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Homework Help: Finding the mass of neptune

  1. Dec 30, 2007 #1
    [SOLVED] Finding the mass of neptune

    1. The problem statement, all variables and given/known data
    The planet Neptune has 8 moons, the largest of which is Triton. The Voyager II spacecraft has photographed large frozen swampts on the surface of Triton, indication it once had a much warmer climate. Triton may have been a planet itself, migration away from the sun where it was later captured by the gravitational field of Neptune. Neptune requires 164.83 Earth years to complete one orbit around the sun. The diameter of the planet Neptune is 49,528km. Triton orbits Neptune at a mean distance of 354,800km and has an orbital period of 5.88 Eath Days.
    Find the mass of Neptune

    2. Relevant equations

    I used the equation:4(pi)(radius of satellite ^3 = (Period of Satellite )^2(Mass Focus)(Gravitational constant) , Using Triton as the Satellite and Neptune as the Focus.
    My Professor adds some irrelevant information to make the problem more difficult, but I found that the most important figures were the Diameter of Neptune and Distance of Neptune's surface to Triton ( for the Radius of the Satellite) and the orbit time of Triton.

    3. The attempt at a solution

    When using the above equation I Divide the diameter of neptune in half to find the radius and add it to the distance of triton to neptune to find the Radius of the orbit path (in km). then i multiply it by 4*pi then divide by period^2(in days) and the gravitational constant (6.67*10^-11) to isolate the variable(mass of neptune), after reworking the problem several times and getting the same answer I always get 2.98*10^26kg which from what I can find is about twice the actual mass, if anyone could help me find out what i'm missing in the problem that would be great
    Last edited: Dec 30, 2007
  2. jcsd
  3. Dec 30, 2007 #2

    Doc Al

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    Staff: Mentor

    (1) Use standard units for time, distance, and mass: seconds, meters, and kilograms.
    (2) Double check your formula; Looks to be off by a factor of pi.
  4. Jan 1, 2008 #3
    I changed the factor of pi in the formula to pi^2 and the answer looks correct, thankyou for you help
  5. Jan 16, 2011 #4
    Re: [SOLVED] Finding the mass of neptune

    I don't seem to get the pi^2...
    when i tried to put that in my calculator the answer would be 2.34*10^26...
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