1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper
    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


    User Avatar
    Homework Helper
    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


    User Avatar
    Homework Helper
    Gold Member

    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
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Help with vector physics problem
  1. Physics Vectors Help (Replies: 4)