Shana is sitting in a rolling chair . Susan is pushing her from the front of the room to the back of the room. How can shana be moving and not moving at the same time?

The man of mass m1 and the woman of mass m2 are standing on opposite ends of the platform of mass m0 which moves with negligible friction and is initially at rest with s = 0. the man and woman begin to approach each other. derive an expression for the displacement s of the platform when the two meet in terms of the displacement x1 of the man relative to the platform.