# Rotating circle

• November 17th 2006, 11:54 AM
Jones
Rotating circle
Hi, i need some help on this.

a) Determine the volume dV of the cylindric(?) shell in the figure.

the curve is y=16-x^2

I took a picture of the image in the book, might help

http://img452.imageshack.us/my.php?i...ture022oj5.jpg
• November 17th 2006, 12:29 PM
CaptainBlack
Quote:

Originally Posted by Jones
Hi, i need some help on this.

a) Determine the volume dV of the cylindric(?) shell in the figure.

the curve is y=16-x^2

I took a picture of the image in the book, might help

ImageShack - Hosting :: picture022oj5.jpg

$
dV=(2\ \pi\ x)\ y\ dx=(2\ \pi\ x)\ (16 - x^2)\ dx
$

RonL
• November 17th 2006, 12:37 PM
Jones
Quote:

Originally Posted by CaptainBlack
$
dV=(2\ \pi\ x)\ y\ dx=(2\ \pi\ x)\ (16 - x^2)\ dx
$

RonL

Hm, could you explain how you did that?
• November 17th 2006, 12:41 PM
CaptainBlack
Quote:

Originally Posted by Jones
Hm, could you explain how you did that?

The circumference of the shell is $2 \pi\ x$ the thickness is
$dx$ the height is $y$. To first order in $dx$ the volume is:

$
dV= (2 \pi x) y dx
$

If you want it more explicit its the difference in volume of the two cylinders of radius $x$
and $x+dx$ of heights $y(x)$ and $y(x+dx)$ or:

$
dV= -\pi x^2 y(x) + \pi (x+dx)^2 y(x+dx)=\pi\ 2 \ x\ dx (16-x^2) +O(dx^2)
$

RonL