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Homework Help: Mastering Physics Vector Question

  1. Jan 26, 2009 #1
    1. The problem statement, all variables and given/known data

    I am having a problem with a mastering physics problem. I am given this image
    41119_b.jpg

    I am to Express C in terms of vector A, vector B, and angle theta, using radian measure for known angles. I know theta is equal to pi - c.

    2. Relevant equations

    law of cosines
    render?tex=C%5E2%3DA%5E2+%2B+B%5E2+-+2AB+%5Ccos%28c%29.gif


    3. The attempt at a solution

    By using the law of cosines the answer i got was

    C = sqrt(A^2 + B^2 - 2ABcos(θ))

    The feedback I got back from the program was to check my signs but everything apperas to be correct. Can someone help me out?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 26, 2009 #2
    Hi,
    Should'nt it be C = sqrt(A^2 + B^2 - 2ABcos(Pi-θ)) ?
    As c is the angle opposite to C.
     
  4. Jan 26, 2009 #3
    Thats right! I do not know how I missed that
     
  5. Jan 26, 2009 #4
    I am having trouble with the second part of the problem. I do not know if it is my input to the problem or if I have the problem totally wrong. I am told to:

    Find the angle phi that the vector C_vec makes with vector A_vec.

    I get arcsin(ABsin(pi - theta) / Csin(a))

    For earlier solutions in the problem I found C = to pi - theta so U used that instad of c. Can anyone help me out?
     
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