Newton's Third Law

• Newton's third law: when two bodies interact, the forces they exert on each other are equal in magnitude and opposite in direction.
• If object A pushes object B, then B pushes A back with the same size of force the other way. The two forces appear together, at the same time. Neither one causes the other.

A true third-law pair passes all four checks:

1. the two forces act on different objects (one on each);
2. they are equal in magnitude;
3. they are opposite in direction;
4. they are the same type of force (both gravitational, both contact, both magnetic, and so on).

• A person standing on the ground is part of two separate pairs. Gravity pair: the Earth pulls the person down, and the person pulls the Earth up. Contact pair: the ground pushes the person up, and the person pushes the ground down.
• Each pair has one force on the person and one force on the Earth. That is exactly what check 1 asks for.

The two forces of a third-law pair act on different objects, so they can never cancel each other and can never hold one object in equilibrium.

• A book rests on a table. Its weight (down) and the table's contact force on it (up) are equal and opposite. Students call them a third-law pair. They are not.
• Both forces act on the same object (the book), which fails check 1. They are also different types (one gravitational, one contact), which fails check 4.
• The real partner of the book's weight is the book's gravitational pull on the Earth. The real partner of the table's push is the book's push on the table.

• Rockets: the engine pushes exhaust gas backwards, and the gas pushes the rocket forwards. The pair is “force of rocket on gas” and “force of gas on rocket” (9702/11/M/J/25 Q10).
• Car wheels: the tyre pushes backwards on the road, and the road pushes forwards on the tyre. That forward friction force is what drives the car (9702/12/M/J/25 Q11).
• Collisions: when X hits Y, the force on Y from X is equal in magnitude and opposite in direction to the force on X from Y, and the two forces act for the same time (9702/23/O/N/24 Q2(c)(ii)).