N bullets each of mass m kg are fired with a velocity v ms-1 at the rate of n bullets per second, upon a wall. The reaction offered by the wall to the bullets is given by

The reaction force offered by the wall to the bullets = the force exerted by bullets on the wall (third law of motion) = the rate of change of momentum of bullets (second law of motion). Now, total mass of N bullets = Nm. Momentum of N bullets = Nmv. Time  t=NnSo, the change of momentum per second = NmvNn=nmv