dynamics. A body has a constant velocity unless it is acted on by a net force. Many individual forces may act on a body, but the resultant or net force must be equal to zero if the body is not to accelerate. The assertion that a body on which there is no net force experiences no acceleration implies that a moving body will maintain a constant velocity forever unless acted on by a net force. Newton's first law, however, does not apply to all reference frames, the coordinate systems. The application of Newton's laws usually means solving dynamics problems, which are commonly stated as word problems. The basic issue in dynamics problems is solving the fundamental dynamic equation, ∑F = ma. Perhaps the most common form of problem in dynamics requires that the acceleration of a given mass be determined when the mass is subjected to known forces. Sometimes more than one mass is involved; occasionally the mass is the unknown quantity to be determined