# Work and Energy

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## Work and Energy

#### Why do we have to do work?

Work is done whenever energy is changed from one form into another.

If a football is kicked along the ground it has friction and air resistance trying to slow it down. The ball loses its kinetic energy as it slows down because it has to work against these forces. The kinetic energy is changed into heat energy.

If a cyclist travels up a hill, without pedalling, the kinetic energy the cyclist had at the bottom of the hill is changed into gravitational potential energy as he travels up the hill. The cyclist slows down as he loses kinetic energy.

The cyclist is doing work against the gravitational pull of the Earth trying to stop him going up hill.

#### Calculating work done

The amount of energy that has changed form is called the work done.

A crane lifts up a car. The higher it lifts the car the more work it will do. The heavier the car is the more work the crane will do.

So:

The bigger the force the greater the work done.

The further the distance the greater the work done.

The amount of energy or work done can be found using:

work done (or energy) = force x distance travelled

Work done (or energy) is measured in joules, J.

Force is measured in newtons, N.

Distance is measured in metres, m. (Not cm!!)

Note: The force must be in the same direction as the distance travelled.

Example 1:

Example 2:

Note: In example 2 many students would make the mistake of using the wrong distance. Remember the distance must be in the same direction as the force.