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Gravitation between objects.

  1. Mar 17, 2009 #1
    1. The problem statement, all variables and given/known data

    Each mass is 4kg find the magnitude and direction of the net gravtitational force on mass A due to the other masses. each side of the square is 10cm
    http://img13.imageshack.us/img13/3815/1012447.jpg [Broken]
    I am having trouble with (b) the square

    2. Relevant equations

    F = Gm1m2/r^2

    3. The attempt at a solution

    what I did was used the formula above to find AB, AC, and then used pythagorean theorm to find radius from A to D. then I used the above equation, and added the values together getting 2.17x10^-7 the correct solution is 2.04x10^-7 What did I do wrong?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 17, 2009 #2
    Is it possible that you used a different number of significant figures than the answer key?
    The two values are very close.

    Edit:
    You shouldn't add the values at the end, you should find the resultant of AC with AB, and then add AD. Is this what you did?
     
    Last edited: Mar 17, 2009
  4. Mar 17, 2009 #3
    yes I did do it that way, and no I am pretty sure I used the same amount of significant figures as the book did.
     
  5. Mar 17, 2009 #4
    Well I get the same answer as the key, 2.04*10^-7.
    What are some intermediate values you got?
     
  6. Mar 18, 2009 #5
    for AC and AB i did (6.674E-11)(4)(4)/.1^2 =1.068E-7

    for AD i used (6.674E-11)(4)(4)/.5657^2 = 1.89E-9

    I believe these are the correct values, now this is where I run into problems.
     
  7. Mar 18, 2009 #6
    I used 6.67 for G, not 6.674. This will yield 2.04 E-7.
    Also r for AD is 0.1414213562, not .5657. Hope you can figure out why.
     
  8. Mar 18, 2009 #7
    yeah I have .1414 also, I got the .5657 from the problem I had written beneath it. I figured out what I was doing wrong. I was not taking the resultant of AC and AB like you earlier suggested. I tried it this way, and I got the correct result. Thank you for all of your help.
     
  9. Nov 7, 2009 #8
    I've been trying to tackle a similar problem for longer than I would like to admit (again part b).

    All sides are equal, all masses are equal, have to show that magnitude = (Gm^2/a^2)(sqrt(9/4+sqrt(2)))

    the 9/4 looks like (3/2)^2, or (1+1/2); so I believe it is a creation of 60 degrees; which would make me think i need to tackle the sqrt(2) from a 30 degree standpoint.. but I don't see how I can generate sqrt(sqrt(2)) so that the first cancels out while developing magnitude.

    I really didn't want to ask.. but kinda desperate to get this done.
     
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