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Gravity between 2 objects touching

  1. Jun 5, 2009 #1
    1. The problem statement, all variables and given/known data
    Two spherical objects have a combined mass of 138.07 kg. The gravitational attraction between them is 1.48×10-6 N when their centers are 43 cm apart. What is the larger mass?


    2. Relevant equations
    F=Gm1m2/r^2
    138.07=M1+M2

    3. The attempt at a solution

    F=1.48X10^-6
    G=6.67X10^-11
    r=.63 m
    m1 and m2 is what we are solving for.

    so i used the second equation and solved for M1. and plugged it into the first equation for M1.
    F*r^2/G=(Mtot-M2)

    then i solved for M2 and got a quadratic equation. m^2 -138.07m+4102.728. then used the quadratic equation. and came up with 86.568. What is wrong with my logic or math?
     
  2. jcsd
  3. Jun 5, 2009 #2

    D H

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    Please show your work on how you used the quadratic equation.
     
  4. Jun 5, 2009 #3
    no problem
     
  5. Jun 5, 2009 #4
    -b+/- sqrt(b^2-4ac)/2a

    138.07+/-sqrt(138.07^2-(4*1*4102.728))/2

    (138.07+/-51.5012)/2


    then i get 94.78, and 87.058.
     
  6. Jun 5, 2009 #5

    D H

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    (138.07+51.5012)/2 is 94.78.
    (138.07-51.5012)/2 is not 87.058.
     
  7. Jun 5, 2009 #6
    94.78 didnt work maybe his C++ language is wrong.....I think its wrong for another one I am about to post. For some reason gravity is not making any since to me, everytime i head in a direction I get an anwser that ends up wrong. ill post a link after I make it.
     
  8. Jun 5, 2009 #7

    LowlyPion

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    Here is a quadratic calculator that you might find useful:
    http://www.math.com/students/calculators/source/quadratic.htm

    I would concur with the 94.79 number.

    As to your method it seems fine to me.

    Though I think your expression left off your factor of M2
    ...would really be

    F*r/G = (138.07 - m)*m
     
    Last edited: Jun 5, 2009
  9. Jun 5, 2009 #8
    Last edited by a moderator: Apr 24, 2017
  10. Jun 5, 2009 #9

    D H

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    What does this have to do with C++?
     
  11. Jun 5, 2009 #10
    Oh the homework I have to submit online and it tells me whether or not my answer is right or not. It is saying that 94 is wrong. So I am thinking that the C++ language he used to formulate the anwser is wrong because he created it himself
     
  12. Jun 5, 2009 #11

    D H

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    Why did you enter 94? You know that isn't the answer.
     
  13. Jun 5, 2009 #12
    You get multiple tries at each question, that is how I found out that 94.78 is wrong.
     
  14. Jun 5, 2009 #13

    D H

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    Did you try 94.8?
     
  15. Jun 5, 2009 #14
    yes, and it was wrong.
     
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