Newton's third law

Forces always come in pairs, acting on two different objects.

• For every action there is an equal and opposite reaction.
• The pair acts on DIFFERENT objects, so they never cancel each other.
• Both forces are the same type (both gravitational, both contact).
• Name pairs precisely: "the force of the book on the table" and "the force of the table on the book".

• Action-reaction pair — two forces on two different bodies.