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Scientists use the word work to describe the energy needed to do a task, by making a force move through a distance. The amount of work done is equal to the energy used and both are measured in JOULES (J). It takes energy to lift a weight a certain distance, because you have to do work against the force of gravity. POWERFUL machines can do lots of work in a short time. EFFICIENT machines waste relatively little energy when doing work.

Efficiency is a measure of how much of its energy a machine converts into useful work. No machine ever converts all its energy into work: some energy is always wasted in the process. Car engines convert fuel into the energy they need in order to move, but get hot as they do so. This heat does not help the car to move, so a car is relatively inefficient, compared to other machines.

Bicycles are efficient machines. They allow riders to convert muscle power into movement with little wasted energy. Racing cyclists wear aerodynamic clothing. Less energy is wasted overcoming air resistance, so more energy is used to move the bicycle.

Some machines can do work more quickly than others, and these are said to be more powerful. Power is the amount of work that something can do in a certain amount of time. Cars with bigger engines can go faster, which means they cover more distance in the same time. This means faster cars do work more quickly than slower cars, so they are more powerful machines.

The amount of work done when a force acts over a distance equals the size of the force (measured in newtons) times the distance through which it moves (measured in metres). The work done is measured in joules, named after English physicist James Prescott Joule (1818–1889). An amount of work takes the same amount of energy to do it, so energy is also measured in joules.

One joule is the work that has to be done to make a force of one newton act over a distance of one metre. One joule of energy is needed to do one joule of work. It would take two joules of work to apply the same force for a distance of two metres.

When a tennis player hits the ball, he does work. If he eats a banana before the match, his body can use the energy it contains to do this work. The energy value of food is measured in kilojoules or kilocalories (Calories for short). The body does not convert all the energy in food into useful work, so it is not 100% efficient.