This document explains Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. It provides several examples to illustrate this law, including how forces act on objects in contact, a rocket launching, and weightlessness in orbit. It also discusses how Newton's third law explains why astronauts appear weightless in space, even though gravity is still pulling on them. The document concludes by explaining the law of conservation of momentum, which states that the total momentum of a system remains constant unless an outside force acts on it.