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Find orthogonal vector to current vector in 3D

  1. Dec 17, 2012 #1
    Hi,

    In 2D I know a simple answer: vector (a,b) is orthogonal to vector (-b,a)

    Is there anyway similar to that to find an orthogonal vector in 3D?
     
  2. jcsd
  3. Dec 17, 2012 #2
    You can use the dot product. For example, if you have a vector v and want to find vector c that is orthogonal to v, then use the dot product <v,c> and set it equal to 0.

    Example:
    v = (4,2,3)
    c = (x,y,z) = ?

    (i) Set the dot product to zero:
    <v,c> = 4x + 2y + 3z = 0

    (ii) Choose some values for x and y, e.g. x=0 and y=-3

    (iii) Solve the equation in (i) for z:
    z = 1/3*(-4x-2y) = 1/3*(0+6) = 2

    Result: c = (0,-3,2)

    ---

    Another possibility is to use the cross product.
    If vector v is given, choose some vector p (not parallel to v) and form the vector c = v x p.

    Example:
    v = (4,2,3)

    (i) Choose an arbitrary vector p (not parallel to v):
    p = (0,0,1)

    (ii) Form the vector c = v x p (cross product):
    c = (4,2,3) x (0,0,1) = (2,-4,0)

    ---

    Note that there are infinitely many vectors that are orthogonal to a given vector.
     
  4. Dec 17, 2012 #3
    Many thanks

    I knew dot and cross product, but because I write code so I need the simplest way to boost performance. As in 2D case I don't need to calculate anything, just use the trick. Also that it works for normalized vectors which doesn't need square root, a slow operation. I hope there are some tricks like that in 3D

    Regards
     
  5. Dec 17, 2012 #4

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    There isn't a unique vector orthogonal to a given vector in 3D. If the vector doesn't need to have any other properties, the same "trick" works. A vector orthogonal to (a, b, c) is (-b, a, 0), or (-c, 0, a) or (0, -c, b).

    But if you want a unit orthogonal vector, you will have to use something like a square root.
     
    Last edited: Dec 17, 2012
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