Free Body Diagrams
Translational Mechanical Systems
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
Procedure for Drawing Free Body Diagram
- Sketch an outline of each object isolating it from its surroundings.
- 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.
- 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.