14.1 Work and Power 14-1 cornell p. 412 What is work Calculating work What is power? Calculating...

14.1 Work and Power

14-1 cornell• p. 412 What is work

• Calculating work

• What is power?

• Calculating power?

• James Watt and Horsepower.

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

I. Work and PowerA. Work- the product of force and distance.

1. Work Requires motiona. 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.

b. 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

2. Work Depends on Direction

a. If all of the force acts in the same direction as the motion, all of the forces do work.

b. If part of the applied force acts in the direction of motion, that part of the force does work.

c. If none of the force is applied in the direction of the motion, the force does no 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


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


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

14.1 Work and Power

2. Using the Work Formula

a. 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

14.1 Work and Power

C. Power is the rate of doing work.

1. Doing work at a faster rate requires more power.

2. To increase power you can:

a. increase the amount of work done in a given time,

b. 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

14.1 Work and Power

2. 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?

?

72 N

1m

2 sec

fxd(w)

P t

P=

f=

d=

t=

2

172 WsJ 3636

14.1 Work and Power

P=

f=

d=

t=

s

mN

25

5000,11 W200,2fxd(w)

P t

14.1 Work and Power

E. James Watt and Horsepower

1. Another common unit of power is the horsepower.

a. One horsepower (hp) is equal to about 746 watts.

2. James Watt (1736-1819) used horsepower to compare the power outputs of steam engines he had designed.

a. Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s.

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


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


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


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


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


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


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


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

14.1 Work and Power 14-1 cornell p. 412 What is work Calculating work What is power? Calculating...

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