Feynman teaching note

Newton's Third Law

SubjectPhysics

Area tags

Physics

Every push comes with a push-back of equal size in the opposite direction. Not sometime later, not a little less: exactly equal, exactly opposite, at the exact same moment. Stand on a skateboard and throw a heavy ball forward. The ball goes one way; you roll the other way. You pushed the ball, and the ball pushed you back just as hard. The classic trap: 'if forces are equal and opposite, why don't they cancel?' They don't cancel because they act on different objects. Your hand pushes the ball (force on the ball); the ball pushes your hand (force on you). Cancellation only happens when two forces act on the same object. Rockets work this way too: the engine throws gas downward, the gas throws the rocket upward.

Same topic, fresh practice—does not count as an adaptation.

Practice building on this note. If you publish, it can show “Based on” and add an adaptation.

Feedback & gaps

Clarity score: 82/100How well you know it: Can explain it simply

Use Newton's third law to explain how a swimmer moves forward in water.

Doesn't connect the pair of forces to acceleration via F=ma and different masses.
The rocket example could clarify that it works even in a vacuum.

How I worked through the gaps

Why don't the forces cancel?

Confusing equal-and-opposite with cancellation.

Q1If the two forces are equal and opposite, why does anything move at all?

YouBecause each force acts on a different object, so neither object feels both forces.

CoachExactly. Cancellation needs both forces on the same object; here they're split across two bodies.

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