Statement

Newton's third law: When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude and opposite in direction. In short: for every action there is an equal and opposite reaction.

Mathematical form: F₁ = − F₂

• Forces occur in pairs.
• Example: two spring balances hooked together and pulled give identical readings: the force of the first on the second equals that of the second on the first.