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The weight of a body on the earth is defined as "the force with which it is attracted by the earth". The weight (W) of a body of mass 'm' on the surface of the earth is given by the law of gravitation, i.e.


where, M is the mass of the earth and R its radius. We know that the value of acceleration due to gravity is

Thus, the weight of a body on the earth is also given by
W = mg               ------------(1.3)


To keep a body from falling, one has to exert on it an upward force equal to its weight W so as to make the total force equal to zero. The body remains at rest because the two forces act on it - the downward pull of its weight and the upward pull of the string-add up to zero. If the pull of the string is removed the body would be accelerated downwards under the action of a single force, mg. The fact that a freely falling body moves with a constant acceleration, g, and independent of its speed implies that this downward force is always present whether the body is moving or not.


Mass and weight are different quantities. Mass is the quantity of matter contained in a body. By definition, the mass of a given object is the same anywhere in the universe.
Weight is the force with which a body is attracted by the earth. The weight W of a body and its mass m are related as W = mg

where, g is the acceleration due to gravity and is given by g = GM/R2.

Now g is inversely proportional to the square of the distance. Hence, g is less on a mountain than at sea level, though only slightly smaller. Thus, weight changes from place to place.

Mass and Weight

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