The linear momentum of a system remains conserved if no external force is applied on it. If a system is at rest (or in uniform motion) initially, then its centre of mass remains at rest (or in uniform motion) if linear momentum of the system is conserved.
You need a light weight plywood sheet, round pencils, toy car (which runs with spring or battery).
Arrange the pencils parallel to each other and place the plywood sheet on these `rollers'. Place the car at one end of the platform. Switch it on and let it run across the platform.
The car runs forward and at the same time, the platform moves back.
Observe carefully, the rolling wheel push the platform back at the point of contact. Since the platform is light weight, the small force applied by the wheels can make it move. Observe the location of centre of mass of the system (platform+car) as the car moves from one end to the other end. What is the role of friction between the floor and the pencils?
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