# Mass of a material

1. Aug 31, 2004

### rumaithya

Hello,

How can we find the mass of a material with density p is required to make a hollow spherical shell having inner radius r1 and outer radius r2 ?

Thanks

2. Aug 31, 2004

### pmb_phy

Find the volume of the shell by taking the volume of a sphere or radius r2 and subtract out the volume of a sphere of radius r1. Then multiply the mass density times the volume.

Pete

3. Aug 31, 2004

### rumaithya

What is the volume of r1 and r2 ?!! there isn't any given numbers

4. Aug 31, 2004

### HallsofIvy

Yes, there are! r1 and r2 are numbers.

The volume of the hollow shell is $\frac{4\pi}{3} (r_1^3-r_2^3)$. Now multiply by ρ to get the mass.

5. Aug 31, 2004

### pmb_phy

Hint: The volume of a sphere of radius r is $V = \frac{4}{3}\pi r^3$.

Pete

6. Aug 31, 2004

### Cyrus

you can think of r1 and r2 as just representations of numbers. It is a way of representing ANY number rather than one particular number. By doing this, if you have any sphere of any radius, you can just plug in the number you want to calculate in place of r1 and r2. This gives you a convienet equation that is applicable to any situation.