Math Is Fun Forum

markosheehan

Member

Registered: 2016-06-15

Posts: 51

moment of inertia

Find the moment of inertia of a disc of mass m and radius r with a point mass of m on its circumference. i cant work this out can someone help me. i can work out that the moment of inertia of the disc is 1/2mr^2 but i dont know what to do with the point mass. i think it could have to do with some thing with the parallel axes theorem. the answer at the back of the book is 11/2 ml^2

thickhead

Member

Registered: 2016-04-16

Posts: 1,086

Re: moment of inertia

Where does l come into picture? Did you look at the answer of another problem? M.I. of point mas at radius r is mr^2. Add it to that of disc and you get 3/2 m r^2.

Last edited by thickhead (2016-08-30 15:34:17)

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{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}

thickhead

Member

Registered: 2016-04-16

Posts: 1,086

Re: moment of inertia

It is a nice idea to use parallel axis theorem for the point mass.
I_o=I_g+mr^2  For a point mass I_g=0. So I_o=mr^2.

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{1}Vasudhaiva Kutumakam.{The whole Universe is a family.}
(2)Yatra naaryasthu poojyanthe Ramanthe tatra Devataha
{Gods rejoice at those places where ladies are respected.}