# Moment of inertia of a cone

1. ### ehilge

162
1. The problem statement, all variables and given/known data

Calculate the moment of inertia of a uniform solid cone about an axis through its center. The cone has mass M and altitude h. The radius of its circular base is R. (see attached photo)

2. Relevant equations
I know I need to somehow use the equation I=$$\intr2dm$$
also, I have an equation from my proffessor, dm=$$\rho$$dv I'm not sure if I need this though since its unifrom density so it doesn't seem like $$\rho$$ should matter.

3. The attempt at a solution
I don't have a solution right now. I know the answer is 3/10 MR2 but I don't know how to het there. From class, since we did some examples, I think I need an equation that has a triple integral in it but I don't know what to integrate and to where.

Thanks for your help. Also, if you have any general suggestions on how to complete moment of inertia problems like this that would be great. I know we're gonna be doing a lot of them.

Thanks again!!!

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2. ### alphysicist

2,246
Hi ehilge,

To put this in tex, don't use the [noparse][/noparse]. Use the caret ^, and put a space before the r, like this:

[noparse]$$\int r^2 dm$$[/noparse]

which gives:

$$\int r^2 dm$$

Yes, you'll need some form of the density. The integral has a dm in it, and you need to use the density function to change that to a dx, dA, dV, etc. (depending on the type of shape) so that you can perform the integration.

If you look here

http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html#mig

and scroll to near the bottom you will see links to three examples. Does that help?

If you get stuck on the cone calculation, post your work and where you are getting stuck and maybe someone can help.

3. ### ehilge

162
I was able to get the problem figured out. Thanks for your suggestions and also with the help with formatting.