It is useful in certain calculations to consider an imaginary point in a body of matter where, the total mass or weight of the body seems to concentrate at. The imaginary point is termed as the centre of mass.
Definition: The centre of mass of a body of matter is an imaginary point at which the entire mass of the body seems to act.
The centre of gravity of a body of matter is an imaginary point at which the entire weight of the body seems to act.
- When a body is in an uniform gravitational field, its centre of gravity is also its centre of mass.
- The location of a body’s centre of gravity may coincide with the geometrical centre of the body, especially in a symmetrically shaped body composed of homogenous material.
- For hollow bodies or irregularly shaped objects, the centre of gravity (or centre of mass) may occur in space at a point external to the physical material (the centre of gravity can lie OUTSIDE the object!) – for example, in the centre of a tennis ball or between the legs of a chair.
Note: Although the term centre of gravity is widely used, the same imaginary point in a body may also be called the centre of mass, since weight and mass are proportional in an uniform gravitational field. Because the centre of mass does not require a gravitational field, many physicists prefer the term centre of mass to centre of gravity.
Example: The centre of mass of an uniform rod is the centre of the rod.
Centre of gravity of common shapes:
Is the centre of gravity of an object the same whether it is placed near the surface of the Earth or near the surface of the Moon?
Yes! The centre of gravity of an object does not depend on the strength of the gravitational force on the object. The position of the centre of gravity is only dependent on the distribution of mass within the object.