The soapbox car kept moving on the smooth road even after Rahul stopped pushing due to the momentum it had gained. Once the car is in motion, its mass and velocity give it momentum, allowing it to continue moving forward. On a smooth road, there is less friction to slow it down, so the car can keep rolling for some distance without needing additional force. This is a demonstration of Newton's first law of motion, where an object in motion stays in motion unless acted upon by an external force like friction or a barrier.