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Homework Help: Mass of a planet?

  1. Jun 16, 2010 #1
    1. A newly found planet with a density of 4950 kg/m3 has no atmosphere and is orbited by a low altitude satellite with an orbital speed of 3.55 km/s. What is the mass of the planet?

    2. density*velocity=mass
    G= 6.67E-11

    3. After a process of equations, I ended up with mass= sqrt(V^6)/(G^3*density*4/3*pi). Subbing in the values and converting 3.55 km/s to 3550 m/s, I get sqrt(3550^6)/(6.67E-11)^3(4950)(4/3*3.14) to get an answer of 3.3*10^47 kg

    This isn't the right answer, and I'm not sure where I screwed up. Much appreciated..
  2. jcsd
  3. Jun 16, 2010 #2


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    Homework Helper
    Gold Member

    You might try to process the equation again. If you show your work we can help more.

    As you have written your formula, the only thing under the √ sign is the v6 term. According to my calculations, there should be more than that. Although it might just be your typing notation. Perhaps [itex] \LaTeX [/itex] would be of good use here.

    For your convenience I have written your calculated formula in LaTeX (from what I think you meant, anyway -- it's difficult to tell with just text):

    [tex] m = \frac{\sqrt{v^6}}{\frac{4}{3}G^3 \pi \rho} [/tex]

    You might wish to modify that formula.

    [Edit: If you don't know [itex] \LaTeX [/itex], but whould like to use my equation as an example, click on it and it should open up the LaTeX code which you can copy, and modify in your own post. Or, we can just stick with normal text if [itex] \LaTeX [/itex] doesn't suit you. Alternately, you can click on the "[itex] \Sigma [/itex]" symbol in the upper right hand corner of the editing toolbox (make sure you "Go Advanded" if you don't see it), and there is a menu there that you can use to generate LaTeX code from items on the menu.]
    Last edited: Jun 16, 2010
  4. Jun 16, 2010 #3
    Yep you caught my error. I simply missed taking the square root of the solution because there was quite a bit going on. I'll try using Latex next time...I was unaware of it. Thank for you help collinsmark.
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