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 angle through cross and dot product

  1. Jan 31, 2013 #1
    By using both cross and dot products, the angle between 2 vectors can be found. But there is 1 question that I tried for countless times that the result of cross product and dot product are not the same.

    Here is the vectors that I am talking about

    A= 2i+3j=k
    B= -4i+2j-k

    The result by using dot product = 100.08°
    The result by using cross product = 79.92°
  2. jcsd
  3. Jan 31, 2013 #2
    What is 2i+3j=k?

    Show your steps.
  4. Jan 31, 2013 #3
    My mistake. Its A= 2i+3j+k


    abs(AxB) = 16.88

    abs(AxB)=abs(A) x abs(B)sinθ

    ∴ abs(AxB)/(abs(A) x abs(B))= sinθ
    ∴ θ = 79.92


    A.B/(abs(A) x abs(B))= cos(theta)

    ∴θ = 100.08

    The result of dot and cross product should be the same but its not in this case. Please help.
    Last edited: Jan 31, 2013
  5. Jan 31, 2013 #4
    Are you using matrices to solve the cross product?
  6. Jan 31, 2013 #5
    Yeap.. I used the determinant method to solve the cross product..
  7. Jan 31, 2013 #6
    Observe that sin 79.92 = sin 100.08. So the cross product per se cannot distinguish between such angles.
  8. Jan 31, 2013 #7
    Im sorry but I still have a question here.. What is per se?
  9. Jan 31, 2013 #8
  10. Jan 31, 2013 #9
    Thanks a lot guys.. Appreciate you help.. :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook