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Homework Help: Unit Conversion (mass, radius )

  1. Sep 2, 2007 #1
    1. The problem statement, all variables and given/known data

    In January 2006, astronomers reported the discovery of a planet comparable in size to the earth orbiting another star and having a mass of about 5.5 times the earth's mass. It is believed to consist of a mixture of rock and ice, similar to Neptune. Take mass earth 5.97 x 10 ^24 and radius earth 6.38x10^6.

    If this planet has the same density as Neptune (1.76 g/cm^3), what is its radius expressed in kilometers?

    What is its radius expressed as a multiple of earth's radius?

    2. Relevant equations


    3. The attempt at a solution

    My attempt:

    I got (5.97x10^24kg earth's mass / 6380 km earth's radius) x (.00176 kg mass of unknown planet mass / x the radius of the unknown planet)

    = 1.068676422 x 10^-21

    The answer does not seems right, I did convert each unit accordingly.

    Please and thank you in advance.
  2. jcsd
  3. Sep 2, 2007 #2


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    Homework Helper

    Well for one thing, density is M/V, not M*V. I don't understand what you are doing. Where's your use of the density equation?
  4. Sep 2, 2007 #3
    Nevermind, I solved it using the formula for finding the volume of sphere. In which it lead me to the final radius of 1.65 x 10 ^4 km.

    Thank you anyways. :wink:

    I just noticed the mistake of the D=M/V formula I stated before, :surprised can't believe I did that.
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