Work and Power

WorkWork is the product of force

and distance.

Work is done when a force acts on an object in the direction the object moves.

Work is done by the weight lifter when he exerts an upward force to raise the barbell over his head.

WorkFor 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.

WorkWork depends on the direction of the force and

direction of the motion

Force and work have to be in the same direction

WorkA force does not have to act

entirely in the direction of movement to do work.

The force acts upward and to the right along the handle, whereas the suitcase moves only to the right along the ground.

There is a horizontal portion of the force acting in the direction of motion.

Only the horizontal part of the applied force—the part in the direction of movement—does work.

WorkAny part of a force that

does not act in the direction of motion does no work on an object.

The vertical lifting force does not act in the direction of motion.

Thus, this vertical force does no work on the suitcase.

Calculating WorkWork = Force X Distance

 When using SI units in the work formula, the force is in Newtons, and distance is in meters.

The product of force and distance results in the units of newton-meters, also known as joules

The joule (J) is the SI unit of work.

When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.

Calculating WorkCalculate the work done by a weight lifter who

lifts a 1600-newton barbell over his head. Assume that the barbell is lifted to a height of 2.0 meters.

PowerOften people are not just concerned

about getting work done.

They want it done fast, and that requires power.

Power, like work, has a precise meaning in science.

Power Power is the rate of doing work.  

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.

Power• A person using a shovel and a person using a

snowblower can both do the work needed to remove the snow.

• The snowblower can do the job much faster. The fact that the snowblower can do more work in less time means that it has more power.

Calculating PowerPower = Work ÷ Time

When using SI units in the power formula, work is in joules (J) and time is in seconds (s).

The SI unit of power is the watt(W), which is equal to one joule per second. Thus, a 40-watt light bulb requires 40 joules each second that it is lit.

Calculating PowerYou 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?

Practice1. 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?

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?

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.)

HorsepowerHorsepower is another unit of power

One horsepower is equal to about 746 watts.

Horsepower is based on the power output of a very strong horse.

Both do work at a rate of 4 horsepower