# How to calculate this angular velocity problem

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1. Mar 5, 2015

### karzy

A 2.3kg , 20-cm-diameter turntable rotates at 100 rpm on frictionless bearings. Two 500g blocks fall from above, hit the turntable simultaneously at opposite ends of a diagonal, and stick.

What is the turntable's angular velocity, in rpm, just after this event?

I first cacluted the angular momentum as .120428 using I*W so I then set that equal to I*W where I=.5(3.3)(.1)^2
and w is 2pi(frequency) and tried solving for frequency. and converting that back to rpm. But 69.7 rpm wasn't correct so I'm missing something or approaching this wrong!

Any suggestions?

Last edited by a moderator: Mar 5, 2015
2. Mar 5, 2015

### Staff: Mentor

What's the new moment of inertial after the blocks stick to the turntable?

Chet

3. Mar 5, 2015

### karzy

i used the formula (1/2)(M)R^2 which I used 3.3 for the mass and .1 for the R which gave me .0165.

4. Mar 5, 2015

### Staff: Mentor

This formula applies to the disk, but not to the blocks. The blocks are at the outside radius, so their contributions to the moment of inertial (per unit mass) are greater. Do you know the formula for the moment of inertial of a mass at a distance r from the center of rotation?

Chet

5. Mar 5, 2015

### karzy

I=MR^2 where m is the mass and r is distance from the center?

6. Mar 5, 2015

### karzy

Ahhh i just got it! I needed to add the three Ineritas together then solve for the velocity.Thanks for the help!