When we lift up a weight, stretch a piece of elastic or push a box across the floor we are doing WORK and changing energy from one form to another. In each case we are pushing or pulling against a force, in the first case gravity and in the second case the tension of the elastic and so on. The further we lift or pull and the bigger the force we use the more work we do. When a car brakes to a stop work is being done by the frictional forces in the brakes and between the wheels and the road. If a clockwork motor is used to lift up a stone then the energy stored in the wound up spring is changed into stored energy of the stone. (We called this gravitational potential energy). The stone has been raised up against the force of gravity and so work has been done on it.When work is done a force is moved through a certain distance and the formula for the work done is:
When work is done energy must be changed from one form to another. This work is a direct result of the energy change and so energy is measured in the same units as work – joules. If 48J of work are done then 48J of energy are changed from one form of energy to others.
Note: Strictly speaking the formula should also mention the direction of the force. Work is force x distance moved in the direction of the force. Therefore for a planet going round the Sun no work is being done because the gravitational force of the Sun is always at right angles to the motion of the planet.