- gravity, centre of
Imaginary point where the total weight of a material body may be thought to be concentrated.Since weight and mass are proportional, the same point may also be called the centre of mass, but the centre of mass does not require a gravitational field. A body's centre of gravity may coincide with its geometric centre, especially if the body is symmetric and composed of homogeneous material. In asymmetric, unhomogeneous, or hollow objects, the centre of gravity may be at some distance from the geometric centre or even at a point in space external to the object, such as between the legs of a chair.
* * *▪ physicsin 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) or in predicting the behaviour of a moving body when it is acted on by gravity.In a uniform gravitational field the centre of gravity is identical to the centre of mass, a term preferred by physicists. The two do not always coincide, however. For example, the Moon's (Moon) centre of mass is very close to its geometric centre (it is not exact because the Moon is not a perfect uniform sphere), but its centre of gravity is slightly displaced toward the Earth because of the stronger gravitational force on the Moon's near side.The location of a body's centre of gravity may coincide with the geometric centre of the body, especially in a symmetrically shaped object composed of homogeneous 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 geometric 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.Published tables and handbooks list the centres of gravity for most common geometric 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 means of integral calculus.The centre of gravity of any body can also be determined by a simple physical procedure. When an object is suspended from any single point, its centre of gravity lies directly beneath that point.
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