Obtaining the correct free body diagram is considered as a major step in obtaining the mathematical model of a mechanical system. It shows all forces and torques that act on an object. We need to draw free body diagrams for each mass or a junction point of the system that moves with a velocity that is unknown beforehand.

Procedure for Drawing Free Body Diagram

1. Sketch an outline of each object isolating it from its surroundings.
2. Indicate on the sketch all forces acting on the object. Note that there are two different type of forces: active forces which tends to set the object in motion and reactive forces which tends to prevent motion.
3. Label known forces with their magnitude and direction. You can assume a direction to each unknown force. By definition the magnitude of a force is positive. If the solution yields a negative value, then the actual direction is opposite to the originally assumed direction.