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Homework Help: 3D moment

  1. Oct 12, 2009 #1
    1. The problem statement, all variables and given/known data
    http://books.google.ca/books?id=nYR... you want the magnitude of the moment&f=false
    The above link has a copy of the question. It is question 4.65 on page 144

    If you cant see the picture here is the needed information:
    pointA(0,8,0)-on tree

    There is a cable from A to B with a tension of 100lb and onother cable from A to C with an unknown tention. If the magnitude of the moment about the origin(0,0,0) is 1500 ft-lb, what is the tension on rope AC?

    2. Relevant equations
    M=r * F

    3. The attempt at a solution
    I know that the moment is equal to the position vector *(cross) the force in cartesian form, but how do you express the 100lb foce in rope AB in cartesian form? Do i even need to?

    Also to find the final answer im thining you have the equation:

    1500 = the sum of both moment forces
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 12, 2009 #2


    User Avatar
    Homework Helper

    Hi sapphire4770, welcome to PF.
    Position vector OA = 0i+8j+0k
    Position vector OB = 0i +0j + 10k
    Position vector OC = 14i + 0j + 14K
    Vector AB = OB - OA. Similarly
    Vector AC = OC - OA.
    Now find the moments,
  4. Oct 12, 2009 #3
    Thanks for the help, but I'm still kinda confused.

    Ok, so the vectors would be:
    AB: [0,-8,10] or -8j +10k
    AC: [14,-8,14] or 14i - 8i + 14k

    To find the moment caused by AC do i simply multiply? 100[0,-8,10]? Or do i have to split up the 100 lb tension into its x y and z components and find the cross product?

    Thanks again, any help is greatly appreciated!
  5. Oct 12, 2009 #4


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    Homework Helper

    Find the vector OA. = 8(0i + j + 0k)
    Find unit vector along AB and AC.
    So the vector AB = 100/sqrt(164)[0i -8j + 10k]
    The moment of AB about o is OAXAB.
    Similarly find the vector AC and moment of AC about O. Since both of them are in the counterclockwise direction, add them to get net momentum.
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