# Homework Help: MI of rectangular and triangular lamina

1. Feb 21, 2009

### Mansi Khanna

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

can anyone please send me the link to calculate the moment of inertia of cuboid,rectangular and triangular lamina ??(along with figure)
2. Relevant equations

3. The attempt at a solution

2. Feb 21, 2009

### LowlyPion

Welcome to PF.

Just use the formula:

I = ∫ r2 dm

Taken over all the mass elements.

Since you are interested in lamina then you're only interested in two dimensional area density.

3. Feb 22, 2009

### Mansi Khanna

thank you so much for your help..
while deriving the formula for rectangular lamina,of sides 2a and 2b(i.e.,length 2a and breadth 2b),when i consider a thin horizontal strip parallel to x-axis at distance y from x-axis and infinitesimally small thickness dy,
after using the formula you have given..what limits of the integral should i take??please help..
one last thing i need to know is that in the link which you have sent,under extended explanation someone's written
WARNING: there is no such thing as moment of inertia about a point
and then the definition of moment of inertia about a point is mentioned...if there's no such thing as moment of inertia about a point then how do we define MI in 1 dimension??
like MI in 2-d is about a line and MI in 3-d is about coordinate axis..then in 1-d is it not supposed to be about a point??