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.

The gravitational force between two bodies. The two arrows are the same length and act in opposite directions along the same line - as in the diagram on the left. This reflects the fact that forces are of the same origin and have the same magnitude. This is Newton's Third Law.

Examples are an ice skater pushing another, or two particles repelling each other. The forces are equal and opposite. No external forces act. This is another example of Newton's Third Law.