Gravity & Newton’s Third Law By: Charles A. Petersen Newton’s third law states that, “Every action (or force) gives rise to a reaction (or opposing force) of equal strength but opposite direction. In other words, every object that exerts a force on another object is always acted upon by a reaction force. The recoil of a gun, the thrust of a rocket, and the rebound of a hammer from a struck nail are all good examples of this law. The way the solar system is affected by Newton’s third law is easily shown using the sun and earth as examples. The sun is exerting gravity upon the earth which, by Newton’s Third Law, causes the earth to fall towards the sun. But the earth is moving laterally away from the sun so it falls around it in an ellipse. The sun is also falling towards the earth but its mass is so much greater that this is barely recognizable. The reason the earth never falls into the sun is that by Newton’s third law there has to be an outside force for the object to ever be at rest, since there’s no friction in space there is no outside force and the earth continues to move. When the moon goes around the earth it is basically the same thing that happens with the earth going around the sun, but the moon exerts a much more recognizable appearance on the earth than on the sun. When asteroids go around the sun they do not form good ellipses like the planets do. Instead because of their high velocity and strange shape some may actually travel outside the Oort Cloud and not come back for several thousand years. However the forces with asteroids and the sun are still basically the same except that asteroids are a whole lot smaller and hardly shift the sun a millimeter. What this means is that while the earth may be going around the sun the sun is also going around the earth. The moon is going around the earth but also the earth is going around the moon and asteroids are going around the sun but the sun is also going around the asteroids. What turns out happening is everything, in the entire universe, is going around everything else. The way we deal with this is by saying there is a certain of all gravity. At this center there is probably nothing at all except space, but everything in the universe, however slightly, revolves around this place. The center of gravity can even be made for just or own solar system, planetary system, room, whatever. Another application of Newton’s Third Law is when an object, such as an asteroid, hits another object, such as a planet, they will react with an equal and opposite force. This means that if an asteroid hits the earth it will have the force in which it is hitting repelled backwards. The way this actually works out is the asteroid, after going through the atmosphere, hits the earth and causes force to be released against the earth which is directed not only downward but laterally as well. The asteroid is reacted on with an equal and opposite force which tears it to shreds and sends pieces of it all over. Newton’s Third Law is what proves the Law of Universal Gravitation, the binding force of the universe. The most obvious effect of Newton’s third law, in relation to planetary bodies, is the tides that occur every day on earth. Just more evidence that Newton is right.