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Homework Help: 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.

    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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