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Compute the length of ||u||

  1. May 7, 2012 #1
    1. The problem statement, all variables and given/known data

    u =
    [-.6]
    [ .8]
    compute the lengths ||u||


    3. The attempt at a solution

    I thought to compute ||u|| you multiply the absolute value of u * u then take the square root. That would be .6 * .8 which is .48 The square root of that is roughly .7 The book says the answer is 1 what am I doing wrong.
     
  2. jcsd
  3. May 7, 2012 #2
    Yes, assuming by '*' you mean the dot product.

    No.
     
  4. May 7, 2012 #3
    Why not? Saying no, doesn't help me. I already knew it was wrong. And yes I mean dot product.
     
  5. May 7, 2012 #4
    never mind about this. i got the answer now.
     
  6. May 7, 2012 #5

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    The dot product of two vectors, <a, b> and <c, d>, is ac+ bd.

    If [itex]u= <u_x, u_y>[/itex], u.u is NOT [itex]u_xu_y[/itex], it is [itex]u_x^2+ u_y^2[/itex]
    u.u= <-.6, -.8>.<-.6, -.8>= (-.6)^2+ (-.8)^3.

    The length of vector <a, b> is [itex]\sqrt{a^2+ b^2}[/itex].
     
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