John Meeker, age 13, of Seattle, Wash., forms question:
What exactly is gravity?
When you drop a stone, it falls to the ground. It does not sail off Into a tree or travel in any other direction. It falls down. We tend to take this sort of thing for granted. But in the year 1666, young Isaac Newton stopped taking it for granted and figured out why things tend to fall down, rather than in any other direction. Newton did all this when he was only 25 years old, though he did not publish his findings until 20 years later.
Newton found out why a dropped stone falls to the ground, why we stay on the face of the earth instead of floating off into the sky and why the moon and the planets stay in their places. For he discovered what he called the laws of gravitation and worked them out in f inert detail. Isis discovery is used to calculate missiles and orbiting satellites.
The universe, it seems, is composed of vast oceans of space, spattered with conglomerations of atoms. Stars and planets, moons and comets, hazy nebulae and filmy clouds of cosmic dust are made of atoms. And these objects all have a built‑in force which Newton called gravitation. It is a pulling force and all the atoms in an object, whether it is a large sun or a tiny comet, all pull together.
The more atoms pulling together, the greater their force of gravity. The number of atoms in an object gives it its mass. In the Solar System, about 99.9 per cent of the mass is centered in the sun. The pulling power of its gravity is much stronger than that of the planet earth. It is strong enough to hold all the planets in the Solar System and prevent them from floating off alone into space.
Newton also discovered that the power of gravity decreases with distance. The farther we get from an object, the weaker its pull of gravity becomes.
The planet Mars is about 14.1 million miles from the sun and earth about 92 million miles. The earth feels the sums gravity four times stronger than does Mars. The planets have gravity of their own, though it is less than that of the massive sun. They pull back at the sun and at each other.
The gravity of the moon pulls at the earth, causing the tides. The gravity of the earth pulls at the moon, keeping it in orbit around us. There is gravity, too, in every speck of dust, in everysolid object including your own body. The gravity of the earth, however, is so much stronger that it pulls them down when they fall and keeps them from flying off into space. The earth even holds its blanket of air by the force of gravity.
Why does not the mighty force pull all the planets into the sun and why doesn't the earth pull down the less massive moon? There is a force in the universe working against the pull of gravity. It is speed, the kind of speed at which a. planet travels around its orbit. We call it centrifugal force and it pulls an object away from a central force of gravity, such as the sun. The speed of the orbiting moon keeps it from falling onto the earth.