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Encyclopedia > 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. Image File history File links Stop_hand. ... Image File history File links Stop_hand. ... Wikipedia does not have an article with this exact name. ... It has been suggested that this article or section be merged with Center of gravity. ... A Superconductor demonstrating the Meissner Effect. ... In the physical sciences, weight by Definition VIII, per Newtons Mathematical Principles of Natural Philosophy or Principia, is an upward force exerted on matter to deny the body from entering freefall as a result of gravity, a centripetal accleration field. ... It has been suggested that this article or section be merged with Center of gravity. ... Mass is a property of a physical object that quantifies the amount of matter it contains. ...

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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 objects environment.


Note that 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.


Thought experiment showing CG different from CM

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 and center of inertia and center of gravity

There is no factual difference between centre of mass (note: not centre of gravity) and centre of inertia. For most "everyday" purposes, i.e. those where centre of gravity and centre of mass may be considered equivalent, centre of gravity can thus be considered to be the same as the centre of inertia.


References

  • Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.), Brooks/Cole. ISBN 0534408427.
  • Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.), W. H. Freeman. ISBN 0716708094.

See also


  Results from FactBites:
 
Where Is Your Center Of Gravity? (551 words)
In a regular geometric solid with the uniform density, the center of gravity is the object's geometric center.
The center of gravity of the board is its geometrical center because it is a uniform rectangular solid.
The position of the center of gravity of the person is what we are trying to determine and will be defined by X, the distance from A, the axis we have chosen.
Center of gravity - Definition, explanation (727 words)
Most astronomical objects are radially symmetric, causing both the center of gravity and the center of mass to coincide at the center of the sphere.
In a radially uniform gravitational field (such as the one formed by a typical star), the center of mass and the center of gravity of a radially symmetric object (such as planets and stars) are both at the center of the sphere.
In locations where earth's gravity dominates, the center of gravity of a vertical, long, uniform beam is closer to the earth than the center of the beam (although it is still inside the beam).
  More results at FactBites »

 
 

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