Question

A horizontal disc rotating freely about a vertical axis makes 100 rpm. A small piece of wax of mass 10 g falls vertically on the disc and adheres to it at a distance of 9 cm from the axis if the number of revolution per minute is thereby reduced to 90. Calculate the moment of inertia of disc.

Answer 1

`I_(1)omega_(1)=I_(2)omega_(2)`
`thereforeI_(1)(100)=(I_(1)+(10)(9)^(2))(90)`
or `I_(1)+810=1.11I_(1)`
`thereforeI_(1)=7290g-cm^(2)`
`=7.29xx10^(-4)kg-m^(2)`