Question

A body of mass m moves with velocity v and on the way meets (joins) another body of mass 2m, which was at rest.

 After merging the two bodies move rapidly:

 A) v2 = v / 3

 B) v2 = v / 2

 C) v2 = v / 4

 D) v2 = v

Answer 1

According to the momentum conservation law, the momentum before the collision:

$$p_1 = mv + 2m\cdot 0$$

(since the second body did not move before the collsion) is equal to the momentum after the collision:

$$p_2 =(m + 2m)v_2$$

Thus, obtain:

$$p_1 = p_2\\ mv = (m+2m)v_2\\ v_2 = \dfrac{v}{3}$$

Answer. A.