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Gravitaional attraction between two objects

  1. Apr 14, 2014 #1
    1. The problem statement, all variables and given/known data
    If the mass of the Earth were twice as big, and the mass of the moon were three times bigger, by what factor would the gravitational attraction between the two be multiplied?

    2. Relevant equations

    3. The attempt at a solution
    Ive used this formula and failed twice to get the correct answer. please help.
  2. jcsd
  3. Apr 14, 2014 #2
    Show us what you did.

  4. Apr 14, 2014 #3
    6.67×10−11 N·(m/kg)2 x [(1.194438 x 10 ^19) x (2.204301927 x 10^14)/147765666409]

    Mass of earth: 5.97219 × 10^24 kg
    Mass of moon: 7.34767309 × 10^22 kg
    Distance between centers of masses: 384,403 km
  5. Apr 14, 2014 #4
    Rather than put all the numbers in, just note that F depends on the product of the masses. So if one mass doubles and the other triples, you can replace m1 with 2xm1 and m2 with 3xm2. Write both equations (the original one and the one with the new masses) side by side, and the answer should be obvious...
  6. Apr 14, 2014 #5
    nope. still not getting it.
  7. Apr 14, 2014 #6
    So, in the original equation, you have

    F = G*m1*m2/r^2

    In the new equation, you have

    F' = G *(2xm1)*(3xm2)/r^2 = 6*G*m1*m2/r^2 = 6*F

    So the force is increased by a factor of 6. Does that make sense?

    (Apologies for the badly formatted equations, I'm useless with this stuff... will have to learn).
  8. Apr 14, 2014 #7
    OOOHHHH. wow. Yeah i get it now. Sorry it took me so long. I thinking way to much into it. Thank you!
  9. Apr 14, 2014 #8


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    Staff: Mentor

    Please check your PMs. It is against the PF rules to do other students' homework for them.
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