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Constant vector?

  1. Jan 29, 2004 #1
    Okay, here's the question:

    For r=xi + yj +zk evaluate grad x (k x r), where k is a constant vector.

    I know how to do the cross product of k x r and I know how to do
    grad x (k x r). But I don't know what k is. What's k?
     
  2. jcsd
  3. Jan 29, 2004 #2

    Doc Al

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    Staff: Mentor

    Try assuming an arbitrary k vector: ai + bj + ck (or whatever). Then see what happens.
     
  4. Jan 29, 2004 #3
    does this look right?

    The answer I got is: -2ai-2bj-2ck
     
  5. Jan 29, 2004 #4

    Doc Al

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    Re: does this look right?

    I got 2k (where k is the constant vector) but I did it quick. (So my answer differs by a minus sign from yours.) My main point was that you didn't have to know the value of k (the a, b, & c) to do the problem.
     
  6. Jan 29, 2004 #5
    question

    how did you just get 2k? i don't understand
     
  7. Jan 29, 2004 #6

    Doc Al

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    Re: question

    By "2k" I mean: 2ai + 2bj + 2ck. (The same as you, but with a + instead of -.) [itex]2\vec{k}[/itex] is what I should have written. Make sense?
     
  8. Jan 29, 2004 #7
    thanks

    i get it now
     
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