# Can I get Surface Density from Volume Density?

1. Nov 15, 2004

### Farina

I'm working on a vibration frequency problem
involving a thin, circular aluminum membrane

I know the volume density of Al.

How do I arrive at a surface density for this circular
membrane -- especially since I'm not given the
thickness (I'm told that frequencies for thin membranes
are independant of thickness).

I could see how to do this if I had a rectangular membrane,
but I have a circular membrane instead.

??

2. Nov 15, 2004

### Gokul43201

Staff Emeritus
Yes, you can calculate the surface density from the volume density. It's just $\sigma = \rho ^ {2/3}$

3. Nov 15, 2004

### AKG

No, I don't think you can. If something has surface density $\sigma$, and you stack 3 thin sheets on top of each other, the total mass will be the mass of the three sheets. Now, something with finite thickness would be like having an infinite number of thin sheets stacked on top of each other, so the mass would be infinite (and so would the volume density).

Conversely, assume something has volume density $\rho$. Let's say that we take a very bad approximation of it's surface density by taking a 1cm thick piece of the substance, and approximating it's surface density to be it's mass/surface area = mass/(volume/1cm) = 1cm * $\rho$. Now, the "true" surface density would be this number as the thickness approches zero. If we start with a thickness t = 1cm, then we have that it's "bad-approximate" surface density is $t\rho$. What we need to do, obviously, is evaluate the limit as t approaches zero, and since $\rho$ is just some positive finite number, the limit is zero, so it's surface density is zero, which is what we have in real life (because objects are 3-d).

I'm not sure how to go about solving your problem, but the best suggestion I can give is to treat "thin" as having the thickness of the atomic radius of aluminum. You can then treat the membrane as a zero-thickness membrane with surface density (approximated to) $t\rho$, where t is the radius of aluminum atom, and $\rho$ is its density.

Gokul is saying something else, I'm not sure where he's getting that from.

4. Nov 15, 2004

### rayjohn01

I assume you mean a sperical membrane not circular -- the volume density then tells you the mass of the membrane -- thickness assumed at some value -- so you have the details the rest is up to you -- I would not know how to solve this offhand.