Key Concepts: Newton’s Third Law

Newton's Third Law

For every action, there is an equal and opposite reaction.

• If object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction.
• Action-reaction pairs act on different objects — they never cancel each other.

Examples

• You push a wall → the wall pushes you back with equal force.
• Earth pulls you down (gravity) → you pull Earth up with the same force (but Earth barely moves due to its huge mass).
• A rocket expels exhaust gases downward → the gases push the rocket upward.

Common Misconceptions

• Action-reaction pairs do NOT cancel because they act on different objects.
• The "heavier" object accelerates less because a = F/m (same F, bigger m).
• Newton's third law always applies — there are no exceptions.