Free-body diagrams are one of the most useful tools in mechanics. They catalog all the forces acting on an object, and provide you with the necessary representation to make use of the Momentum Principle (i.e., to find the direction of the net force acting on the object). In these notes, you will read how to construct a free-body diagram and how it can help you reason about the direction of the net force on an object.

When you are trying to determine the motion of a system, it is important to determine the net force acting on the system. In physics, we often model systems as “point-particles”, that is, we neglect the internal structure and extent of the system – crushing it down to a single point that experiences the same net force as the real system. It is for this point particle that we draw the free-body diagram – a representation of all the forces acting on the system.

To be concrete, consider a book lying on a table as shown in the figure to the right.