Mass needed to rotate wheel and axle (rotational motion)

[MinaHany]

I'm sorry for posting another thread but I'm totally lost here..

Homework Statement

A wheel and axle having a totel moment of inertia of 0.002kg^m is caused to rotate about a horizontal axis by means of an 800g mass attached to a cord wrapped around the axle.
The radius of the axle is 2cm. Starting from rest, how far must the mass fall to give the wheel a rotational rate of 3rev/s?

 

[Sethric]

Use energy. Spinning the wheel and axle combo will impart a kinetic energy. The falling mass will have kinetic energy. The state of the system before the mass falls relies on its (the mass) potential energy. Since the mass is attached to the axle, you know that the kinetic energy of the spinning axle and the falling mass are somehow tied together. How so? From there, conservation of energy is your answer. You know the rotational speed of the axle in the final state.

 

[MinaHany]

Thank you sethric..
I wrote that final KE = inital KE
rotational KE + KE of the mass = PE of the mass

and v=wr to get the v of the mass when the w is 3 rev/s

Best thing around here is nobody gives the answer.. everybody gives you the ideas you need to understand to be able to solve the problem.

I understand it now.. Thanks again.