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

Proportion or No?

  1. Feb 19, 2007 #1
    Proportion Help. (from Ratio, Proportion, and Variations)

    1. The problem statement, all variables and given/known data
    A ball 4.50 in in diameter weighs 18.0 oz. What is the weight of another ball of the same density that is 9.0 in in diameter?

    2. Relevant equations
    Is this set up as a proportion? Or something else? I read the similar figures things but that does not seem to work.

    3. The attempt at a solution

    I set it up like a proportion: 4.50/9 = 18/x this comes up with 36 oz. But this is not the right answer. The answer is supposed to be 144 oz. But they dont show how they got it. The book doesnt have anything like this in its examples so I am stuck.

    Is this not set up like a proportion? Or is there another type of formula I should be using?
    Last edited: Feb 19, 2007
  2. jcsd
  3. Feb 19, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    What's the relation between the volume of a sphere and its radius?
  4. Feb 19, 2007 #3


    User Avatar
    Science Advisor

    Yes, it is a proportion- but not just "volume is proportional to radius"- volume is proportional to a power of the radius: which power? As Dick said, "What is the relation between the volume of a sphere and its radius?"
  5. Feb 20, 2007 #4
    ok, so V= 4/3piD/2^3

    then 18 = 4/3pi (4.5/2)^3
    2 = 4/3pi (4.5)^3 right?

    cancellation gives me 18 / x = 1/8 / 1 which is 144 ! Great! thanks guys!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook