2.3 Newton's Third Law

Newton's Third Law

For every force one object exerts on another, the second object exerts a force equal in magnitude and opposite in direction on the first. These action-reaction pairs always act on *different objects*, occur simultaneously, and are of the same type of force. They never cancel each other because they act on separate bodies.

Identifying force pairs

To find a third-law pair, swap the subject and object: if A pushes on B, then B pushes on A. Pairs must match in type (e.g., both gravitational, or both normal) and act along the same line but in opposite directions. Internal forces within a system (like tension pulling two blocks together) still obey this law.

Third law vs. net force

Don't confuse action-reaction pairs with balanced forces on one object. A book resting on a table has weight (Earth on book) and a normal force (table on book) — these act on the *same* object and are NOT a third-law pair, even though they may be equal. The true pair to weight is the book pulling Earth upward via gravity.

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 simultaneously on two different objects.

How to find a third-law pair

Swap the subject and object of the force statement: if A pushes B, then B pushes A equally and oppositely.

Common misconception

Weight and normal force on the same object are NOT a third-law pair — they act on the same body, not different ones.

Why third-law pairs don't cancel motion

They act on different objects, so each object can still accelerate independently based on its own net force and mass.

Try an AP-style question

A 4 kg block pushes a 1 kg block along a frictionless floor with a 10 N push on the 4 kg block. What force does the 1 kg block exert on the 4 kg block?

1. 10 N backward
2. 2 N backward
3. 2 N forward
4. 8 N backward