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Homework Help: Help with vector physics problem

  1. Jan 24, 2008 #1
    1. The problem statement, all variables and given/known data

    Let vectors [tex]\vec{A}[/tex]=(2,1,-4) [tex]\vec{B}[/tex]=(-3, 0, 1), and [tex]\vec{C}[/tex]= (-1, -1, 2) .
    Calculate the following:

    What is the angle[tex]\theta_{AB}[/tex] between [tex]\overline{A}[/tex] and [tex]\overline{B}[/tex] ?

    2. Relevant equations

    [tex]\vec{A}[/tex] * [tex]\vec{B}[/tex]= magnitude of A * Magnitude of B * Cos[tex]\theta[/tex]

    magnitide of X= [tex]\sqrt{a^{2}+b^{2}+C^{2}}[/tex]

    3. The attempt at a solution

    magnitude of A= 4.58
    Magnitude of B= 3.16

    [tex]\theta[/tex]=arccos[( A * B)/ magnitude of A * magnitude of B)

    I tried that but the incorrect answer. Maybe i did the arithmetic wrong. Please Help..
  2. jcsd
  3. Jan 24, 2008 #2


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    Gold Member

    I can't tell you if you did the arithmetic wrong if you don't show your calculations...

    Also, it seems you posted this problem in the forum 3 times. Please do not flood the forum with repeat posts.
  4. Jan 24, 2008 #3
    sorry if i posted it three times. It is my first time using this program.

    magnitude of a= [tex]\sqrt{2^{2}+ 1^{2}+ 4^{2}}[/tex]= 4.58

    Magnitude of b= [tex]\sqrt{-3^{2}+ 1^{2}}[/tex]= 3.16

    and the vector A * the Vector B gives me -10
    Last edited: Jan 24, 2008
  5. Jan 24, 2008 #4


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    Gold Member

    OK, you calculations for the magnitudes are correct. Can you show me what you get for [tex]\theta[/tex] and the full calculation involved?

    I ask because you have the correct formula, so I think the error is probably in your calculation somewhere.
  6. Jan 24, 2008 #5
    a ha!!! i just saw my mistake!! But i will show you my calculations that i just re-did

    [tex]\theta[/tex][tex]_{AB}[/tex]= arccos[ -10/ (4.58* 3.16)]

    = arccos(-10/ 14.47)
    = 133.7

    Thanks a lot!!!
  7. Jan 24, 2008 #6

    I can't tell you the number of times I begin to spend the time typing up a problem here, and then all of a sudden I figure out the answer to my question.
  8. Jan 24, 2008 #7


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    Good Job!
  9. Jan 25, 2008 #8
    i know i couldn't believe it! I was so frustrated trying the problem out over and over. I lauphed so hard when i figured it out lolz... I thought it was such a hard problem, turned out pretty easy
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