Centre Of Gravity (O Level)

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:

Self-Test Question

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?