Easy physics?

Question

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/M

b. -Mv/m

c. -mv

d. -mv/M

e. -v

a is not correct so...

Answer 1

Use conservation of momentum:

pi = pf    (initial momentum = final momentum)

Initial momentum is zero since everything's stationary.

0 = mv + MV   (V is the recoil velocity of the rifle)

V = - mv / M

so the answer's (d).

Answer 2

the answer is d it is moving in the opposite direction of m and v

your on the right track