LAW III: to every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. — whatever draws or presses another is as much drawn or pressed by that other. if you press a stone with your finger, the finger is also pressed by the stone. if a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. if a body impinges upon another, and by its force changes the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. the changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. for, as the motions are equally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies. this law takes place also in attractions, as will be proved in the next scholium.
"principia" on page 20 of volume 1 of the 1729 translation
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