Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Need help with word problem

  1. Apr 9, 2006 #1
    Question: A spherical bubble of radius 1 is surmounted by a smaller, hemispherical bubble, which in turn is surmounted by a still smaller hemispherical bubble, and so forth, until n chambers including the initial sphere are formed. What is the maximum height of any bubble tower with n chambers?

    Ok, The answer is 1 + sqrt(n) , but I can't figure out how that is possible. Any help would be appreciated.
  2. jcsd
  3. Apr 9, 2006 #2


    User Avatar
    Science Advisor

    Hint 1: you can optimize the 2 dimensional cross section of the bubble tower instead and it gives the same result.

    Hint 2: First optimize the radius of a bubble hemisphere r2 that sits atop a bubble hemisphere r1.
  4. Apr 18, 2006 #3


    User Avatar
    Science Advisor
    Homework Helper

    Interesting. The OP says that you can get 1 + sqrt(n). I agree for n=2. For n=3, the formula gives 1+sqrt(3), but choosing hemispherical bubble sizes of 1, sqrt(0.5) and 0.5, I only get a height of 2+\sqrt(0.5), slightly smaller. In other words, I found the optimal sequence to be [tex]r_n = 2^{-n/2}[/tex], where n starts at zero rather than the OP's one, and the height formula to be

    [tex]1 + r_n + \sum_{k=1}^{k=n}\sqrt{r_{k-1}^2 - r_k^2}[/tex].

    Did I screw up the calculus or did I read the problem wrong or what?

    Well it's late.

    Last edited: Apr 18, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook