1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Compute the length of ||u||

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

    u =
    [ .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.

  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


    User Avatar
    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].
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook