When a body falls in a gravitational field, it loses potential energy. If it falls freely, it gains an equal amount of kinetic energy (See "FALBAL" for a video method of measuring the speed of a falling ball). If the body is attached to a spring, the decrease in gravitaional PE is offset by an increas in the sum of kinetic and elastic potential energy. The body will oscillate about a new equilibrium point, which, except for frictional losses, will be halfway between its release point (zero KE) and its point of extreme travel.