A machine gun has a mass of 20kg. It fires 30g bullets at the rate of 400 bullets per second with a speed of 400 m/s. What force must be applied to the gun to keep it in position ?

 In the given problem, 

 m _g =20kg ; v _B =400m/s 

 mass of the bullet = 30g = 30×10 ^-3 kg 

 no. of bullets fired/sec = 400 

 For t = 1sec. 

 Total mass of bullets in 1second , = (400×30)/1000=12 kg