# Moment of Inertia formula

1. Dec 13, 2006

### PSOA

Why is the end area of shell $$2\pi rdr$$?

2. Dec 13, 2006

### marlon

It's the area expressed in differential form. To acquire the correct formula, you need to calculate the integral with respect to r.

Intuitively, the differential form means this : to describe an infinitesimal small area, take the circumference 2 times pi times r and make it a bit thicker, expressed by the dr. Integrating over r means that you sum up the area of all this different, very thin, circumferences to get the total circular area.

marlon

Last edited: Dec 13, 2006
3. Dec 13, 2006

### PSOA

Oh, I got that! Thank you marlon! I am giving my first steps over integration. And congratulations for the award!!

4. Dec 13, 2006

### marlon

What award ?
I just lost my guru badge to somebody else, lol.

marlon