Energy stored by position
A book on a high shelf is not moving, so it has no kinetic energy. It can still do work — drop it and it will. The energy it is holding has a different name.
Potential energy
The energy stored in a body because of its position, or because its shape has been changed, is called potential energy.
Two ways to store it
The definition names two routes, and they behave quite differently even though both end in stored energy.
By position
Raise an object above the ground and it stores energy simply by being up there. Let it go and the energy is released.
By change of shape
Stretch a rubber band or bend a bow and it stores energy in the deformation. Release it and the material springs back.
Where the stored energy came from
When you raise an object to some height, you do work on it — energy is spent to bring it to that position. That energy is not lost; it is stored in the object as potential energy, and it stays there until the object is released.
The work you put in is the energy you get out
Nothing is created by lifting. The store is exactly the size of the work that was done to fill it, which is why the next page can find the potential energy by calculating the work instead.