A van is standing on a frictionless portion of a horizontal road. To start the engine, the vehicle must be set in motion in the forward direction. How can the persons sitting inside the van do it without coming out and pushing from behind ?
The persons sitting inside the van can move the van forward to start the engine without coming out if they simultaneously move backward. In absence of an external force like friction force by the road, the center of mass of van and persons together will remain stationary. So if the persons move backward simultaneously the van will move forward to keep the center of mass at the same position and the van will start.