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!

Parallel vectors

  1. Jul 10, 2012 #1
    Reading through a book and this question popped up, but the question didn't actually cover parallel vectors so i'm not sure what to make of it:

    Given that c = 3i + 4j and d = i -2j find

    t if d-tc is parallel to -2i + 3j

    I done d - tc and got (1-3t)i + (-2-4t)j but i'm not quite sure how to equate this to the parallel vector -2i + 3j

    in the solutions for some reason they have done the coefficient of i in vector -2i + 3j and multiplied it by the coeffecient of j in d-tc and done the same for j, so they have

    -2(-2-4t) = 3(1-3t)

    but I don't really know how they got that, if anyone could explain thanks.
     
  2. jcsd
  3. Jul 10, 2012 #2
    If two vectors are parallel, what can you say about the coefficients of i and j ?
     
  4. Jul 10, 2012 #3
    I have no idea

    edit: searched around and if two vectors are parallel then one of them is multiplied by a scalar, so

    (1-3t)i + (-2-4t)j = k[-2i+3j]

    so 1-3t = -2k , -2-4t = 3k

    k = (1-3t)/-2

    -2-4t = 3((1-3t)/-2)
    t = -1/17
     
    Last edited: Jul 10, 2012
  5. Jul 10, 2012 #4
    Right! :smile:

    Alternatively, you could say that they have the same "slope", so

    (-2-4t) / (1-3t) = 3 / (-2)

    ie the ratio of the i and j coefficients are the same.

    (One has to be careful if one of the coefficients is zero, though.)
     
  6. Jul 10, 2012 #5
    thanks
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Parallel vectors
  1. Parallel Vectors (Replies: 4)

  2. Parallel Vectors (Replies: 13)

Loading...