What is a free-body diagram?
Free body diagram - a physical model which represents the forces acting on a system. Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

The size of the arrow in a free-body diagram reflects the magnitude of the force. The direction of the arrow shows the direction that the force is acting. Each force arrow in the diagram is labeled to indicate the exact type of force.

The free-body diagram shown above depicts four forces acting upon the object. Objects do not necessarily always have four forces acting upon them. There will be cases in which the number of forces depicted by a free-body diagram will be one, two, or three. There is no rule about the number of forces that must be drawn in a free-body diagram. The only rule for drawing free-body diagrams is to put all the forces that exist for that object in the given situation. So, to construct free-body diagrams, it is extremely important to know the various types of forces.

By: Tawnie DeJong and Tomilyn Trask