Given, mass of bullet, m=50 g=0.05 kg
Speed of the bullet, u=200 m/s
Mass of the gun, M=2 kg
Since for (gun + bullet) the system, the external force is zero.
Hence by momentum conservation
Initial momentum = Final momentum
0=mu+Mv
0=50×10−3×200+2v
v=−5 m/s
where the −ve sign indicates that the velocity of gun is opposite to that of the bullet.