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: Vectors problem

  1. Oct 28, 2004 #1
    It's given that vector c is perpendicular to both vectors a and b. Which of the following is true:

    1) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_01.gif [Broken]
    (a x b) x c
    2) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_02.gif [Broken]
    (a x c) dot b
    3) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_03.gif [Broken]
    (a x c) x b
    4) http://phstudy.technion.ac.il/~wn114071/physweb/question/1_4_04.gif [Broken]
    (a x b) dot c

    Can someone please just confirm the answer is (1) because I have only one attempt left at submitting an answer for this question.

    Thanks :smile:
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Oct 28, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    I can't open the files. You need authorization for that...
  4. Oct 28, 2004 #3
    I knew that might happen, so I wrote the expression in each link below. (1) is (a x b) x c, i.e (a cross b) cross c.

  5. Oct 28, 2004 #4

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It's impossible to say which one is true, because those are all expressions, not equations.
  6. Oct 28, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper

    Answers 2 and 4 give you a scalar number. The absolute value is the same for both, but the sign changes.

    Answer 1 gives you a null vector (all components equal to 0) since the cross product of A and B is either in the same direction as C or the exact opposite direction of C.

    Answer 3 gives you a vector.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook