1. Not finding help here? Sign up for a free 30min 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!

Vectors - basic help

  1. Mar 9, 2005 #1
    vectors -- basic help

    Two vectors are given by A=-3i+4j and B=2i+3j. I need to find the angle between A and B

    I know AxB is 17k because:
    -3i+3j + 4j*2i = -9k-8k = 17k

    I'm not sure how to find the vector
  2. jcsd
  3. Mar 9, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Do you know how to find the dot product? (It's very similar to the cosine difference identity in trigonometry)

    Edit: By the way, AxB is -18k, not 17k. You have to cross multiply (-3*2)-(4*3) to find the cross product.
    Last edited: Mar 9, 2005
  4. Mar 9, 2005 #3
    my book says -17k for an answer. It might be wrong or something.

    To find the angle between A and B...
    where AXB is the cross product... and AB is ??

    not sure what the equation is says
  5. Mar 9, 2005 #4


    User Avatar
    Science Advisor
    Homework Helper

    Yeah, it's -17k. What ever gave you the idea it wasn't? (Okay, I admit, the second time I looked at it, I looked at your -3i + 3j and thought I was looking at your two vectors).

    Yes, that would work, since your cross product only has one component - otherwise you'd need the norm. A dot B over the product of the norms also works, but gives you the cosine of the angle, instead.

    In other words, you'd wind up with (-3*2)+(4*3)/(5*√13) = cos (θ)
  6. Mar 9, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    The first question you said was " I need to find the angle between A and B"
    then "my book says -17k for an answer." You do understand that "-17k" is not an angle, don't you?

    Yes, it is true that the length of uxv is |u||v|sin(θ) and you can use that to find the angle. But the cross product is much more complicated than the dot product and u.v is |u||v|cos(θ) that's a much simpler way to find the angle.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Vectors - basic help
  1. Vectors help! (Replies: 13)

  2. Vectors help! (Replies: 2)

  3. Vectors Help (Replies: 12)

  4. Vectors Help (Replies: 12)

  5. Help with vectors? (Replies: 5)