# Rotational Physics

1. Aug 6, 2011

### scifan

So the question is this:

A disc is released from rest. A block is causing it to rotate. After a time t the block has fallen a height h and the disc has rotated through an angle theta. (in rad)
Express h in terms of r (the radius of the part of the hub around which the string is wound) and theta.

I came up with this answer:

PE lost = KE gained
mgh = (1/2)mv^2 + (1/2)I(omega)^2
mgh = (1/2)m(r*omega)^2 + (1/2)*(mr^2)omega^2
gh = (1/2)r^2*omega^2 + (1/2)omega^2*r^2
gh = omega^2*r^2
omega = d(theta)/dt
subs for omega and solve for h: h = (r^2*theta^2)/(t^2*g)

The question asks me to express h in terms of r and theta but my answer has g and t in it. Any help?

2. Aug 7, 2011

### tiny-tim

welcome to pf!

hi scifan! welcome to pf!

(have a theta: θ and try using the X2 icon just above the Reply box )

energy has nothing to do with it …

this is just geometry!

3. Aug 7, 2011

### scifan

Re: welcome to pf!

Would that be correct if I say h = r*theta then?

4. Aug 7, 2011

### tiny-tim

Last edited by a moderator: Apr 26, 2017