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Suppose the mass of the Earth were doubled (Physics 12)

  1. Feb 18, 2013 #1
    1. The problem statement, all variables and given/known data
    Suppose the mass of the Earth were doubled , but it kept the same density and spherical shape. How would the weight of objects at the Earh's surface change?


    2. Relevant equations
    Fg=Gm1m2/r^2


    3. The attempt at a solution
    g=G2m/r^2
    g=19.59817612

    g/g(on earth - 9.8)
    =2

    I know this is wrong because the textbook says the answer is 1.26 or 2^1/3.
    I have no idea how to get this answer. I did find an online solution but they used equations that I have not been taught.

    Here is a link to the online solution:
    http://archives.evergreen.edu/webpages/curricular/2005-2006/PhyAstro/0506/HW/G6solns.pdf

    If anyone can explain how to get this answer step by step that would be great.
     
  2. jcsd
  3. Feb 18, 2013 #2

    Simon Bridge

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    If you double the mass, but keep the same density, what happens to the radius?
     
  4. Feb 18, 2013 #3
    It would increase?
     
  5. Feb 19, 2013 #4

    Simon Bridge

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    Did you increase the radius in your calculation?
     
  6. Feb 19, 2013 #5
    No I didn't. So it would double as well?
     
  7. Feb 19, 2013 #6

    Simon Bridge

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    Would it?
    What is the relationship between mass and volume? volume and radius for a sphere?
     
  8. Feb 19, 2013 #7
    I get it now. My Physics teacher explained it to me after school. Thank you!
     
  9. Feb 19, 2013 #8

    Simon Bridge

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    Cool. Of course it would have been better if you'd reasoned it out yourself.
    If you posted the answer here, you'd help someone else stuck on the same problem...?
     
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