Donald Edwards, age 12, of Chilliwack, B.C., Can., for his question:
HOW DOES GRAVITY WORK?
The mysterious force of gravity reaches out from every star, from every planet and every speck of dust. Though it gets weaker as it goes, this overwhelming energy reaches on and on to span the entire universe. In fact, astronomers suspect that gravity is one of the main forces that holds the far flung universe together.
The gravity we know best belongs to the planet earth. It is, of course, the mighty tug that hugs us to the global surface and causes loose stones to roll down the slopes. We know that it is on the job day and night, every day of the week. We know its strength and at least one way to outwit it. But the basic nature of gravity is still quite mysterious.
Actually it is a built in quality of matter. Every star, every planet, every speck of dust has its own built in quota of gravity.
Each quota, weak or strong, behaves like all the others. It exerts an attraction or pull on all other matter solids, liquids and gases. In each case it works like the force of gravity between the earth and the moon.
Gravity acts as if it is in the center of a mass. Since mass is the amount of matter packed into a certain volume, the earth's center of gravity is down in the middle'of the globe. From there it reaches up, and we cope with it as surface gravity. If the same mass were concentrated into a smaller volume, the surface gravity would be stronger and we would weigh more. The smaller moon is much less massive and its surface gravity is only one sixth of the earth's.
The mysterious tug reaches out to link the earth and its orbiting moon, growing weaker with distance at a set rate. For example, if the distance between their two centers of gravity were doubled, the tug between them would be only one rul¬fourth. Then the moon's gravity would have less strength to pull up the tides in our oceans.
Naturally the mass, the size and the distance of each heavenly body is different. This makes a difference to th strength of gravity they exert upon each other. .
If this were the whole story, the enormous gravity of the massive sun would pull in all the members of the solar system. However, the force of gravity is counteracted by spiraling motion. The earth orbits at the right speed to maintain its distance from the sun. The earth cannot drag down the orbiting moon. Hence the subtle working of gravity keeps all the heavenly bodies in their places, which is very nice to know.