ABSTRACT
It may be appropriate to recall at this point that, at the macroscopic level, the forces available to drive mechanical systems can be put into two classes: forces associated with physical contact between two bodies, and the "mysterious" actionat-a-distance forces, namely, gravitational, magnetic, and electrostatic forces. When
using Newton's law, :E forces= (mass)(acceleration), the terms entered into the force summation must arise either from physical contact or else from magnetic, gravitational, or electrostatic origin; there are no other kinds of forces. If you have encountered D'Alembert's method of dynamic system analysis, you may at this point be thinking about the so-called inertia force, but should recall that this is a fictitious force mentally added to convert what is really a dynamics problem into an equivalent statics problem. Inertia force is not a real force capable of causing a body initially at rest to move.
