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Homework Help: Another question Gravitaional Force ?

  1. Sep 18, 2004 #1
    Another question!! Gravitaional Force ?

    I know this is easy, but for some reason I am getting stuck!
    The mass of a robot is 5450kg. This robot weighs 3620N more on Planet A than on Planet B. Both planets have a radius of 1.33^7. What is the difference Ma-Mb in the masses of these planets.
    I think that I need to set the gravitaional force on A(minus 3620) equal to gravitaional force on B. I know teh G and the r's are the same. I am confused on how to put it into the formula and figure out what I am looking for?
     
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  3. Sep 18, 2004 #2

    arildno

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    Hint:
    Let the weight of planet A be given by [tex]W_{A}[/tex], while the mass of planet A by [tex]m_{A}[/tex] (similarly for planet B; mass of robot:[tex]m_{R}[/tex]
    We then have, for example:
    [tex]W_{B}=\frac{Gm_{B}m_{R}}{r^{2}}[/tex]
    which you may rewrite as:
    [tex]m_{B}=\frac{r^{2}}{Gm_{R}}W_{B}[/tex]
     
  4. Sep 18, 2004 #3
    I see where you are going, but what are you using for W of planet A or B. We only know the weight and mass of the robot. Are you saying to do each planet seperately and then subtract the answers?
     
  5. Sep 18, 2004 #4

    arildno

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    You do know the weight difference, don't you?
     
  6. Sep 18, 2004 #5

    arildno

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    Oh, I'm sorry, I just saw my mistake:
    I meant "Weight of robot on planet A" and so on..
    Shame on me..
    :redface:
     
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