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Need help with forces!

  1. May 20, 2009 #1
    1. The problem statement, all variables and given/known data

    The asteroid Ceres has a mass of 7x10^20 and a radius of 500km

    a. what is the gravity on the surface
    b. How much would a 85kg astronaut weigh in ceres

    2. Relevant equations

    Fg = (Gm1m2)/r^2

    3. The attempt at a solution

    None, where do I get the seccond mass value from? or am I using the wrong equation?

    pls help, all constructive input appreciated :)
     
  2. jcsd
  3. May 20, 2009 #2

    diazona

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    Homework Helper

    Well, you can use that equation, but you'll need another one: F = mg. You might have seen this before with w (for "weight") in place of F, but it's the same thing. Part (a) is just asking for the value of g, the gravitational acceleration.

    Can you see how to put this equation together with the one you have to find g?
     
  4. May 20, 2009 #3
    So something like

    w=mg
    w=(7.20x10^20)(6.67x10^-11)
    w=4.669x10^10

    then

    fg=(Gm1m2)/r^2
    fg=(6.67x10^-11)(4.669x10^10)(7.20x10^20)/500^2
    fg=2.24x10^21

    like that?
     
  5. May 20, 2009 #4

    LowlyPion

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    In figuring gravity for Ceres, the mass of the object that you want to determine the value of gravity for drops out.

    W = m*g = G*M*m/r2

    g = G*M/r2

    To find W then

    W = m*g
     
  6. May 20, 2009 #5

    diazona

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    Homework Helper

    No, not that... putting equations together is not the same thing as plugging in numbers. I was trying to point you towards the answer LowlyPion wrote out.
     
  7. May 20, 2009 #6
    got it
     
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