# Moment of Inertia, lab

Ok i need on a lab related to calculating the moment of inertia of a disk and a disk + ring.

The lab looks similar to this one i found on the web

http://qbx6.ltu.edu/natsci/physics/labs/t1lab08.shtml

We did it two seperate ways, first we has a solid disk and did 3 trials with 3 different weights attached to the pulley system. Then we LEFT the disk on and simply put a thin hoop on top, doing 3 trials again with the same 3 weights.

I have the correct equation for the measured value, but my theory value is not coming out very close.

trial weights = 0.05kg,0.10,0.15
The mass of the disk = 1.4kg
the mass of the ring = 1.43kg
The radius of the ring = .0575m
radius of the disk = 0.1075m

I was thinking Itotal = I(ring) + I(disk)

though my values don't seem correct, also wouldn't the weight on the string/pulley system have to factor in somehow?

#### topsquark

Forum Staff
Ok i need on a lab related to calculating the moment of inertia of a disk and a disk + ring.

The lab looks similar to this one i found on the web

http://qbx6.ltu.edu/natsci/physics/labs/t1lab08.shtml

We did it two seperate ways, first we has a solid disk and did 3 trials with 3 different weights attached to the pulley system. Then we LEFT the disk on and simply put a thin hoop on top, doing 3 trials again with the same 3 weights.

I have the correct equation for the measured value, but my theory value is not coming out very close.

trial weights = 0.05kg,0.10,0.15
The mass of the disk = 1.4kg
the mass of the ring = 1.43kg
The radius of the ring = .0575m
radius of the disk = 0.1075m

I was thinking Itotal = I(ring) + I(disk)

though my values don't seem correct, also wouldn't the weight on the string/pulley system have to factor in somehow?
Sorry, I managed to miss the post earlier somehow.

Yes, the moment of inertia of the ring + disk will be the sum of the moments of inertia of the separate pieces in this case. The different weights produce different angular accelerations for the system. I suppose you are to calculate I for each weight and then average them.

As to how well the data come out I couldn't say until you give me the details of the experiment and the data.

-Dan