The resultant force in the vertical direction of the system is not zero, and the magnitude of the supporting force n cannot be directly known, so whether the resultant force in the vertical direction is zero can be analyzed from the perspective of motion.

Obviously, the momentum of the initial state of the system in the vertical direction is zero, but at the moment when the shell is launched, the shell has a velocity component in the vertical direction, the magnitude of which is usinα, while the speed of the gun car in the vertical direction is still zero, so the momentum of the system in the vertical direction is musinα at the moment when the shell is launched. It can be seen that the momentum change in the vertical direction is: musinα during the interaction between the shell and the cannon car. So the resultant force of the system in the vertical direction is not zero. That is to say, the supporting force is greater than gravity.