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Homework Help: Vector Problem

  1. Nov 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A vertical building was built on a horizontal plane. The plane is then tilted to A=(0,0,0) B=(1,2,0) and C=(0,3,1). By what angle (in radians) does the building now deviate from the vertical?

    2. Relevant equations

    Dot Product
    Cross Product

    3. The attempt at a solution

    So what I did was take the cross product of the B and C, relative to A.

    BxC = |2 0|, |1 0|, |1 2| = (2, -1, 3)
    ........|3 1|, -|0 1|, |0 3|

    Now this is the part where I dont know what to do.

    I took the Dot product of (BxC) with (0, 1, 0)

    theta = arccos (u.v)/(|u||v|) = (-1/[tex]\sqrt{}14[/tex][tex]\sqrt{}1[/tex])

    and i got an angle to be 1.84 in radians, which was wrong. I dont know what the answer is, I'm using webwork
    So if anyone can help me, that would be great.

  2. jcsd
  3. Nov 7, 2009 #2


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    Science Advisor
    Homework Helper

    Why do you think the vertical direction is (0,1,0)? My guess would have been that they meant it to be (0,0,1).
  4. Nov 8, 2009 #3
    Your Right! I must of been thinking in 2D..
    Thank You Dick!!
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