Newton's third Law

Meaning: Forces come in interaction pairs. If A pushes on B with force F, B pushes on A with a force of the same magnitude in the opposite direction, at the same instant and of the same type (both gravitational, both contact, both electrostatic). Because the two forces act on different objects, each appears in only one object's free-body diagram and they do not cancel. The objects can still accelerate differently, since a = F/m.

Equation: F(A on B) = −F(B on A), where each F is a force vector in newtons (N), and the minus sign means equal magnitude in the opposite direction.

On the MCAT: Typical setups are two carts or skaters pushing apart, a collision between a large and a small object, gun or rocket recoil, and a person pushing on a wall. You are asked to compare forces (equal) and then accelerations or speeds (the smaller mass changes velocity more). The law also leads to conservation of momentum in an isolated system, because the impulses on the two objects are equal and opposite.

Common confusion: A book resting on a table has weight down and a normal force up, and students call these a third-law pair. They are not, because both act on the book and they are different types of force. The partner of the book's weight is the book's gravitational pull on Earth. The partner of the normal force is the book pressing down on the table.