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: Linear independancy/dependancy

  1. Apr 25, 2005 #1


    User Avatar

    hi guys just want to clarify something...

    If we are givin (vectors w u and v) w=au +bv what can we say about w
    a) if u and v are independant?
    b) if u an v are dependant?

    this is what i got so far.
    a) w cannot be writtin as a scalar multiple of u and v and are therefore not coplanar.
    b)w can be written as a scalar multiple of u and v and are therefor coplanar.

    but i think there is more to it. I don't think my answer explains enough.

    in a) can i say that w=0?... just trying to find more properties of w for a) and b)... any help would be appreciated. thnks.

    - Tu
    Last edited: Apr 25, 2005
  2. jcsd
  3. Apr 25, 2005 #2


    User Avatar
    Science Advisor

    No, w is coplanar with u and v by virtue of being a linear combination of them. What is true is that if u and v are independent, then w is not a multiple of one or the other and is not "co-linear" with either. If u and v are dependent, then one is a multiple of the other, w is a multiple of each, and w points in the same direction as both u and v (which are "co-linear" since they are dependent).
  4. Apr 25, 2005 #3
    We should know that any two vectors in a N-Dimensional space where N>1 should be coplanar since Plane is 2-dimensional , that is, any of the 2 vectors should lie on the same plane.
  5. Apr 25, 2005 #4
    what? im pretty sure thats wrong dude, what about two vectors in three space? your thinking is limited to 2 dimensions or 2-space..BTW Nx2, does that happen to be in the Harcourt mathematics 12 geometry and discrete math text book?
  6. Apr 25, 2005 #5
    In order to define a plane (through the origin), you need two (linearly independent) vectors. So it doesn't make much sense to talk about two vectors being coplanar. Sure, they're always both in some plane together, though, specifically, the plane that they define (if they're independent)!
    Last edited: Apr 25, 2005
  7. Apr 25, 2005 #6


    User Avatar
    Science Advisor

    No, Guppy is completely correct and you are completely wrong. Any two vectors define either a line (if one is a multiple of the other) or a plane (if one is NOT a multiple of the other).
  8. Apr 25, 2005 #7
    Questions of that form appear in numerous texts, MiniTank, though it would be odd if you were right.
  9. Apr 25, 2005 #8
    ya your right. my mistake
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook