Motion of a particle in a rotating tube.

1. The problem statement, all variables and given/known data
Hello everyone,
How do I go about this problem; Describe the motion of the particle, m, inside the rotating tube. (See attachment please.)
At time [itex]t=0, r(t=0)=b[/itex] and [itex]\dot{r}(t=0)=v_{0}[/itex]

2. Relevant equations
I think it would be practical to use polar coordinates to describe the system.
Then maybe find the kinetic energy and the potential, to find the Lagrangian?

3. The attempt at a solution
I don’t really know where to start, any hints or tips are really appreciated.
In class we just started with holonomic constraints, therefore I’m a little new to this concept. I guess it has something to do with this problem?

Attached Images
File Type: png 123.png (31.6 KB)

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