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 the two coincides? A building, a pond, a lake, a mountain?
The centre of gravity is the geometric centre whereas the centre of mass is the mass of the point where the entire mass of the body is considered. The object is said to be
small when the vertical height of the object is close to the earth’s surface. If the distance between the object and the surface of the earth is large then the object is
extended object.
Building and pond are considered to be small objects while a mountain and a lake are considered to be extended objects.