GRAVITY, CENTRE OF

in physics, imaginary point in a body of matter where, for convenience in certain calculations, the total weight of the body may be thought to be concentrated. The concept is sometimes useful in designing static structures (e.g., buildings and bridges) and in predicting the behaviour of a body when it is acted on by external accelerating forces. Although the term centre of gravity is widely used, the same imaginary location in a body may also be called the centre of mass, since weight and mass are proportional. Because the centre of mass does not require a gravitational field, many physicists prefer the term centre of mass to centre of gravity. When a body is in a gravitational field, its centre of mass and centre of gravity share a common location. An exception is a pair of large cosmic bodies that exert gravitational force on each other as each orbits around the other. In binary star systems, for example, the stars' mutual attraction may cause a separation of the centre of mass from the centre of gravity in each of the bodies. The location of a body's centre of gravity may coincide with the geometrical centre of the body, especially in a symmetrically shaped object composed of homogenous material. An asymmetrical object composed of a variety of materials with different masses, however, is likely to have a centre of gravity located at some distance from its geometrical centre. In some cases, such as 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-e.g., in the centre of a tennis ball or between the legs of a chair. Centre of gravity Published tables and handbooks list the centres of gravity for most common geometrical shapes. For a triangular metal plate such as that depicted in the Figure, the calculation would involve a summation of the moments of the weights of all the particles that make up the metal plate about point A. By equating this sum to the plate's weight W, multiplied by the unknown distance from the centre of gravity G to AC, the position of G relative to AC can be determined. The summation of the moments can be obtained easily and precisely by the integral calculus. The point G can be located by suspending the plate by a cord attached at point A, and then by a cord attached at C. When suspended from A, the line AD is vertical; when suspended from C, the line CE is vertical. The centre of gravity is at the intersection of AD and CE. When an object is suspended from a single point, its centre of gravity lies directly beneath that point.