A body may be considered to be made up of an indefinitely large number of particles, each of which is attracted towards the centre of the Earth by the force of gravity. These forces constitute a system of like parallel forces. The resultant of these parallel forces known as the weight of the body always acts through a point, which is fixed relative to the body, whatever be the position of the body. This fixed point is called the centre of gravity of the body.
The Centre of gravity is a theoretical point in the body where the body’s total weight is thought to be concentrated. It is important to know the centre of gravity because it predicts the behavior of a moving body when acted on by gravity. It is also useful in designing static structures such as buildings and bridges.
In a uniform gravitational field, the centre of gravity is identical to the centre of mass. Yet, the two points do not always coincide. For the Moon, the centre of mass is very close to its geometric centre. However, its centre of gravity is slightly towards the Earth due to the stronger gravitational force on the Moon’s near side.
The centre of gravity of a body is the point at which the resultant of the weights of all the particles of the body acts, whatever may be the orientation or position of the body provided that its size and shape remain unaltered.
In the Fig., W1,W2,W3….. are the weights of the first, second, third, … particles in the body respectively. If W is the resultant weight of all the particles then the point at which W acts is known as the centre of gravity. The total weight of the body may be supposed to act at its centre of gravity. Since the weights of the particles
constituting a body are practically proportional to their masses when the body is outside the Earth and near its surface, the centre of mass of a body practically coincides with its centre of gravity.
In a symmetrically shaped object formed of homogenous material, the centre of gravity may match the body’s geometric centre. However, an asymmetrical object composed of various materials with different masses is likely to have its centre of gravity located at some distance away from its geometric centre. In hollow bodies or irregularly shaped bodies, the centre of gravity lies at a point external to the physical material.