Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

4.2 FREE BODY DIAGRAMS (FBD)

· Up to this point we assumed very simple forces acting on very small particles.

· In reality mechanical systems have many parts, and we draw an FBD for each part.

· We should divide forces on free body diagrams into two categories,

Internal - these forces act only within a free body, and cancel out, unless we are looking at a section of a free body.
External - these forces act on a free body, and they induce reaction forces. Examples are gravity, and other free bodies.

· An example of using free body diagrams for a system is given below with a system of masses, ropes, pulleys and anchors.

4.2.1 Pulleys and Springs