David Arroyo, age 13, of Columbia, Tenn., for his question:
HOW DOES GRAVITY WORK?
Gravity is a field of force or of attraction that surrounds every material object, no matter how large or small. Its true nature isn't known, but it is presumed to have the nature of electromagnetic propagation.
Gravitation holds the universe together. objects with a large mass, like the sun, have gravitational forces strong enough to hold in orbit planets that are hundreds of millions of miles away.
Less massive objects, like the moon, have weaker fields. The moon's gravity is only one sixth that of the earth, yet it is strong enough to cause tides here.
The earth is so big that its attraction does not weaken until you are far from the surface. Two hundred miles above the earth, gravitation still has 90 percent of its ground level strength.
The earth's gravity force still exerts 50 percent of its full force when an object has risen to about 3,000 miles above the surface.
If an object weighing 16 ounces `on earth were to travel to the moon, it would weigh 4 ounces at 8,000 miles, 1 ounce at 16,000 miles and 1/16th of an ounce at 64,000 miles. When it reached a distance of 210,000 miles, it would have no weight at all. Higher than that, the moon would start its gravitational pull.
When the 16 ounce object landed on the moon, it would weigh 2.66 ounces. The reduced weight, of course, would be caused by the moon's lesser mass and consequent lower gravity.
The ancient Greek astronomers studied the motions of planets and the moon and came up with some theories about gravity. But it wasn't until the 1600s that an English scientist named Isaac Newton found at least part of the answer.
In the late 1600s, Isaac Newton correctly showed a connection between the forces that attract objects to earth and the way planets move.
Newton based his explanation on observations of planetary motions by two astronomers of the late 1500s and early 1600s, Tycho Brahe of Denmark and Johannes Kepler of Germany.
Newton's theorized that the gravitational force between two objects is proportional, or related directly, to their mass. The larger the mass, the larger the force between the two objects.
Albert Einstein's 1916 theory of gravitation involved a major change in ideas about gravitation. But it explains rather than contradicts Newton's theory.