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Homework Help: Vector component perpendicular

  1. Sep 8, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose v = <-3,-2, 1> and w = <-1,1,1>. Then
    projw(v) = < -2/3, 2/3, 2/3 > ,
    and the component of v perpendicular to w is?

    2. Relevant equations

    3. The attempt at a solution

    I understood how to get the projection, and tried using the component formular Compaonb= a*b/magnitude( A )

    But did not get the correct answer which is w2 = < -7/3, -8/3, 1/3 >

    Where did I go wrong?
  2. jcsd
  3. Sep 8, 2012 #2
    Is that projection given? Because that doesn't jive with what I get.

    If you find the part of v that is parallel to the direction of w, then the only other part of v that remains must be perpendicular to w, no?
  4. Sep 8, 2012 #3
    That is the answer to the projection yes, I don't understand what you are trying to explain though?
  5. Sep 8, 2012 #4
    Okay, right, that projection is good. That is indeed the projection of [itex]v[/itex] onto [itex]w[/itex]. You could call it [itex]v_\parallel[/itex].

    The point I'm making is that, if [itex]a + b = c[/itex], then [itex]b = c - a[/itex], so if [itex]v_\parallel + v_\perp = v[/itex], then [itex]v_\perp = ?[/itex]
  6. Sep 9, 2012 #5
    Perfect I got the correct answer. So what your saying is by getting the projection of v onto w. It gives me a vector V which is parallel to W, and to get the component of that vector we subtract the original vector v by the parallel component " if we want the perpendicular component".

    Where I am confused is that there is a formula in my book for getting the component, and its in the form Comp v onto w = v*w/norm of v

    Why would using that be incorrect?

    Thank you for your help btw
  7. Sep 9, 2012 #6
    The component of v parallel to w is obtained by first calculating the unit vector in the direction of w (dividing w by its own magnitude), and then dotting v with that unit vector. The component perpendicular to w is obtained by subtracting the component parallel to w (times the unit vector in the direction of w) from v.
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