# Homework Help: Hollow Sphere

1. Dec 14, 2006

### PSOA

How do I get rid of infinitesimal mass element dm?

2. Dec 14, 2006

I assume you're trying to derive the moment of inertia of a hollow sphere, but you should really be more specific when posting questions.

3. Dec 14, 2006

### PSOA

I am not determining the moment of inertia. I didn't specify what I was doing because I just wish to know of to solve this particularly problem. How to get rid of dm?

4. Dec 14, 2006

### BobG

Your differential mass is the rate of change in the mass. It will depend on the object's density (g/cm^3, kg/m^3, etc).

In your case, you have a hollow sphere, so the mass will change in relation to the area (assuming the sphere has an infinitely small thickness). That would be g/cm^2, kg/m^2, etc.

That should allow you to change your variable to dr, the differential radius, since the volume and/or the area will depend upon the radius.

5. Dec 14, 2006

### PSOA

But I need the constant sigma M/A (equivalent to density) which I do not know.

6. Dec 14, 2006

### billiards

Maybe Integrate? I don't really understand your problem.

7. Dec 14, 2006

### PSOA

I need to $$\int dm$$ for a spherical shell.

8. Dec 14, 2006

### OlderDan

Look at the thread that started this.

Last edited: Dec 14, 2006