# Homework Help: Find moment of inertia

1. Apr 11, 2013

### Sneakatone

for part a I used the equation I=m*r^2 ---> 120(58/100)^2=40.36 kg*m^2

b)I know how to do this using the equation K=1/2Iw^2 but my moment of inertia is incorrect.

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2. Apr 11, 2013

### SammyS

Staff Emeritus
What is the moment of inertia of a uniform disk of mass, M, and radius, R ?

3. Apr 11, 2013

### Sneakatone

I used the equation 1/2MR^2 and got =20.18.
when I plugged everything into the KE equation I had 305.22 J but that is incorrect.

4. Apr 11, 2013

### SammyS

Staff Emeritus
What are the units for ω ?

5. Apr 11, 2013

### Sneakatone

rev/s

6. Apr 11, 2013

### haruspex

The rotation rate is given you in rev/s, but what units does it need to be in to plug it into the formula for KE?

7. Apr 11, 2013

### Sneakatone

should it be rev/min or rad/s?

8. Apr 11, 2013

### Sneakatone

I got it now it should be in rad/s for some reason I thought It was suppose to be rev/s.
Thank you all!