The ability to focus attention on important things is a defining
characteristic of intelligence.
--Robert J. Shiller, Irrational Exuberance
System: a body or group of bodies which can be isolated from all
Free-body diagram: a sketch which isolates a system from its
surroundings and shows all forces applied to it.
Boundary: an imaginary line encompassing the isolated system.
A joint is an interface between the system and the environment.
How to construct a FBD step-by-step (do these steps every time).
1. Decide which body or system of bodies to isolate.
2. Choose the boundary.
3. Apply all forces which act on the body.
4. Indicate coordinate axes.
Rule of Joint Resistance
If a joint isolated from the system could resist a translational
displacement along or rotational displacement about a certain axis,
then a corresponding force or moment must be represented on the