1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Proving t^2/r^3

  1. Nov 18, 2015 #1
    1. The problem statement, all variables and given/known data
    In our unit on centripetal force, a large portion of the unit has been focusing on gravity and how the orbit is related to centripetal force. Our teacher told us to prove t^2/r^3 using F=(mv^2)/r and F=G(mM/r^2) as a small side part of our homework

    2. Relevant equations

    where G = 6.67*10^-11
    M and m are the objects' masses

    3. The attempt at a solution
    (mv^2)/r = G(mM/r^2)
    mv^2 = G(mM/r)
    v^2 = GM/r


    4(pi^2)(r^2)/t^2 = GM/r
    GM = v^2 / r = 4(pi^2)r

    GM/r = (4pi^2)r / r = 4pi^2

    4(pi^2)(r^2)/t^2 = (4pi^2)

    leaving the ratio
    r^2 / t^2

    When it should be r^2 / t^3. What did I do wrong? Was there something I forgot?
  2. jcsd
  3. Nov 18, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    It's just bad algebra.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted