# Homework Help: Find the volume of a sphere with respect to time

1. Jan 27, 2012

### bobsmith76

1. The problem statement, all variables and given/known data

I guess I should point out that the volume was known, which is 100 cm.

3. The attempt at a solution

I don't see why using the chain rule compels you to take the inverse of 4∏r^2. Any help would be appreciated.

I have a feeling that the solution is something like

100 = (4pir^2)/x

but that still is not right.

Last edited: Jan 27, 2012
2. Jan 27, 2012

### SammyS

Staff Emeritus
They're simply solving $\displaystyle \frac{dV}{dt}=4\pi r^2\frac{dr}{dt}\ \text{ for }\ \frac{dr}{dt}\,.$

3. Jan 27, 2012

### bobsmith76

Ok, now I got it. The way the book proceeded was a bit bewildering. I didn't see how dV/dt = 100, and r = 25, but now I do.