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Homework Help: Vector Calculus

  1. Oct 1, 2005 #1
    Hey... i was hopeing somebody can help me with a homework question... its about vector calc.... taking the deriviative.

    d/dx[r(t) dot r(t)] = r'(t) dot r(t) + r(t) dot r'(t) = 2r'(t) dot r(t)

    I know it sounds sillly, but i was just wondering how on earth they got
    2r'(t) dot r(t) ????
     
  2. jcsd
  3. Oct 1, 2005 #2

    Hurkyl

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    Well, go through the list of properties of the dot product -- any of them look useful?
     
  4. Oct 1, 2005 #3

    robphy

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    Another approach... what must be true for that last equality
    r'(t) dot r(t) + r(t) dot r'(t) = 2r'(t) dot r(t)
    to happen (assuming that each side is correct)?
    This should lead to the property hinted by Hurkyl.
     
  5. Oct 4, 2005 #4
    Okay i see it now.

    It comes from the property....

    a dot (b + c) = a dot b + a dot c

    :)

    Thanks for poinitng that out...
     
  6. Oct 4, 2005 #5

    Hurkyl

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    Ah, right -- that one's so fundamental I forgot it even needed to be invoked! (I had only noticed you needed a.b = b.a)
     
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