The force acting on a car in vertical direction cancel out(the gravitational force and the normal force) such that it doesn't move in vertical direction. If the car were to move to the right on a road, we are supposed to provide a force to the car by imagining that we can use our hands to push the car from the back. However, I find it odd to think of how can the engine provide the force to the car since the engine is in the system. The force provided by pushing using hands from the back are from external agent. It's confusing to think of how can the internal system-the engine supplies force to itself and make itself moves. Just like an astronaut floating in the space is unable to move unless someone else push/pull him. Thank you.