# Homework Help: A rim unwinding from a string due to gravity.

1. Nov 15, 2011

### Eats Dirt

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

A string is wrapped around a small hoop several times. the radius is 8cm and mass 0.18 kg. the string is released from rest after the hoop has descended 75 cm what is the angular speed of the rotating hoop and the speed at its centre?
2. Relevant equations

mgh 1/2 Iw^2

3. The attempt at a solution

MGH energy is conserved so mgh=1/2Iw^2
i sub in and solve except this does'nt give the right answer.

2. Nov 16, 2011

### grzz

If I is the moment of inertia of hoop about its centre then,
intial energy = final energy
GPE = KE of translation + KE of rotation

3. Nov 16, 2011

### grzz

mgh = (1/2)mv$^{2}$ + (1/2)I$\omega$$^{2}$