Susan Fanuzzi, age 13, of Staten Island, N. Y., for her question:
WHAT IS GRAVITY?
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 the vertical line passing through its center of gravity, the object will remain balanced. But if the center of gravity is to one side of the pivot, the object will turn in that direction. You can prove this on a child's seesaw.
Gravity is the force of attraction that acts between all objects because of their mass. Mass is the amount of material they are made of.
Gravitation holds the universe together. It holds together the hot gases in the sun and it also keeps all of the planets in their orbits. The moon's gravitation causes the ocean tides on earth.
Because of gravity, all objects on or near the earth are pulled toward it. The gravitational attraction a planet has for objects near it is called the force of gravity.
The true nature of gravity is not known but it is presumed to have the nature of electromagnetic propagation. It is visualized as a field of force, or of attraction, surrounding every material object, no matter how small.
The earth is so big that its attraction does not weaken immediately as you leave the surface. At 200 miles' altitude, gravitation still has 90 percent of the ground level strength it had. It is not reduced to half until a spacecraft has risen to about 3,000 miles.
The ancient Greek astronomers studied the motions of planets and the moon but these motions were not correctly explained until the late 1600s when the English scientist Isaac Newton showed that there is a connection between the force that attracts objects to the earth and the way the planets move.
Newton's theory of gravitation says that the gravitational force between two objects is proportional, or related directly, to the size of their masses. This means that the larger either mass is, the larger the force is between the two objects.
The theory refers to mass rather than the weight because the weight of an object on the earth is really the strength of the earth's gravity on that object.
In 1915, the German born American physicist Albert Einstein advanced general theory of relativity which also outlined theories on gravitation.
Einstein's theory involved a complete change in the ideas about gravitation but it explained rather than contradicted Newton's theory.