# Homework Help: Finding the inertia with a thin rectangular sheet

1. Nov 15, 2012

### kgianqu2

A) A thin, rectangular sheet of metal has mass M and sides of length a and b. Find the moment of inertia of this sheet about an axis that lies in the plane of the plate, passes through the center of the plate, and is parallel to the side with length b.
Express your answer in terms of given quantities.

attempt at answer: M(a^2+b^2)/12 (mastering physics said the correct answer does not depend on the variable b)

B) Find the moment of inertia of the plate for an axis that lies in the plane of the plate, passes through the center of the plate, and is perpendicular to the axis in part A.
Express your answer in terms of given quantities.

I did not attempt an answer on this part because of the confusion in part A.

Please help, I am very confused about how to input this into mastering physics.

2. Nov 15, 2012

### SammyS

Staff Emeritus
That's the moment of inertia about the axis through the center of mass and perpendicular to the sheet.
For part A), think of the sheet as being broken up into thin strips of length a and width Δx .

By the way, you really didn't show your work. You just gave an answer without giving the reasoning behind it.

3. Nov 16, 2012

### kgianqu2

You honestly didn't help at all. I figured it out on my own, thanks a lot. The worst help ever. The answer that I had was basically right, I just had to take out the variable b for the first part, and a for the second part. But thanks for asking where I got the equation from.

4. Nov 16, 2012

### SammyS

Staff Emeritus
I glad to have helped.