what is means by centre of mass considerd to be whole mass of object concentrated?
The centre of mass is the point where the entire mass of the system could be considered to be concentrated.
For homogenous solid bodies that are symmetrical, the centre of mass is concentrated at the centre of body's symmetry, that is, its geometrical centre.
The centre of mass is the point about which a solid will freely rotate.
A solid body could be suspended from its centre of mass and it would not become unbalanced.
The centre of mass of a solid body need not always lie within the body. The center of mass of a ring is at its centre where no matter of ring is present.
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