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.
Table 4. HOW EFFICIENT ARE OUR MACHINES?
| Bicycle | 90% |
| Power station’s steam turbine | 35% |
| Human body | 24% |
| Car’s petrol engine | 20–25% |
| Electric light bulb | 5% |
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.
