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Homework Help: How to solve planet orbital question

  1. Oct 8, 2008 #1
    Ok I remeber a question on my physics test. where theres a planet orbiting a star in some other solar system.

    heres what was given to me.
    the radius,and the time it took for it to finish 1 revolution. I was asked to find the mass of the sun, that the planet was orbiting around

    so heres what i did.

    I calculated the distance, which was 2 pi r.

    I divided that value by time and got the speed.

    then i used Fc=FG formula.
    mv^2/r = Gm1m2/r^2

    with some cancellations, im left with

    so i just isolated for m and solved it. I remember i got like some number x 10^30kg.

    but did i do this question right? was this the right method of solving it?
  2. jcsd
  3. Oct 8, 2008 #2


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    Sounds reasonable, the sun's mass is around 2^30 kg
  4. Oct 8, 2008 #3
    i asked my friend how he had solved it, and he said he used
    4pi^2mr/T^2 = Gm1m2/r^2

    does my method and his method work in this question?
  5. Oct 8, 2008 #4


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    It's the same thing, you can also use T = 2 [tex]\pi \sqrt{ ( r^3 / GM )}[/tex]
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