Pushing a carEdit
Have you ever helped to push start a car? The hardest part is getting the car to move, but once it is rolling it takes much less effort, (especially if it is on a hill rolling down). To change the velocity of an object we have to do work on it.This means we have to supply energy to it. Once an object is moving it requires no further work, to keep it moving. Objects only slow down in everyday life because of friction forces which do work on the object.
To do work on an object you have to apply a force to it, the amount of work you do depends on the acceleration of the object. Consider pushing a car quickly versus slowly. Pushing a car quickly requires more work.
The rate of change in an objects velocity with respect to time is its acceleration.
When you do work to push start a car, your energy is transferred to the kinetic energy of the car. When objects go faster they have more kinetic energy. We can slow an object down by removing energy from it. For instance if we use a spinning wheel to drive machinery, such as a generator, it will slow down as the energy is transferred to the generator and converted to electricity.