The 42nd Period | STEM Education Hub

Newton's Third Law

Newton's Third Law: for every force one object exerts on a second object, the second object exerts an equal-magnitude force back on the first, in the opposite direction.

F(A on B) = −F(B on A)

This isn't a special case that happens sometimes — it's true for every force, every time, with no exceptions. Gravity, contact pushes, magnetic attraction, all of it.

Two Objects, Two Forces

A valid Third-Law pair always has the same shape: two objects, two forces of the exact same type (both pushes, or both pulls, or both gravitational), equal in size, opposite in direction, each one acting on a different object than the other.

Flip through four real Third-Law pairs, then look at the classic trap that ISN'T one.

The wall doesn't move because it's attached to the whole building — a huge effective mass. You feel the pushback because YOUR mass is much smaller.

The Balanced-Pair Mix-Up

The single most common Third-Law error: looking at a book sitting still on a table, seeing its weight and normal force are equal and opposite, and calling them a Third-Law pair.

⚠️They're not. Weight and normal force both act on the book. A Third-Law pair requires two different objects. Weight and normal being equal here is Newton's First Law (equilibrium) — a completely different fact that happens to also involve equal and opposite forces, by coincidence of this specific scenario.

Use the "Book Trap" toggle in the explorer above to see the book's actual Third-Law partners: it pulls the Earth upward (paired with its weight), and it pushes the table downward (paired with the normal force) — two forces on two objects that are not the book at all.

Why Nothing Cancels

If every push has an equal, opposite partner push, why does anything ever move? Because those two forces act on two different objects, and different objects usually have different masses.

🔑A rocket and its exhaust gas feel exactly equal forces — but the rocket has far more mass than the tiny amount of gas it expels each moment, so the same force produces very different results for each object. (Exactly how much different is a question for Newton's Second Law, coming up soon.)

Real-World Reaction Pairs

Once you know what to look for, Newton's Third Law shows up everywhere: walking, swimming, rocket propulsion, a gun's recoil, a jump off a diving board. In every case, something pushes against something else, and that something else pushes right back — and it's the "pushing back" force that actually moves you.

💡A common phrasing trap: "I push off the ground when I walk." True, but the force that actually propels you forward is the ground pushing back on your foot — you don't move yourself forward, the ground does, in response to what your foot did to it.

Looking Ahead

You now have all three of Newton's Laws in some form: equilibrium and inertia (First), the two-object structure of every force interaction (Third), and a first hint of how mass determines how much a given force actually changes an object's motion. Project 2-1-4 puts the First and Third Laws to direct use on a real force table — and Newton's Second Law, which finally makes the force-mass-acceleration relationship exact, is coming up soon after.