Answer is: The potential energy decreases as kinetic energy increase.
A book at the shelf has only potential energy.
Potential energy is type of stored energy; it can be increased by getting into a higher position and/or stretching an object.
When the book is falling from the book shelf it gains kinetic energy.
Kinetic energy has to do with the speed of an object and how much mass it has; basically how the object is moving.
The kinetic energy of a non rotating object of mass m traveling at a speed v. The standard unit of kinetic energy is the joule.
Ek = 1/2 · m(book) · v²(book).
