Answer:Mass of bullet, m=20g= 0.02kg
Velocity of bullet, v=100m/s
Mass of gun, M=5kg
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=
=2+5v
we know Final momentum = Initial momentum
i.e., 2+5v=0
5V=−2
v=-0.4m/s
Negative sign shows that the direction of recoil velocity of the gun is opposite to the direction of the velocity of the bullet.