Newton’s Third Law: Action and Reaction

Having established that a force—the action of another body—was necessary to cause a body to change its state of motion, Newton made one further crucial observation: such forces always arise as a mutual interaction of two bodies, and the other body also feels the force, but in the opposite direction.

To every action there is always opposed an equal and opposite reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Newton goes on:

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 impinge upon another, and by its force change 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, towards the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made towards contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions.

The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force. UP, UP, and AWAY!   All this maybe sounds kind of obvious. Anyone who’s had a dog on a leash, especially a big dog, is well aware that tension in a rope pulls both ways. If you push against a wall, the wall is pushing you back. If that’s difficult to visualize, imagine what would happen if the wall suddenly evaporated. Newton’s insight here, his realization that every acting force has a reacting force, and that acceleration of a body only occurs when an external force acts on it, was one of the big forward steps in our understanding of how the Universe works.