# Homework Help: Finding the volume

1. Dec 26, 2005

### jacy

Hi,
Is there a guideline that one can follow when finding the volume of a solid obtained by revolving over a region. The three methods that i know are disk, washer, and shell method. But i dont know when to apply which method. My book doesn't explain it clearly. If anyone could help me with this, i will appreciate it. Thanking you, and have a safe and wonderful holiday.

2. Dec 26, 2005

### marlon

Depends on the symmetry of the object and how it is rotated, but you can apply both if you want. Actually, this is a good exercise...

Check out :

http://mathdemos.gcsu.edu/shellmethod/ I used this website when i tutored students in college. I recommend it...
disk and shell method

regards
marlon

Last edited by a moderator: Apr 21, 2017
3. Dec 26, 2005

### Tide

You could use Pappus' Second Theorem.

4. Dec 27, 2005

### jacy

thanks marlon, i will check it out.

Last edited by a moderator: Apr 21, 2017
5. Dec 27, 2005

### jacy

I am not familiar with Pappus' Second theorem. We never learned that theorem. Thanks

6. Dec 28, 2005

### kant

You use the disk method when you know you can distinquish a inner and outer fuction, and want to produce a glass china.
To use the waster method, you are trying to find many A(x), to do a intergral of many A(x)dx s. you see, A(x) can have different geometries( If you were to do a disk erercise with and waster method, then A(x) is always a damn circle)
The shell method is sort of like the waster method, but all you are doing is rotating a figure accross a given axis.
My recommendation is to read that section 5 times, and try to understand it intuitively. My advice cannot do a damn thing for you, but it helps me to reflect more deeply.

Last edited: Dec 28, 2005
7. Dec 28, 2005

### siddharth

http://mathworld.wolfram.com/PappussCentroidTheorem.html" [Broken]
The theorem is very interesting, is it not?

Last edited by a moderator: May 2, 2017
8. Jan 2, 2006

### jacy

Thanks Siddharth, Tide, Marlon, and Kant. All u guys have a Happy New Year.

Last edited by a moderator: May 2, 2017