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Math Help - PLEASE HELP! (Matrix problem!)

  1. #1
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    PLEASE HELP! (Matrix problem!)

    Hey everyone, I feel pretty ridiculous for asking help with a simple(?) problem, but it's been too long since I've worked with matrices and to be honest, I'm stumped. That and the way it's worded...

    Here it is:

    Bob is making a mobile. He wants to suspend three objects from a lightweight rod, as shown. Suppose the weighs of the objects are w1= 2.5 oz, w2, 1.2 oz, and w3, 2.0 oz.

    a. Write three equations involving the unknown distances, d1, d2, and d3 based on the following facts:
    - The length of the rod is 14.5 in.
    - To balance the mobile, he must position the objects so that w1d1= w2d2 + w3d3.
    - He wants the third object to be 1.5 times as far from the support as the second object is.

    The only equation I'm sure of is d1 + d2 + d3= 14.5
    How would I set up an equation for the 2nd fact so I can put it in a matrix equation? Wouldn't it be 2.5d1 = 1.2d2 + 2d3? Since I have to put all the variables on one side, would the other side just be "0"?

    b. Write the system of equations as a matrix equation.

    This is all I have:
    a x b
    [ 1 1 1 ] [ d1 ] [ 14.5 ]
    [ ] [ d2 ] = [ ]
    [ ] [ d3 ] [ ]
    Sad, I know.

    c. Find d1, d2, and d3.


    Ahh, I'm so bad at math. If someone can guide me through this problem, I will be eternally grateful. :')

    EDIT: Nevermind, I got it. : P
    Last edited by miyosuke; November 14th 2007 at 07:35 PM. Reason: SOLVED!
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  2. #2
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    Quote Originally Posted by miyosuke View Post
    ...

    a. Write three equations involving the unknown distances, d1, d2, and d3 based on the following facts:
    - The length of the rod is 14.5 in.
    - To balance the mobile, he must position the objects so that w1d1= w2d2 + w3d3.
    - He wants the third object to be 1.5 times as far from the support as the second object is.

    ...
    Hello,

    only a hint:

    you wrote:
    - He wants the third object to be 1.5 times as far from the support as the second object is.

    According to your sketch that means: d_3 = \frac32 \cdot d_2
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