Center of gravity
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In physics, the centre of gravity of an object is the average location of its weight in a particular environment (and relative to a particular reference point). In a (hypothetical) uniform gravitational field, it coincides with the object's center of mass. For this reason (i.e. that in everyday experiece the Earth's gravitational field may be considered uniform) the centre of gravity (CG) of an object is a point at which the object's mass can be assumed, for many purposes, to be concentrated.
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Comparison with centre of mass
Unlike the centre of mass, the center of gravity of an object can vary, depending on the object's environment.
The center of gravity of a body is not a point such that the gravitational field due to that body is equal to the gravitational field if all mass were concentrated there. Such a point usually does not exist. For example for two equal spherical bodies the center of gravity of the system is forced by symmetry, and lies midway between the centers; but gravity due to the system is not very large near that point.
To illustrate this, consider two identical large masses (or indeed, small ones), connected by a light rigid bar several hundred miles long (like a large set of dumbells). Its centre of mass will be exactly half-way along the connecting bar in all circumstances.
It is suspended high above the surface of a "flat" (i.e. perfectly spherical) planet by a string attached to a loop on the side of the middle of the connecting bar, such that it is level. The balance point, the centre of gravity, will be in line with the string, at the same position as the centre of mass.
Now place a large mountain directly underneath one end, such that it is very close to one of the large masses. Because of the greater mass beneath it, and in accordance with the inverse square law of gravitation, the mass above the mountain will experience a greater downward pull - i.e. its "weight" will increase. In order to make it balance, the position of the string on the bar will have to be shifted towards the end above the mountain. In other words, the centre of gravity of the object in that environment is different from its centre of mass.
Differences between center of mass, center of inertia and center of gravity
There is no factual difference between center of mass (note: not center of gravity) and center of inertia. For most "everyday" purposes, i.e. those where center of gravity and center of mass may be considered equivalent, center of gravity can thus be considered to be the same as the center of inertia.
References
See also
- The barycenter is the center of mass of two objects.
- Center of mass
- centroidde:Schwerpunkt
es:Centro de gravedad ja:重心 pl:Środek ciężkości pt:Centro de gravidade sl:Težišče tr:Ağırlık merkezi