A rifle of mass M is initially at rest but free to recoil.It fires a bullet of mass m and velocity v (relative to the ground).After firing, the velocity of the rifle (relative to the ground)is:
A)-mv
B)-Mv/m
C)-mv/M
D)-v
E)mv/M

Question

Quizplus · Accepted Answer

According to the law of conservation of momentum, the total momentum before firing is zero since the rifle is at rest. After firing, the total momentum is mv (momentum of the bullet) and -MV (momentum of the rifle, since it moves in the opposite direction). Thus, MV = mv, and the velocity of the rifle is -mv/M, which is equivalent to -(1/M)(mv), which gives us choice C.

A Rifle of Mass M Is Initially at Rest but Free