Conservation of momentum velocity problem

[kozis]

Homework Statement are included in the attached image.

I've solved the first two parts, my question is about part c. It makes more sense for me to think that the tube has gained some velocity after the collisions and moves to one direction. Probably on the left because the velocity gained from fragment A is bigger than B. But the answer provided says that the velocity should be zero because nothing moves inside the tube and the system's momentum is zero. can someone please explain this to me? I really don't get it. As I mentioned before my thinking is that at first tube gains some velocity after fragment A collides on it and that after collision of fragment B,tube will still be moving with less velocity.

 

[Doc Al]

It makes more sense for me to think that the tube has gained some velocity after the collisions and moves to one direction. Probably on the left because the velocity gained from fragment A is bigger than B.

What counts is the momentum, not just the velocity. And the fragments have equal and opposite momenta.

 

[kozis]

Ok I see now.. Thank you for your quick reply.