Topic: Collisions
Concepts: 1D inelastic collision

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Solution:

The initial momentum of the system is

Since the collisions are completely inelastic, all the masses stick together so the final momentum is

Conserving linear momentum $p_i=p_f$,

so the answer is A.

Topic: Collisions
Concepts: 1D elastic collision

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Solution:

Recall that in an elastic collision between mass $m$ moving at velocity $v_0$ and mass $M$ at rest, the velocity $v_2$ of $M$ after the collision is given by

In our case, we have after $m$ collides with $3m$,

After $3m$ collides with $9m$, we have

Thus, the answer is B.