or

Define centre of mass and centre of gravity? When will they be equal?

A hypothetical point at which the whole mass of a body is considered to be concentrated is known as centre of mass.

While the hypothetical point at which whole weight of a body is considered to be concentrated is known as centre of gravity.

As we know weight is g times the mass of the body and g decreases with altitude, if we ignore this variation of g  over the vertical dimension of body(I e. in uniform gravitational field), then it's centre of mass and centre of gravity will be equal.