The quantity of matter contained in a body is known as its mass. It remains unchanged no matter at which place the body is carried. Two bodies of the same mass are attracted by the earth with the force of the same magnitude but if they are of different masses then the force exerted by the earth on each body will be of different magnitude. Thus, if it is required to lift two bodies of different masses, the downward pulls experienced by a man will be different. The force (gravitational force) with which a body is attracted by the earth determines its weight. The weight of a body is, therefore, the force with which it is attracted towards the centre of the earth. It depends on two factors-
mass of the body and
the acceleration due to gravity (g), i.e. acceleration on a body due to earth’s attraction.
The weight (W) of a body of mass (m), according to the second law of motion, is given by the relation; W = mg.
Knowing the value of ‘g’, which varies from place to place on the earth, we can obtain ‘W’ from the known value of ‘m’. The value of ‘g’ on the surface of the earth is approximately taken as 9.8 m/sec2. The weight of a body of mass 1 kg, referred to as 1 kilogram force or 1 kilogram weight, is therefore,
W = (1 kg) (9.8 m/sec2) = 9.8 newton i.e. 9.8 N
Since the attraction of the earth on a body depends on the distance of the body from the centre of the earth, the weight of the body varies from place to place on the earth. It is more at the sea level than at the top of the mountain, though the difference is much less due to the little difference in height.
Distinction between Mass and Weight:
The following points make a distinction between mass and weight.
Mass of a body is the quantity of matter contained in it.
Weight of a body is the force with which it is attracted by the earth towards its centre.
It is constant and does not vary with the change of the distance between the body and the centre of the earth.
It varies from place to place depending on the change in the distance of the body from the centre of the earth.
There is no place in the universe where a body does not have its mass.
A body has no weight at the centre of the earth and at a place somewhere between the sun and the earth, where the force of attraction exerted by the sun is equal to that exerted by the earth.