Dynamics is the study of systems that are not in equilibrium. The set of forces acting on the system under consideration reduces to either a single resultant force or a resultant couple, and the system is no longer in a state of rest or constant speed in a straight line. In our discussion of the statics of systems, making use of Newton’s first law, we assumed the particle or the particle system to be at rest, and for the most part disregarded the possibility of constant speed in a straight line, since we did not have at our disposal the necessary criteria for establishing the nature of motion in general. However, if the system is not in equilibrium, then motion of some kind is certainly taking place and the study of that motion becomes all-important.
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