# Need help with this moment about the y

1. Nov 30, 2006

### stinlin

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

Determine by direct integration the moment of inertia about the y-axis in the figure shown.

2. Relevant equations
See Iy on attached.
Equation for curve also on attached.

3. The attempt at a solution
My answer didn't include pi, and the answer apparently does. I'm trying to do this via a double integral as opposed to using one integral and doing something funky. My problem I THINK is in finding dA.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

File size:
15.3 KB
Views:
60
2. Dec 1, 2006

### HallsofIvy

Staff Emeritus
You are right. There is no "$\pi$" in the solution.

The moment of inertia of that figure, about the y-axis is
$$\int_{x=0}^a\int_{y= -b\sqrt{1-\frac{x^2}{a^2}}}^{b\sqrt{1-\frac{x^2}{a^2}}} y dydx$$
That's easy to integrate and has no "$\pi$".

3. Dec 1, 2006

### stinlin

Hmm - that's what I thought. In fact, I think that's the exact integral that I had. I'll give it another go. Thanks!

4. Dec 1, 2006

### stinlin

I lied - that's a y^2, not just a y in the integral. The answer is pi*a*(b^3)/8...Still no luck though.