**1. The problem statement, all variables and given/known data**

A ball is attached to a rod at one end. The rod rotates about a point at the other end.

A second ball hits the first ball with a horizontal velocity u, and then the two balls travel together with the same horizontal velocity v. The collision is inelastic.

What is conserved in the system after the collision?

The mechanical energy? Horizontal momentum? Angular momentum about the CM? Or angular momentum about the rod’s axis?

**3. The attempt at a solution**

I do not believe the mechanical energy is conserved. Since this is an inelastic collision the kinetic energy after the collision is less than before, thus the mechanical energy is not conserved.

I’m not sure if the momentum is conserved. In my book it says the momentum is conserved if the external forces can be neglected but I’m not sure if the second ball is an external force or not?

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