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A mass m moving with speed v collides inelastically with a second identical mass that is initially at rest. After the collision the first mass moves with speed v/sqrt(3) in a direction perpendicular to its original direction of motion. Find the speed of the second mass after the collision.

1. 2v/sqrt(3)
2. v/sqrt(3)
3. v*sqrt(2/3)
4. The situation described is impossible without an external impulse

Correct answer: 2v/sqrt(3)

Solution

By momentum conservation the second mass has velocity components (v, -v/sqrt3), so its speed is sqrt(v² + v²/3) = 2v/sqrt(3).

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