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

Given radius, find orbit

  1. Nov 16, 2011 #1
    1. The problem statement, all variables and given/known data
    The International Space Station (ISS) circles the earth at an altitude of 347 km.
    What is the period of the orbit of the ISS expressed in minutes?
    G=6.67x10^-11 N * m^2 /kg^2
    M(Earth)=5.98*10^24 kg

    2. Relevant equations
    T^2/R^3 = (4Pi^2)/(GM)
    So: T^2= Sqrt(((4Pi^2)/(GM))*(r^3))

    3. The attempt at a solution
    Alright, so I've been plugging in these numbers for awhile, and I keep getting the wrong answer: 1.1 seconds. The right answer is 91.3 min.
    T is in seconds when initially calculated, right? I'm still doing something wrong, but I'm hoping to just double check on that.
  2. jcsd
  3. Nov 17, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to PF, :smile:

    I'm not sure what numbers you're plugging in, but keep in mind that the 'R' in Newton's law of gravitation is the distance between the centres of mass of the two bodies. What is the distance between the ISS and the centre of mass of the Earth? Hint: it is not 347 km.
  4. Nov 17, 2011 #3
    Ahh, 347000 m + 6.37x10^6 m! Perfect, thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook