# Homework Help: How far must the mass fall to give a rotational rate of 3 rev/s?

1. Dec 4, 2011

### skysunsand

1. The problem statement, all variables and given/known data
A whee and axle having a total moment of inertia of .0002 kg*m^2 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 2 cm. Starting from rest, how far must the mass fall to give the wheel a rotational rate of 3.0 rev/s?

I changed all the values here to 800g -> 8 kg
2 cm -> .02 m
3 rev/s -> 6pi rad/s

2. Relevant equations

ωi = 0, ωf= 6pi rad/s

Downward force on the object would be mgh- Ft, but the problem doesn't mention tension at all so I don't know about using that equation.

I can figure out v, if I use v=rw, but I don't know if that's even useful to this problem.

KE total = 1/2 mv^2 + 1/2Iw^2 is likely going to be important, but I have no clue how it would relate to figuring out the height this thing has to fall.

3. The attempt at a solution

^ Above is the thought process of the equations. Otherwise, I haven't got a clue.