A ball kept in a
we know that, Centre of Mass of a body changes only when an non zero external force is applied. Now, if we consider the ball to be system, there is external normal forces from wall of the box, so conservation can’t be applied. In the box also, the ball gives collision impulses, so here also external force is present. In combined system of box and the ball, there is no external force present, so here the velocity of centre of mass must be constant.
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