A force of attraction exist between every body in the universe. It is called gravity and it has been investigated by many scientists including Galileo and Newton. This gravitational force depends on the masses of the bodies involved. Normally it is very small, but when one of the bodies is a planet, like the earth, the force is considerable. Everything on or near the surface of the earth is attracted by the mass of the earth. The greater the mass, the greater is the earth’s forces of attraction on it.
We call this force of attraction gravity.
Because of the gravity, bodies have weight. We can perceive weight only when a body resists gravity. For example, when we pick up a stone there are two forces involved. One is the lifting force we exert, and the other is the force of gravity which attracts the stone downwards and thus gives it weight. When a body escapes from the influence of the earth’s gravitational pull, it can become weightless. For example, the centrifugal force of a spacecraft spinning in orbit round the earth cancles the effect of gravity. The crew therefore experience weightlessness. One of the minor disadvantages of weightlessness is that normal pens will not write, because the ink is not attracted by gravity to flow out of the pen.
If the space crew land on the surface of the moon, they experience the much weaker force of gravity exerted by the moon. On the moon they weigh less than on the earth. Special training is necessary to help them to walk on the moon’s surface.
To simplify engineering calculations, it is assumed that gravity is the same everywhere on the earth’s surface, and that for every kilogram of mass the earth exerts a force of 9.81 Newton on a body. In fact gravity differs slightly from place to place because of the shape of the earth. It is greatest at the poles where the earth is flattest and is the least at the equator. (adapted from English in mechanical engineering by Eric H. Glendinning)
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