Free Body Diagrams
Free diagrams are diagrams that show all the forces acting on a body. These can typically be:
Friction, gravity, tension, pushing forces, pulling forces or reaction forces.
Typical examples of free body diagrams are:
A man falling from a height. The man is shown as a particle. Sometimes air resistance can be neglected. Gravity is larger than air resistance is this diagram so the arrow that represents the force of gravity is longer.
A particle on a plane.
In the diagram, the particle is about to slip down the plane so the friction force acts up the slope, against the direction of motion.