# Moment of Inertia

dross
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Hi I am struggling with the following question.

A cylinder of radius 20cm is mounted on a horizontal axle coincident with its axis and is free to rotate. A light chord is wound onto it and a 50g mass hung from it. After release the mass drops 1m in 12seconds. What is the moment of inertia?

With energy conversation I know the gravitational potential energy lost must equal that of the kinetic energy gained my the mass + the rotational energy gained by the cylinder

0.5mv^2+0.5Iω^2 = mgh

I can rearrange this to find moment of Inertia, I. However is ω = v/r ? with v being 1/12 m/s ?

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Merlin3189
Homework Helper
Gold Member
v is not 1/12 m/s. The average speed is 1/12 m/s, but since it starts from rest, the final speed must be higher.
Otherwise you seem on the right track.

dross
Yep got it. Thanks for your help!