# How would I do this problem?

1. Feb 27, 2004

### jlmac2001

Question: Find the solid angle subtended at the origin by a thin circular disk of radius a, whose cente is a distance b from the origin and where the normal to the disk is parallel.

Do I have to find the center of mass to solve this question?

2. Feb 27, 2004

### HallsofIvy

"the normal to the disk is parallel"?? Parallel to what?

The problem would be relatively easy if the normal to the disk were directed at the origin. It's quite a bit harder if the normal is parallel to the xy-plane or parallel to the z-axis.

3. Feb 28, 2004

### Dr Transport

If the normal of the disk is parallel to the direction from the orgin, the incremental solid angle is defined by

$$d\Omega = {dA_s}\over{r^2)$$

where the numerator is the surface area of the shape and the demoninator is the distance from the orgin. For a cylindrical plate the surface area would be $$\pi a^2$$. For your problem, the solid angle could be approximated by

$$\pi {a^2}\over{b^2}$$.

4. Feb 28, 2004

### Dr Transport

should be

$$d\Omega = {\frac{dA_s}{r^2}}$$