PHYSICS: variation in value of 'g'

Asked by 24-02-1994 | 5th Jun, 2009, 09:30: PM

Expert Answer:

Apparent gravity is weaker at lower latitudes (nearer the equator), for two reasons. The first is that the surface of the Earth is rotating (i.e. not an inertial frame of reference): some of the gravitational force, therefore, provides centripetal force, keeping objects on the surface (and indeed the surface itself) moving in a circular motion. Consider that if the gravity of the Earth were to shut off, objects would fly off into space in the direction of their motion in accordance with Newton's First Law of Motion. Alternatively, if Earth's gravity were weakened so as to provide only the centripetal force (at, say, the equator where rotational velocity is largest) then objects there would appear to float. At the poles, of course, there is effectively no rotation and so this weakened gravity would contribute only to weight and objects would not float. In this sense, local gravity (gravity at a particular point on the surface of the Earth) felt as weight is gravity due to the Earth's mass minus the centripetal force. A scale or plumb bob measures only this effective gravity. Because rotation velocity (and, thus, centripetal force) decreases as one moves towards the poles local gravity, g, increases from 9.789 m·s−2 at the equator to 9.832 m·s−2 at the poles.

The second major cause for the difference in gravity at different latitudes is that the Earth's equatorial bulge (itself also caused by centrifugal force) causes objects at the equator to be farther from the planet's centre than objects at the poles. Because the force due to gravitational attraction between two bodies (the Earth and the object being weighed) varies inversely with the square of the distance between them, objects at the equator experience a weaker gravitational pull than objects at the poles.

Answered by  | 7th Jun, 2009, 09:54: AM

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