BELIEVE   ME   NOT!    - -     A   SKEPTIC's   GUIDE  

The Free Body Diagram

  

Figure: A man standing on the Earth (left) and his FBD (right). The man is pulled downward by the force of gravity W which is spread out over all his individual atoms but can be treated as if it were concentrated at his centre of gravity [CG] indicated on the diagram at about belt-buckle position. He is prevented from accelerating [falling] toward the centre of the Earth by the "normal force" N exerted upwards by the ground against his feet. These are the only two forces we need to consider to treat the problem of his equilibrium - i.e. the fact that he is not accelerating. The FBD on the right is perhaps a rather extreme example of a "simplified sketch" but it does serve the purpose, which is to show just the object in question and the forces acting on it from outside.

A good way to keep track of this (and cathect the right hemisphere in the process) is to draw what is universally known in Physics as a Free Body Diagram [FBD]. When you need to analyze the forces acting on a body [there are usually more than one!] the first step is to decide upon the boundary of "the body" - i.e. an imaginary surface that separates "the body" from "the outside world" so that we can talk unambiguously about who is applying which force to whom. Having done this in our imagination, it is usually wise to actually draw a little sketch of "the body" isolated from the rest of the world; it needn't be a good sketch, just a blob of approximately the right shape so we know what we are talking about. Then we draw in each of the vector forces acting on the body from other entities in the outside world; forces are always pictured as little arrows pointing in the direction of application of the force.⁸ A rather trivial example is shown in Fig. 1. We call N a "normal" force because it is normal (perpendicular) to the horizontal surface on which he stands; this terminology (and the N symbol) will be extended to describe any force exerted by a frictionless surface [yes, I know, another idealization...], which can only be perpendicular to that surface. Think about it.