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Homework Help: Using Kepler's Laws to find the mass of a star

  1. Oct 1, 2012 #1
    1. The problem statement, all variables and given/known data
    In recent years, a number of nearby stars have been found to possess planets. Suppose, the orbital radius of such a planet is found to be 4.9 1011 m, with a period of 1280 days. Find the mass of the star.

    2. Relevant equations

    3. The attempt at a solution

    I thought this was the right solution; however, I am not getting the correct answer as I am doing it online and it tells me I am doing it wrong. Am I using the wrong equation?
  2. jcsd
  3. Oct 1, 2012 #2
    I see at least two mistakes. The equation is wrong (c.f. Wikipedia); the period must be expressed in seconds, not days.
  4. Oct 1, 2012 #3


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    welcome to pf!

    hi james! welcome to pf! :smile:
    days !! :wink:
  5. Oct 1, 2012 #4
    I tried switching days to seconds and i got T=110592000 seconds so i changed my equation....


    I changed it to seconds, but I still don't seem to be getting the right answer.
  6. Oct 1, 2012 #5
    The equation is start with is wrong. I mean this one: T^2=4pi√((r^3)/Gm)) As I said. look up the correct equation.
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