Volume of a sphere

Discussion in 'Math' started by mentaaal, Feb 11, 2009.

  1. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Hey guys, I have a question relating to integration which is puzzling me.

    I figured that I would try and derive the volume of a sphere myself. Having never done this before I started out with a strategy that to me seems perfectly logical. If it can be seen from the 10 second scribble in paint, I imagine the circle i've drawn to be a 3-d sphere. If i cut the sphere up in cross-sectional slices with cross sectional area πr^2 and integrate from 0 to r, taking the zero point to be the left most point on the circle, I would have found the volume of a hemisphere. Therefore I would need to multiply this integral by two to get the total volume.

    Like this: 2∫πr^2 dr (from 0 to r) = [2πr^3]/3

    Obviously this is wrong so could someone point out the flaw in my approach as I am off by a factor of 2.
    Last edited: Feb 11, 2009
  2. Alexei Smirnov

    Active Member

    Jan 7, 2009
    When you integrate from 0 to r, the radius of the circle is not equal r,
    it is equal to sqrt(1-r^2), so you have to take
    2*∫π(1-r^2)dr = 2πr^3(1-1/3) = 4πr^3/3.
  3. mentaaal

    Thread Starter Senior Member

    Oct 17, 2005
    Yes I can see now how I was wrong. Thank you very much!
  4. nene biggie

    New Member

    Feb 28, 2009
    volume ofsphere formular =4/3pie r because it has four quaters according to my view.
  5. Mark44

    Well-Known Member

    Nov 26, 2007
    We've already established in this thread that the volume of a sphere is 4/3 * πr^{3}. This formula has been known for hundreds of years.