Mass of bullet, m=100 g=\\dfrac {100}{1000}kg=\\dfrac {1}{10}kg Velocity of bullet, v=100 ms^{-1} Mass of gun, M=20 kg Let recoil velocity of gun =V Before firing, the system (gun+bullet) is at rest, therefore, initial momentum of the system =0 Final momentum of the system = momentum of bullet + momentum of gun =mv+MV=\\dfrac {1}{10}\ imes 100+20 V=10+20V Applying law of conservation of momentum, Final momentum = Initial momentumi.e., 10+20 V=0 20 V=-10 or V=-\\dfrac {10}{20}=-0.5 ms^{-1} .Negative sign shows that the direction of recoil velocity of the gun is opposite to the direction of the velocity of the bullet.