Forces - Forces and Energy

When a force is applied to an object and causes it to move, energy is transferred to the object and work is done. In everyday language, work means a lot of different things but in physics it means only one thing - a force has made something move.

It is calculated by multiplying the force needed to move an object by the distance it moved. The unit of work is the joule (J) and one joule is the amount of work done when a force of one newton (N) moves through a distance of one meter (m). Joules are also the units of energy so the work done must therefore be equal to the energy transferred to or from the object that has moved.

The greater a force that is applied to an object, the more energy the object gains. An example of this is a tennis player making different shots. When they make a smash, they use as much force as they can so the ball gains a lot of energy. When the ball hits the court, it will bounce a long way. Another kind of tennis shot is the drop shot. The player uses much less force so that when the ball lands, it has a lot less energy and the idea is that the ball bounces as little as possible.

As a force causes an object to move, it takes a certain amount of time. Power is a measure of how quickly that work is done and therefore is also a measure of the rate at which energy is transferred. Whenever you come across anything in physics that is described as a 'rate' you know immediately that it is something that is divided by time. In the case of power, it must be the work done (or energy transferred) divided by the time taken. So, if a question asks you to calculate power, it will either talk in terms of work done or energy transferred. The SI units of power are watts (W), one watt of power is equal to one joule of energy being transferred in one second.