ABSTRACT
To make sure that you are including all the pertinent forces, recall that fundamentally only two types of forces can act on a body: forces due to actual contact with another body (solid or fluid), and the "mysterious" gravitational, magnetic, and electrostatic forces which can cause a mass to move with no physical contact whatever. Also note that, if you have been introduced to the inertia force method of solving dynamics problems, the inertia force used there is not a real force, but rather a fictitious force added to the real forces to make a dynamics problem into an equivalent statics problem. Thus in using Newton's law '£F = MA, you never include an inertia "force" in the summation on the left-hand side; inertia force is not a real force that can move a mass initially at rest. It has no place in an analysis which directly uses '£F = M A. You thus need to account only for direct contact forces and gravitational, magnetic, and electrostatic forces in the summation of forces. (We are here restricting ourselves to the "macroscopic" world and thus don't get involved with other types of forces which exist at atomic and nuclear levels.)
