Mass of the rifle, m1=4 kg
Mass of the bullet, m2=50g=0.05 kg
Recoil velocity of the rifle =v1
Bullet is fired with an initial velocity, v2=35 ms−1
Initially, the rifle is at rest.
Thus, its initial velocity, v= 0
Total initial momentum of the rifle and bullet system =(m1+m2)v=0
Total momentum of the rifle and bullet system after firing:
=m1v1+m2v2=0.05×35=4v1+1.75
According to the law of conservation of momentum:
Total momentum after the firing = Total momentum before the firing 4v1+1.75=0
v1=−1.754=−0.4375 ms−1
The negative sign indicates that the rifle recoils backwards with a velocity of 0.4375 ms−1