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Homework Help: Vectors problem

  1. Oct 28, 2004 #1
    It's given that vector c is perpendicular to both vectors a and b. Which of the following is true:

    1) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_01.gif [Broken]
    (a x b) x c
    2) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_02.gif [Broken]
    (a x c) dot b
    3) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_03.gif [Broken]
    (a x c) x b
    4) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_04.gif [Broken]
    (a x b) dot c

    Can someone please just confirm the answer is (1) because I have only one attempt left at submitting an answer for this question.

    Thanks :smile:
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Oct 28, 2004 #2


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    I can't open the files. You need authorization for that...
  4. Oct 28, 2004 #3
    I knew that might happen, so I wrote the expression in each link below. (1) is (a x b) x c, i.e (a cross b) cross c.

  5. Oct 28, 2004 #4

    Tom Mattson

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    It's impossible to say which one is true, because those are all expressions, not equations.
  6. Oct 28, 2004 #5


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    Answers 2 and 4 give you a scalar number. The absolute value is the same for both, but the sign changes.

    Answer 1 gives you a null vector (all components equal to 0) since the cross product of A and B is either in the same direction as C or the exact opposite direction of C.

    Answer 3 gives you a vector.
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