Suppose that you have three clockwork trains, all of the same mass but with their springs all wound up differently. The more the spring is wound up the more energy is stored in it.
Train 1 has 1 unit of stored energy, train 2 has 2 units of stored
energy and train 3 had 3 units of stored energy. All three trains are now allowed to run along a
straight, level track until they stop - they will then have used up all their stored energy.
We
find that the first train with 1 unit of stored energy goes 30 m before it stops.
The train
stops because it has used up its stored energy in doing work against the FRICTIONAL FORCE
that is trying to slow it down. This energy is converted to heat.
How far will the other trains
go before they stop is the force of friction on all of them is the same.
If the trains were
twice as heavy the force of friction would be twice as big and if they were three times as heavy it
would be three times as big.
The energy used up by the train controls the distance
travelled and this distance is also affected by the force that the train has to overcome. The train
has used up some stored energy and has done work against a frictional force.