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The centre of gravity of a body on the e...

The centre of gravity of a body on the earth coincides with its centre of mass for a small object whereas for an extended object it may not. What is the qualitative meaning of small and extended in this regard? For which of the following two coincides? A building, a pond, a lake, a mountain?

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If the vertical height of the object is very small as compared to the earth.s radius, it is called obejct small, else it is extended.
(1) Building and pond are small objects.
(2) A deep lake and a mountain are examples of extended objects.
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