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Homework Help: Vectors- cross product

  1. Feb 28, 2015 #1
    Mod note: Member warned about posting with no effort.

    1. The problem statement, all variables and given/known data

    Expand to the general case to explore how the cross product behaves under scalar multiplication k (a x b) = (ka) x b = a x (kb).

    3. The attempt at a solution
    would this be the right general case to portray the situation?

    Attached Files:

    Last edited by a moderator: Feb 28, 2015
  2. jcsd
  3. Feb 28, 2015 #2


    Staff: Mentor

    Can't you use the definition of vector cross product?

    ie A x B = | A | * | B | * sin ( angleAB ) u

    Where u is the unit vector perpendicular to both A and B such that A, B, and u form a right handed system.

    EDIT amended the definition where I left out the unit vector part.
    Last edited: Feb 28, 2015
  4. Feb 28, 2015 #3


    Staff: Mentor

    You need to show an attempt. Normally I would issue a warning and possibly infraction points. You were hit with several points a day or so ago, and your posts have improved considerably, so I'll treat this one more informally.

    The problem as you state it seems to be to prove that k (a x b) = (ka) x b = a x (kb). What you posted in the Word document is not any kind of attempt -- it seems to be just the statement of some larger problem

    Please put your work directly in the input pane here - not in a PDF or Word file or other attachment. It's frustrating to have to open another window to view the work. Having the work right here makes it easier for us to insert a comment right where there is a problem.

    One other thing. All of these vector problems fall under precalculus, not calculus, so I am moving this thread to that section (and leaving a forward link).
  5. Feb 28, 2015 #4
    alright thanks
  6. Feb 28, 2015 #5


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    Gold Member

    That isn't the definition of the cross product. It is also incorrect. It is ##|A\times B|## that equals the right side but it still isn't the definition because ##A\times B## is a vector, not a scalar.
  7. Feb 28, 2015 #6


    Staff: Mentor

    To do this (i.e., prove that k (a x b) = (ka) x b = a x (kb) ), let a, b, and c be arbitrary vectors such as a = <a1, a2, a3>, b = <b1, b2, b3>, and similar for c, and let k be an arbitrary scalar. Do not use specific numbers. Calculate all three cross products and show that they are all equal.
  8. Feb 28, 2015 #7


    Staff: Mentor

    Sorry folks I left the unit vector part out in my haste to answer the question.
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