The word “kinetic” comes from the Greek word ‘, meaning “to move.” Kinetic energy is one of the two main categories into which physicists divide energy, the other being potential, or stored, energy. The former represents work that has already been done, whereas kinetic energy is energy of movement. A merry- go- round that is not being used has potential energy; when it is spinning, it has kinetic energy.

Energy is constantly changing from one form to the other, and in both directions. When a pitcher swings his arm back to pitch a baseball, his potential energy is transferred to the ball; when he releases the ball, its potential energy is transformed into kinetic energy. And when the batter strikes the ball, he transfers into it more potential energy, which is in turn transformed into kinetic energy.

No object can be said to have absolute kinetic energy; the amount it can be said to possess depends on what frame of reference one is using. For example, a person sitting in a moving train car has no kinetic energy if the car is used as the frame of reference, but he does have such energy if the ground is being referenced.