Mike Bockman, age 8, of Ivanhoe, aCalif., for his question:
HOW DOES GRAVITY WORK?
The center of gravity is the point in an object where the force of gravity appears to act. If an object is balanced at any point on a vertical line, the object will remain balanced. It's like a seesaw. There will be no balance if a light child and a heavy child are at the two ends, yet it will balance again if the heavy child moves toward the center of the board.
Gravity is the force of, attraction that acts toward all objects because of their mass. Mass is the amount of material the object is made of.
Gravity holds our universe together. It holds the hot gases in the sun, and it keeps the planets in their orbits. The moon's gravitational force plays a part in the earth's ocean tides.
Greek philosopher Aristotle was one of the first to come up with a theory of gravity. He said that heavy objects fell faster than light ones. The point was challenged by an Italian scientist named Galileo in the late 1500s. He said that all objects fall with the same acceleration, or change of speed, unless air resistance or some other force slows them down.
By the late 1600s an English scientist named Isaac Newton proved there was a connection between the force that attracts objects to the earth and the way the planets move. He based his theory on the observations of planetary motions found by two astronomers of the late 1500s and the early 1600s Tycho Brahe, from Denmark, and Johannes Kepler, from Germany.
At the age of 23, Newton saw an apple fall from a tree and he realized that the same force that made the apple fall holds the moon in its orbit around the earth. From the laws set out by Kepler, Newton showed what kind of forces must be necessary to keep the planets in their orbits. He established what such a force would be at the earth's surface. The force proved to be the same as the force that gave the apple its acceleration.
Newton's theory said that the gravitational force between two objects is proprotional to the size of their masses. In other words, the larger either mass is, the larger will be the force between the two objects.
In 1915 the American physicist Albert Einstein came up with a theory of relativity. While it changed the ideas about gravitation, it explained rather than contradicted Newton's theory.
Some scientists think that a change in a gravitational field will give off gravitational waves. These waves, if they did exist, would be difficult to detect. Work in this area was done in 1969 by an American physicist named Joseph Weber. Although his experiments were most revealing, many scientists believe it wasn't established that gravitational waves had indeed been observed.