Newton's 3rd law

Meaning: Forces come in interaction pairs. If object A pushes or pulls on object B, then B pushes or pulls on A with the same magnitude, in the opposite direction, along the same line, and of the same type (both gravitational, both contact, both electric). The two forces exist at the same moment. Because the forces are equal but the masses may differ, the accelerations can differ, since a = F/m.

Equation: FAB = −FBA, where FAB is the force of A on B (N) and FBA is the force of B on A (N).

On the MCAT: Expect questions on collisions (a small car and a large truck feel equal force magnitudes, but the car has the larger acceleration), recoil of a gun, a swimmer pushing off a wall, rocket propulsion, and the Earth and a falling object pulling on each other. Third-law pairs also underlie conservation of momentum in an isolated system: the forces are equal and opposite, so the momentum gained by one object equals the momentum lost by the other.

Common confusion: Students call a book's weight and the table's normal force a third-law pair because they are equal and opposite. They are not. Both act on the book, and they are equal because the net force on the book is zero (Newton's first law). The partner of the Earth's pull on the book is the book's pull on the Earth. A true pair always acts on two different objects.