# Rotational inertia again!

1. Nov 3, 2005

Hi,
Please take a look at this:

Calculate the rotational inertia of a section of a right circular cylinder of radius R that subtends an angle of 'theta knot' at the origin when the reference axis is at the origin and perpendicular to the section.

I tried to draw the picture in the attachment.
I tried to use the formula I=Integral(R2*dm), with dm =density*dV
where V is the volume. I would like to know if I am in the good direction

B

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2. Nov 3, 2005

### Galileo

Lol, isn't it 'theta naught'? As in $\theta_0$?

$$I=\int_V R^2\rho dV$$
is always correct. You should go to cilindrical coordinates for this problem.

3. Nov 9, 2005

Help

Hi I am still stuck with with problem of inertia . please can someone help me.
Can you give me more suggestions
$$\int_V R^2\rho dV =\rho \int_V r^2 r dr d\phi dz$$