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The Effects of Gravity There are some people who worry that when they're outside, if they don't keep a good grip on the ground, they'll just go flinging off into space. They needn't really worry about this, because gravity generally keeps that sort of thing from happening. The thing is, no one is really sure what causes gravity, but the effects have been studied by many physicists and astronomers. Three of the more obvious effects of gravity are things falling down, weight, and the the moon and planets staying in their orbits. Things fall down. People have generally grown to accept that if one lets go of one's prized and valuable textbook when walking through a mud puddle, the book will invariably end up in the puddle and therefore be stripped of all value and even legibility. Things fall down because there is a strong gravitational attraction between things of great mass, like the Earth, and things of little mass, like a book. The only problem with this relatively simple explanation is that no one really knows why it's like that. What people have figured out so far is that gravity is a force, and a force is anything that changes the state of rest or motion of an object. In the absence of outside forces, the momentum of a system remains constant. This means that if there was no gravity, when one would relinquish one's hold on the textbook, it would remain at rest in the air. If a force acts on a body, the body accelerates in the direction of the force. In the example of the force of gravity, small things like textbooks are pulled downward toward the center of the large mass of the Earth, not up into space, even if some people think that this might happen. Torgerson 2 Isaac Newton was the first to conceive of weight as the gravitational attraction between a body and the Earth. The force that results from the gravitational attraction of the Earth on bodies at its surface is what we call weight. Science has chosen to measure the mass of objects in units that are roughly equivalent to the weight of those objects on Earth. For example, if a textbook weighs four pounds on Earth, it would have a mass of four pounds in an orbiting spaceship. The textbook would be "weightless" because it does not feel the gravitational attraction of the Earth, but, even in outer space, to push the