Question

define Centre of mass nd give its expression​

Answer 1

Centre of Mass. ... At the centre of mass, the weighted mass gives a sum equal to zero. It is the point where any uniform force applied on the object acts. In other words, a particle's centre of mass is the point where Newton's law of motions applies perfectly.

Answer 2

Centre of mass of a system is defined as the point where the whole mass of the system is supposed to be concentrated.

Suppose there are two particles of mass m1 and m2 and their distance from origin x1 and x2. Them centre of mass of system is expressed as:-

$centre \: of \: mass = \frac{m1x1 + m2x2}{m1 + m2}$

Hope it helps.