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: Finding a scalar such that vectors p and q are parallel

  1. Oct 10, 2011 #1
    1. The problem statement, all variables and given/known data
    p = (2,k)
    q = (3,5)
    Find k such that p and q are parallel

    3. The attempt at a solution

    Well, I know that for two vectors to be parallel we need to have p = kq.

    I know the answer will be kind of obvious but I just can't get it lolll, any help please??

  2. jcsd
  3. Oct 10, 2011 #2


    Staff: Mentor

    What does it mean for two vectors (p and kq, here) to be equal?
  4. Oct 11, 2011 #3
    Hummm.... They need to have the same magnitude and direction?
  5. Oct 11, 2011 #4


    User Avatar
    Science Advisor

    (a, b) and (c, d) are parallel if and only if c/a= d/b.
  6. Oct 11, 2011 #5


    Staff: Mentor

    And what does this say about the coordinates of the two vectors?
  7. Oct 11, 2011 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Thinking geometrically...

    HallsofIvy's statement (essentially a similar-triangles argument) is equivalent to requiring that the slopes are equal.
    Alternatively, consider certain "products" involving vectors and their geometric interpretation.
  8. Oct 11, 2011 #7
    That they must be equal also, I guess...

    So... p = tq (I'm using "t" as the scalar multiplying q):
    tq = 2/3 (3,5) = 2, 10/3

    So --> p = (2, k) = (2, 10/3)

    k = 10/3 ????

    Is that the way to do it? (trying to match the numbers only) or is there a more "pro" approach to it? lolll
  9. Oct 11, 2011 #8


    Staff: Mentor

    That's the answer you want.

    You can check your answer, by confirming that q = <3, 5> and p =<2, 10/3> are multiples of one another.
  10. Oct 12, 2011 #9
    great, thanks for the help!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook