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Homework Help: Mass of a planet given it's satellites orbital radius & period

  1. Aug 27, 2009 #1
    Ah, please, someone help me. I've been working on this problem for half an hour, it's the last question on my review before my test, which i want to write right now (correspondence).

    1. The problem statement, all variables and given/known data

    If a satellite requires 2.5 h to orbit a planet with an orbital radius of 2.6 x 10^5 m, what is the mass of the planet?

    2. Relevant equations

    ac = v^2/r = 4 pi^2 r / T^2

    v = sqrt(GM / r)

    (... at least that's what i think?)

    3. The attempt at a solution

    1. I attempted to find the velocity from the radius (2.6*10^5) and the time (2.5hr*60*60=9000s)

    v^2/r = 4 pi^2 r / T^2
    v^2 = 4 pi^2 r^2 / T^2

    v^2 = 4 * 3.14^2 * 2.6*10^5 / 9000^2

    2. I used v= sqrt(GM/r) to find the mass.

    .356= sqrt(6.67*10^-11m/2.6*10^5)
    m= 5*10^15kg

    the answer key says it's 1.3*10^20kg
    Last edited: Aug 27, 2009
  2. jcsd
  3. Aug 27, 2009 #2
    The answer key is correct, I am unsure about what mistake was made in the attempted solution...maybe a calculator entering issue...
    Mp==1.284x10^20 kg.
    Last edited: Aug 27, 2009
  4. Aug 27, 2009 #3
    i appreciate your help but i think i need a bit more in depth of an explanation.

    "v is actually the square root of GMp/r"... isn't that what i did?... see... a little lost.
  5. Aug 27, 2009 #4
    Yes, when I reread your post, that is what you did...I don't know why you didn't get the right answer...However, if you follow the steps I showed in my previous post, you will get the correct answer.
  6. Aug 27, 2009 #5
    hmm thanks. i wonder if i just screwed up my maths somewhere along the way. i checked it so many times though... weird
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