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Find the volume of a sphere with respect to time

  1. Jan 27, 2012 #1
    1. The problem statement, all variables and given/known data

    Screenshot2012-01-27at25235PM.png

    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. jcsd
  3. Jan 27, 2012 #2

    SammyS

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    They're simply solving [itex]\displaystyle \frac{dV}{dt}=4\pi r^2\frac{dr}{dt}\ \text{ for }\ \frac{dr}{dt}\,.[/itex]
     
  4. Jan 27, 2012 #3
    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.
     
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