... This lesson presents and explains Newton's 3rd law ("To every action, an equal and opposite reaction"), gives some examples as well as an (optional) formulation of Newton's laws that avoid explicitly mentioning forces. ... The formal definition of Newton's 3rd law: " forces always originate in pairs, equal in magnitude and opposite in direction. ... Starting the lesson . ... Newton's 3rd law discusses the origin of forces: they are always created in pairs, equal in magnitude and opposite in...