Cosider a gravity - free hall in which a tray of mass M , carrying a cubical block of ice of mass m and edge L , is at rest in the middle. If the ice melts, by what distance does the centre of mass of "the tray plus the ice" system descent ?
zero
Xcm = m1X1+m2X2m1+m2 = 0
But as there is no gravity , the shape of ice even when it melts completely, will remain the same.
Therefore , COM will not move.
This can be concluded that in absemce of net external force on any system its COM does not change its sate with time.