Aristotle wrote that a force is required to keep a body moving. Everyday experience seems to confirm this misconception. For two thousand years Aristotle’s description of motion was accepted without question. Then an experiment by Galileo resulted in the discovery of friction. Galileo’s experimental approach to investigating Nature helped to establish modern science and led to the invention of calculus and Newton’s laws of motion. Four centuries after Galileo the knowledge of motion enables scientists to predict and control the paths of distant spacecraft with great accuracy.

There are many types of motion: straight line, circular, back and forth, free-fall, projectile, orbital, and so on. This standard set concerns itself with the motion of a body traveling either at a constant speed or with a varying speed that is represented by an average value.