2.4 Newton's Third Law and Free-Body Diagrams

Newton's Third Law

For every action force, there is a reaction force equal in magnitude and opposite in direction, acting on a different object. These forces form an action-reaction pair — they never cancel because they act on different bodies. Example: a book pushes down on a table; the table pushes up on the book with equal force.

Identifying Force Pairs

Third-law pairs are the same type of force exchanged between two objects (e.g., both gravitational, or both normal/contact forces). They act simultaneously, have equal magnitude, opposite direction, and swap subject/object (A on B, B on A). They are NOT the same as balanced forces on a single object in equilibrium.

Free-Body Diagrams (FBDs)

A free-body diagram isolates one object and shows all external forces acting on it as arrows from its center, sized roughly to reflect magnitude. Common forces include weight (mg), normal force (N), tension (T), friction (f), and applied force. FBDs never include forces the object exerts on other things — only forces acting on the chosen object.

Key terms

Newton's Third Law

For every action force, there is an equal and opposite reaction force acting on a different object.

Action-reaction pair

Two forces of the same type, equal in magnitude, opposite in direction, acting on two different objects simultaneously.

Free-Body Diagram (FBD)

A diagram showing all external forces acting on a single isolated object, drawn as arrows from its center.

Why third-law pairs don't cancel

They act on two different objects, so each object still experiences a net force and can accelerate.

Common forces in FBDs

Weight (mg), normal force (N), tension (T), friction (f), and applied force.

Try an AP-style question

A truck collides head-on with a small car. How do the forces they exert on each other compare?

1. It depends on their speeds
2. The truck's force is larger
3. They are equal in magnitude
4. The car's force is larger