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Vector Problem

  1. Jan 23, 2014 #1
    1. The problem statement, all variables and given/known data
    For the following three vectors, what is 2C*(3AxB)
    A=3i+3j-4k
    B=-3i+2j+3k
    C=-6i-8j

    **All of the capital letters are vectors**

    2. Relevant equations



    3. The attempt at a solution
    2A=2(3i+3j-4k)
    =6i+6j-8k

    2AxB=(6i+6j-8k)*(-3i+2j+3k)

    (6i)(-3i)+(6i)(2j)+(6i)(2k)+(6i)(-3i)+(6j)(2j)+(6j)(3k)+(-8k)(-3i)+(-8k)(2j)+(-8k)(3k)

    (12k)+(-12j)+(18k)+(18i)+24j)+(16i)

    34i+12j+30k

    I don't really know what to do next, honestly I don't really understand what I did to get that far. I found an example problem and just kind of plugged stuff in... If you could explain to me how to work this problem that would be great!
     
  2. jcsd
  3. Jan 23, 2014 #2

    SteamKing

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    I'm confused. You ask what is 2C*(3AxB) and then you calculate 2AxB.
     
  4. Jan 23, 2014 #3

    Simon Bridge

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    Question, as written, does not call for 2A.

    Hmmm ... OK: You need quick primer lessons:

    AxB indicates a cross product:


    What the example you used did was start by multiplying out the brackets as usual ... then used the rules for the cross product of the unit vectors. so, using "x" to indicate cross product:
    then ixi=jxj=kxk=0
    and ixj=k, jxk=i, kxi=j
    and kxj=-i, jxi=-k, ixk=-j
     
    Last edited by a moderator: Sep 25, 2014
  5. Jan 23, 2014 #4
    Ohhh that might be part of my problem, I got this question mixed up with the example I was looking at.
     
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