Work and Energy

Work is done only when a force makes something move, or tries to stop it moving.

Whenever work is done energy is transferred.

The amount of work done is a measure of the amount of energy transferred.

work done = energy transferred

For this reason work and energy are measured in the same units, joules (J).

Work done against frictional forces is transferred mainly as heat.

For example when a car brakes and slows down its kinetic energy is transferred as heat and some sound.

The amount of work done depends on the size of the force applied and the distance something moves.

work done  =   force applied    x   distance moved in direction of force
  (joule, J)          (newton, N)                           (metre, m)

If someone pushes a pram 500 m, with a force of 50 N, the work done by the person will be:

     work done = 50 x 500
                       = 25 000 J (or 25 kJ)

Kinetic Energy

Kinetic energy is the energy an object has because of its movement.

As an object speeds up it gains kinetic energy.

As it slows down its kinetic energy decreases.

Elastic Potential Energy

Elastic potential energy is the energy stored in an elastic object when work is done to change its shape.

For example a stretched catapult has elastic potential energy.

When the catapult is released its elastic potential energy is transferred into kinetic (movement) energy.