Calculating Angular Speed of a Yo-Yo

[cecico]

Hi~
This is the question i got
A yo-yo has radius 8.00cm and mass 0.180kg with small hoop.
The yo-yo is released at rest and after yo-yo
descended 75.0cm calculater the angular speed of the rotating yo-yo
and the speed of its center.
I tried to use K=1/2Mv^2+1/2IW^2 but i got weird number...
I have the answer but I need t know how to solve it...
Just in case the answer for angular speed is 33.9rad/s and
the speed of its center is 2.71m/s.
I'm seriously fall into the deep~~~~ocean.

 

[hage567]

Don't forget about gravitational potential energy...

 

[Dick]

The given answer is right. What did you do wrong? You aren't giving us near enough information to tell. What did you use for I?

 

[cecico]

I use MR^2 for I...

 

[MaGG]

Wouldn't I be

<br /> I = \frac{m r^2}{2}<br />

which is the one for a solid disk? Or are we considering the fact that the yo-yo is made up of two disks?

 

[Dick]

Wouldn't I be

<br /> I = \frac{m r^2}{2}<br />

which is the one for a solid disk? Or are we considering the fact that the yo-yo is made up of two disks?

The clue refers to a hoop, so I think the do want you to use mr^2. You get the correct answer that way. But we still don't know enough to tell where cecico went wrong.