Physics · Unit 2: Dynamics ·  Activity 2-1-3

Newton's Third Law — Action-Reaction Pairs

Every push has a partner push, exactly as strong, going the other way — and they're never fighting on the same team  ·  1 class day

StarringF(A on B) = −F(B on A)
Unit 2: Dynamics

The single biggest mix-up in this whole unit: confusing a third-law pair with a balanced pair of forces.

Third-Law Pair
Two forces, on two different objects. Always equal, always opposite.
Balanced Pair (Equilibrium)
Two (or more) forces, on the same object. Might happen to be equal — that's Newton's First Law, not the Third.

🧭 Plot Summary

Newton's Third Law: for every force one object exerts on a second object, the second object exerts an equal and opposite force back on the first. Push a wall, and the wall pushes back on you — same strength, opposite direction, no exceptions.

Here's the part that trips almost everyone up at first: these two forces act on two different objects. They never cancel each other out, because forces only cancel when they're acting on the same object — which is a completely different idea from Unit 1's equilibrium (Lesson 2-1-2).

What you will do in this lesson

  • State Newton's Third Law and learn to find its two forces on its two objects, every time.
  • Untangle the single most common mix-up in dynamics: action-reaction pairs vs. balanced forces.
  • Explain why equal-and-opposite forces still let things move — mass is the missing piece.
  • Recognize Newton's Third Law behind walking, swimming, and rocket propulsion.

Why it matters

Project 2-1-4: Force Table Balancing Act asks you to balance real forces on a ring — and every one of those forces has an invisible Third-Law partner acting somewhere else that you won't be measuring directly. Knowing which forces belong on your diagram, and which ones don't, starts here.

✅ Self-Check Before You Roll On

Check off each item as you get there. These are not grades — they are your own signal.