|MadSci Network: Astronomy|
Well, if nothing had ever had gravity, the universe would have developed in a completely different way from what we see now, and life as we know it would not exist. Let's think, then, about what would happen if gravity just suddenly disappeared all at once today. To think about that, let's think about what gravity does in our solar system. You probably know that the planets orbit around the sun. This orbit is caused by gravity. The planets are constantly trying to fly off in a straight path, but the sun's gravity pulls them into a curved path around it. Look at the picture here, which is obtained from this Web page. It shows a planet orbiting the sun. Its "instantaneous velocity" is the speed and direction it's moving at when you take a snapshot of the solar system. You can see that this velocity points in a straight line from the edge of the circle. This kind of line is called a "tangent" of the circle. The "centripetal acceleration" in the picture is the acceleration caused by gravity pulling the planet inward. You can then see that this inward pull results in the "resultant orbital path" taking the planet around the sun. If gravity disappeared, that centripetal acceleration would disappear also, and the planet would fly off along whatever tangent its instantaneous velocity was pointing on when gravity vanished. So if all of a sudden nothing had gravity, the solar system would fly apart as the planets all went off in straight lines from their orbits.
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