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Homework Help: Finding Orbital Period

  1. Dec 6, 2007 #1
    Jupiter is 5.2 times farther than Earth from the sun. Find Jupiter's orbital period in earth years.

    Equation to use is: T^2 = Kr^3

    Radius of earth is 6378100 m and radius of Jupiter is 71492000 m.

    I rearrange the equation to K = T^2/r^3 and set k of earth equal to k of Jupiter. I enter all of it including 5.2 times radius of earth into the equation. I solve for t of Jupiter but I always end up with 16.457 years when it should be 11.86 years. What am I doing wrong?
  2. jcsd
  3. Dec 6, 2007 #2
    Can anyone help me?
  4. Dec 6, 2007 #3
    I dont know where you got those radii from, but you don't need them.

    Use 1 earth year as T (earth) and 1 earth orbit as R (earth). Use 5.2 earth orbits as R (jupiter).

    Plug these into T(earth)^2 / R(earth)^3 = T(jupiter)^2 / R(jupiter)^3

    and solve for T(jupiter).
    Last edited: Dec 6, 2007
  5. Dec 6, 2007 #4
    Thanks. Looks like it works but the only problem is that I know that I have to use the radii of the planets which is from the center to the edge of the planet. r stands for radius, not orbit so I don't understand why that's working.
  6. Dec 7, 2007 #5

    Shooting Star

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    The r stands for radius of an orbit, considered circular, not the radius of a planet! All planets are considered to be point masses in this treatment.
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