Momentum

Let be the mass of the disc (0.2 kg), be the mass of each bullet (0.05 kg), and be the number of bullets fired per second (10 bullets/s). Let be the speed of each bullet. Since the bullets rebound with the same speed, the change in momentum of each bullet is . The force exerted by the bullets on the disc is equal to the rate of change of momentum: . For the disc to float, this force must balance the weight of the disc: m/s.