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Need universal Gravitation help

  1. Jul 13, 2011 #1
    1. The problem statement, all variables and given/known data
    what is the magnitude of Gravitational force
    for the following picture(s)
    (attached to forum)

    2. Relevant equations
    Fg = G M*M/D^2


    3. The attempt at a solution
    I tried to do some of them I know the equation for the second one is fG = 2G M*M/D^2
    and for the third one it's Fg = 6G M*M/D^2
    But I have no idea where the 2 came from for the second one and the 6 from the third one!
     

    Attached Files:

  2. jcsd
  3. Jul 13, 2011 #2

    berkeman

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    Staff: Mentor

    A better way to write that force is F = G M1 M2 / D^2, where M1 and M2 are the two masses. Does that help?
     
  4. Jul 13, 2011 #3
    Kinda, I still don't understand where the 2 in fG = 2G M1 * M2/D^2
    or the 6 in fG = 6G M1 * M2/D^2 come from
     
  5. Jul 13, 2011 #4

    berkeman

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    Staff: Mentor

    In the 3rd one, what are M1 and M2?
     
  6. Jul 13, 2011 #5
    According to my sheet, it's just simply m1 and m2, there are no mass representing them, I think you just have to write the final equation
     
  7. Jul 13, 2011 #6

    gneill

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    Staff: Mentor

    attachment.php?attachmentid=37175&stc=1&d=1310606192.gif

    There are two masses and a distance indicated. Call the mass on the left M1, and the mass on the right M2. What are M1 and M2 according to the figure? What is the distance?
     

    Attached Files:

  8. Jul 13, 2011 #7
    there is no mass specified it just says M1 and M2 and distance is just represented by d
     
  9. Jul 13, 2011 #8

    gneill

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    Staff: Mentor

    Look more closely at the diagram. I don't see the characters "M1" or "M2" in the figure. How are the masses labelled?
     
  10. Jul 13, 2011 #9
    Never mind my teacher answered the question for me (after several rather painful hours of waiting) we just have to make the equation nothing special, Fg = 6G M1 * M2 / d
    and he also explained to me what the 6 was...

    I am such an idiot >.<
    thanks for the help though
     
  11. Jul 13, 2011 #10
    You just insert the numbers in the pictures into the equation. For example:

    The second one - [itex]F=\frac{G2mM}{{\Delta}d^{2}}[/itex]

    Which also is equal to - [itex]F=\frac{2GmM}{{\Delta}d^{2}}[/itex]

    Basically, you take the number of the specific mass or [itex]{\Delta}d[/itex] and multiply it or divide it by the appropriate amount as represented by the picture.

    The third one is [itex]F=\frac{G2m_{1}3m_{2}}{{\Delta}d^{2}}[/itex] and it simplifies to [itex]F=\frac{6Gm_{1}m_{2}}{{\Delta}d^{2}}[/itex]


    EDIT: Ah whoops, I replied to late...
     
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