Date of this Version
A particle which remains at rest or in uniform motion with respect to its frame of reference is said to be in equilibrium in that frame. Centuries ago it was recognized that the state of rest was a natural state of things, for it was observed that objects set in motion on the surface of the earth tended to come to rest. The maintenance of any horizontal motion on earth was thought to require the continued exercise of a force, hence to be a violent motion, while vertical motion like that of a falling body was thought to be natural. In heavenly bodies circular motion was thought to be natural. That uniform motion in a straight line was a universal equilibrium condition, a natural state of things, was not recognized until the work of Galileo (1564-1642) and Newton (1642-1727), which represented a very significant contribution to the study of mechanics and to our understanding of nature.
Newton summarized his conception of motion in three principles, which are today called Newton's laws of motion, the first of which may be stated as follows: A body at rest will remain at rest, and a body in motion will continue in motion with constant speed in a straight line, as long as no net force acts upon the body.