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Gravity is one
of the fundamental forces in the Universe. Although gravity is the
weakest of all the forces in the Universe, it is the most important force
in the study of astronomy. Originaly defined by Newton, and refined
by Einstein, gravity is essentially the natural force of attraction between
any two objects. Two factors determine the magnitude of the gravitational
force between two objects: (1) their masses and (2) the separation distance
between them. The size of the force is proportional to the product
of the masses of the two objects. For example, the force doubles
in size if either of the masses is increased to twice its mass. On
the other hand, the force grows weaker if the two objects are moved farther
apart. In fact, it is inversely proportional to the square of the distance
between them, so that if the distance is tripled, the force is only one
ninth as strong.
The gravitational force of the sun, acting on the earth, keeps the earth in its orbit, preventing it from traveling away into interstellar space. The gravitational force of the earth, acting on us, holds us to the earth's surface. The gravitational attraction between a person and the earth is proportional the person's mass and inversely proportional to the square of the planet's radius (distance from the person to the center). This number for gravitational attraction is called your weight! Every planet has mass and so every planet exerts a gravitational force on nearby objects. We say that planets have gravity. However, what we really mean is that there is a gravitational force of attraction between the planet and a person standing on the planet's surface. This force depends on the visitor's mass, the planet's mass, and the planet's radius. Accordingly, people have different weights on different planets. For example, a person on the
moon weighs only about 1/6 as much as on earth. The moon's radius
is 25% earth's radius and the moon's mass is 8% of earth's mass.
So, if a student weighs 150 lbs on earth, she would weight only (1/6) *
150 lbs = 25 lbs on the moon.
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