
What Is Work?
Force
Direction of motion
Force and motion in the same direction
14.1 Work and Power
A. All of the force does work on the suitcase.
B. The horizontal part of the force does work.
What Is Work?
Force
This force does work
This force does no work
Direction of motion Direction of motionForce and motion in the same direction
Part of force in direction of motion
14.1 Work and Power
A. All of the force does work on the suitcase.
B. The horizontal part of the force does work.
C. The force does no work on the suitcase.
What Is Work?
Force
This force does work
This force does no work
Force
Direction of motion Direction of motion Direction of motionForce and motion in the same direction
Part of force in direction of motion
Lifting force not in direction of motion
14.1 Work and Power
Calculating Work
14.1 Work and Power
Units of Work
When using SI units in the work formula, the force is in newtons, and distance is in meters.
The joule (J) is the SI unit of work. A joule is equal to 1 newton-meter.
Calculating Work
14.1 Work and Power
Using the Work Formula
A weight lifter raises a 1600-newton barbell to a height of 2.0 meters.
• Work = Force × Distance• Work = 1600 N × 2.0 m• Work = 3200 N·m = 3200 J
Calculating Work
14.1 Work and Power
How are work and power related?
Power is the rate of doing work.
What Is Power?
Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time.
14.1 Work and Power
Work is required to move snow from one location to another. A person using a shovel and a person using a snow blower can both do the work needed to remove the snow.
The snow blower can do the job much faster because it has more power.
What Is Power?
14.1 Work and Power
Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.
What Is Power?
14.1 Work and Power
Calculating Power
14.1 Work and Power
When using SI units in the power formula, work is measured in joules (J), and time is measured in seconds (s).
The SI unit of power is the watt (W), which is equal to one joule per second.
Calculating Power
14.1 Work and Power
Calculating Power
You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
Calculating Power
14.1 Work and Power
Read and UnderstandWhat information are you given?
Calculating Power
14.1 Work and Power
Read and UnderstandWhat information are you given?
Calculating Power
14.1 Work and Power
Plan and Solve
What formula contains the given quantities and the unknown?
Calculating Power
14.1 Work and Power
Plan and Solve
What formula contains the given quantities and the unknown?
Calculating Power
14.1 Work and Power
Plan and Solve
Replace each variable with its known value and solve.
Calculating Power
14.1 Work and Power
Plan and Solve
Replace each variable with its known value and solve.
Calculating Power
14.1 Work and Power
Look Back and CheckIs your answer reasonable?
Calculating Power
14.1 Work and Power
Look Back and CheckIs your answer reasonable?
36 watts is not a lot of power, which seems reasonable considering the box was lifted slowly, through a height of only 1 meter.
Calculating Power
14.1 Work and Power
1. Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?
Calculating Power
14.1 Work and Power
1. Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?
Answer: Work = Force × Distance =
200 N × 1.5 m = 300 J
Power = Work/Time = 300 J/1.0 s = 300 W
Calculating Power
14.1 Work and Power
2. You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s?
Calculating Power
14.1 Work and Power
2. You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s?
Answer: Work = Force × Distance =
15 N × 1.0 m = 15 J
Power = Work/Time = 15 J/2.0 s = 7.5 W
Calculating Power
14.1 Work and Power
3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)
Calculating Power
14.1 Work and Power
3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)
Answer: Work = Force × Distance = 10.0 N × 0.5 m = 5 JPower = Work/Time = 5 J/1.0 s = 5 W
Calculating Power
14.1 Work and Power
Another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts.
James Watt (1736-1819) was looking for a way to compare the power outputs of steam engines he had designed. Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s.
James Watt and Horsepower
14.1 Work and Power
The horse-drawn plow and the gasoline-powered engine are both capable of doing work at a rate of four horsepower.
James Watt and Horsepower
14.1 Work and Power
Assessment Questions
1. In which of the following cases is work being done on an object? a. pushing against a locked door
b. suspending a heavy weight with a strong chain
c. pulling a trailer up a hill
d. carrying a box down a corridor
14.1 Work and Power
Assessment Questions
1. In which of the following cases is work being done on an object? a. pushing against a locked door
b. suspending a heavy weight with a strong chain
c. pulling a trailer up a hill
d. carrying a box down a corridor
ANS: C
14.1 Work and Power
Assessment Questions
2. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer?a. 2,500 Jb. 4,000 Jc. 20,000 Jd. 160,000 J
14.1 Work and Power
Assessment Questions
2. A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer?a. 2,500 Jb. 4,000 Jc. 20,000 Jd. 160,000 J
ANS: D
14.1 Work and Power
Assessment Questions
3. A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a. 5000 W
b. 6000 W
c. 50 W
d. 1000 W
14.1 Work and Power
Assessment Questions
3. A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a. 5000 W
b. 6000 W
c. 50 W
d. 1000 W
ANS: A
14.1 Work and Power
Assessment Questions
4. One horsepower is a unit of power equal toa. 0.746 W.
b. 1.0 W.
c. 746 W.
d. 2,000 W.
14.1 Work and Power
Assessment Questions
4. One horsepower is a unit of power equal toa. 0.746 W.
b. 1.0 W.
c. 746 W.
d. 2,000 W.
ANS: C
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