The correct option is A -0.72
Mass of the bullet, m1=20 g=20×10−3 kg
Mass of the rifle, m2=5 kg
Velocity of the bullet, v1=180 m s−1
If the recoil velocity of the rifle is given by v2, then according to the law of conservation of momentum, the total momentum of a system is always conserved, i.e. the momentum of the bullet = the momentum of the rifle
v2 is taken negative, because the rifle moves in a direction opposite to that of the bullet.
⟹v2=−0.72 m s−1
The recoil velocity of the rifle is −0.72 m s−1