On a roller coaster, energy changes from potential to kinetic energy and back again many times over the course of a ride. Kinetic energy is energy that an object has as a result of its motion. All moving objects possess kinetic energy, which is determined by the mass and speed of the object. Potential energy is the energy an object has as a result of its position. Potential energy is stored energy that has not yet been released. Gravitational potential energy is potential energy that results from an object's position in a gravitational field, and is equal to the object's weight multiplied by its height. For example, a book placed on a shelf possesses gravitational potential energy because of Earth's gravity.