Thomas Cvetkovich, age 13, of Youngstown, Ohio, for his question:
What determines the pull of gravitation?
This cosmic problem was figured out almost 300 years ago by a vacationing college student. Please do not, however, let this news make you feel backward or downhearted. That student happened to be Isaac Newton. No brain in history has surpassed his and few if any have equaled it.
Every object in the universe, from a speck of dust to a giant star, has its own built in quota of gravitation. It exerts its gravitational attraction on every other object. This mutual pulling power is thought to be a cosmic binding force, a vast network that holds the heavenly bodies in place and keeps them moving in orderly paths. The laws of gravitation are cosmic laws that obey the same regulations in every corner of the universe.
One basic law governs the quota of gravitation built into an object. The quota is proportioned by the mass of the object. And its mass is the matter packed into its size or volume. The quota in a speck of dust is infinitesimal. The more massive earth exerts enough pull to hug us all to its surface. The still more massive sun exerts enough pull to hold the earth in its planetary system. But regardless of quota, the universal pull is mutual. The speck of dust exerts a tiny pull on the earth. The earth tugs back at the more massive sun with its less forceful quota.
Suppose the size of the earth stayed the same while its density doubled. The extra matter would increase its overall density from S 1/2 to 11 times more than an equal volume of water. It would become twice as massive and double its gravitational attraction. Now suppose it remained the same size while reducing its density by half. Its mass and its quota of gravitation also would be reduced by half.
The strength of an object's gravitation seems to be exerted from the center of the object's mass. From this point it diminishes with distance at a set rate. As a rule, a heavenly body is a globe and its pull is exerted from a mass center at the core of the big ball. When the distance from this center is doubled, the gravitational force is reduced to one quarter. If the moon doubled its distance from the earth to 447,712 miles, it could exert only a fourth of its present pulling power on our tides.
The mutual pull between two objects is counteracted by motions that keep them from colliding. The speeding earth intends to travel a straight path out beyond the Solar System. The mighty sun tugs this wayward path towards itself. The earth's travel speed reduces this pull and avoids falling into the sun. But the sun's attraction is still strong enough to bend the wayward path into a curve around and around itself. Hence, the laws of gravitation keep the captive planet moving in a curved orbit at a set distance from the sun.
Two neighboring bodies, such as the earth and the moon, exert noticeable influences upon each other. Each exerts tidal pulls upon the other. Both bend in curved paths around a common gravitational center between them. This center is along a line connecting their two mass centers. It is closer to the earth because the planet is more massive than its satellite. This center of mutual gravitation is merely 2,900 miles from the earth's mass center, or 1,056 below surface level. But it is 236,000 miles from the moon's mass center. Around this point, the little moon moves faster than its more massive partner and it also bends in a closer orbital curve.