The Effects of Gravity
There are some people who worry that when they're outside, if they don't keep a
good grip on the ground, they'll just go flinging off into space. They needn't really worry
about this, because gravity generally keeps that sort of thing from happening. The thing is,
no one is really sure what causes gravity, but the effects have been studied by many
physicists and astronomers. Three of the more obvious effects of gravity are things falling
down, weight, and the the moon and planets staying in their orbits.
Things fall down. People have generally grown to accept that if one lets go of
one's prized and valuable textbook when walking through a mud puddle, the book will
invariably end up in the puddle and therefore be stripped of all value and even legibility.
Things fall down because there is a strong gravitational attraction between things of great
mass, like the Earth, and things of little mass, like a book. The only problem with this
relatively simple explanation is that no one really knows why it's like that. What people
have figured out so far is that gravity is a force, and a force is anything that changes the
state of rest or motion of an object. In the absence of outside forces, the momentum of a
system remains constant. This means that if there was no gravity, when one would
relinquish one's hold on the textbook, it would remain at rest in the air. If a force acts on a
body, the body accelerates in the direction of the force. In the example of the force of
gravity, small things like textbooks are pulled downward toward the center of the large
mass of the Earth, not up into space, even if some people think that this might happen.
Isaac Newton was the first to conceive of weight as the gravitational attraction
between a body and the Earth. The force that results from the gravitational attraction of
the Earth on bodies at its surface is what we call weight. Science has chosen to measure
the mass of objects in units that are roughly equivalent to the weight of those objects on
Earth. For example, if a textbook weighs four pounds on Earth, it would have a mass of
four pounds in an orbiting spaceship. The textbook would be "weightless" because it does
not feel the gravitational attraction of the Earth, but, even in outer space, to push the
textbook from one place to another, someone would have to exert a force sufficient to
overcome the inertial mass of four pounds. If that same textbook which weighs four
pounds on Earth, was placed on the surface of the much more massive planet Jupiter, the
book would weigh 15.76 pounds, because of Jupiter's stronger gravitational attraction.
Newton was also the first to assume correctly that the same force of gravity
that causes objects to fall and to have weight, also explains the movement of astronomical
bodies. Newton stated that every particle is attracted to every other particle with a force
directly proportional to the product of their masses and inversely proportional to the
square of the distance between them. It was natural for him to turn his attention to
celestial bodies, since, on Earth, the distance between a body and Earth could not be
varied greatly, and the force of attraction between two different bodies on Earth was too
feeble to be detected by methods available at that time. The astronomer Johannes Kepler
knew that planets moved in ellipses, but he didn't know why. He sensed that planetary
bodies and the sun probably had a natural affinity for each other, and that possibly
magnetism was involved. By studying Kepler's observations, Newton derived laws that
described how the force of gravity acted. He was the first to declare that the pull of the
Earth extended to infinity. He showed that a projectile launched at sufficiently great...
Cited: Goldsmith, Donald. The Astronomers. New York: St. Martin 's Press, 1991
Newman, James R. The Harper Encyclopedia Of Science. 2nd ed. New York: Harper and Row, 1967.
Macmillan. The New York Public Library Science Desk Reference. New York: The Stonesong Press Inc. and The New York Public Library, 1995.
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