In the case of this animation, where both balls have the same mass, the center of mass is always at the midpoint between the two masses. The momentum of the system was zero before the green block move...In the case of this animation, where both balls have the same mass, the center of mass is always at the midpoint between the two masses. The momentum of the system was zero before the green block moved, is zero when the blocks move, and is again zero when the blocks are stationary. Note that the center of mass of the system does not move relative to the ground but does move to the left relative to the right edge of the red block as the red block moves to the right.
