Two possible situations:
- The car is at rest. In this case, there are only 2 forces acting on the car, both in the vertical direction: the weigth of the car (downward) and the normal reaction of the asphalt on the car (upward). The two forces are equal in magnitude but in opposite directions, so the net force is zero.
- The car is traveling at constant velocity. In this case, there are 4 forces: in the vertical direction, as before, we have the weigth of the car (downward) and the normal reaction of the asphalt on the car (upward). The two forces are equal in magnitude but in opposite directions, so the net force in the vertical direction is zero. However, this time the car is also moving, so there are forces acting along the horizontal direction: the driving force, which pushes the car forward, and the frictional force, which goes in the opposite direction. Since the car is moving at constant velocity, its acceleration is zero, so the net force must be zero: therefore, the magnitude of these two forces is equal.
