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Homework Help: Cross product question.

  1. Jun 9, 2006 #1
    heres the problem.

    im supposed to use geometric definition to find. (i+J)cross(i-j)

    I know: v x w=IIvIIIIwIISin[theta] and that The answer comes out to be -2k

    But what i dont get is looking at the solution manual they show [theta]=pi/4 which i have know clue where that comes from.

    I was thinking that i could use (v)dot(w) =Cos[theta] to get[theta] but that ends up being pi/2 not pi/4 like the solution manual says.

    I think the solution maunual is wrong after futher review [Theta} should equal pi/2, not Pi/4
    Last edited: Jun 9, 2006
  2. jcsd
  3. Jun 9, 2006 #2
    I take it you did not preview your post. Or maybe I'm drunk and don't know it.... but what are you saying?
  4. Jun 9, 2006 #3
    well i think i may have wasted an hour due to 2 things,
    1: the soution manual is wrong [theta]=pi/2
    2:the dot product equals 2 and for somereason i was taking the inverse sign instead of the inverse cosine wheni entered it so that was confusing me. I think i can chaulk this mistake up to dyslexia possibly though.

    if im still wrong please correct me(ps but not on my spelling/grammer that has always sucked only slightly improves every yr).
  5. Jun 9, 2006 #4
    Oh wait...

    IIvII = [itex] |\vec v | [/itex]
  6. Jun 9, 2006 #5
    I guess I'm confused with this:

    [tex] (i+J) \times (i - j) [/tex]

    is [itex] i [/itex] supposed to be [itex] \hat i [/itex], or a complex number, or what?

    You also have [itex] J [/itex] and [itex] j [/itex] so that is confusing...

    you're grammar seems to be fine, it the way you express the mathematical statements that I'm unsure...
  7. Jun 9, 2006 #6
    i totally need this latex thing to simplify life
    no but i think this may be right
    [itex] ||\vec v || [/itex]
  8. Jun 9, 2006 #7
    sorry about the confusion i should probally go and figure out that whole latex way of writting stuff out as to make it easier to read.
  9. Jun 9, 2006 #8
    So here is the question re stated what is the angle between the 2 vectors v and w where:
  10. Jun 9, 2006 #9
    How do you find the angle between two vectors in two dimensional space?
  11. Jun 9, 2006 #10
    yeah i guess thats another way of saying it, but i think i figued it out.
  12. Jun 9, 2006 #11
    what did you get?

    In regards to this question:
    So here is the question re stated what is the angle between the 2 vectors v and w where:

    I got pi/2... but shoot, it's been awhile. Maybe I'm doing this wrong.
  13. Jun 9, 2006 #12
    i think your right its the darn solution manual and the people at Wiley publishing that have no clue what they are doing.
  14. Jun 10, 2006 #13
    I hate crap like that. It makes you second guess what you are doing, even when you are doing it right.
  15. Jun 10, 2006 #14


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    Staff Emeritus
    Science Advisor
    Gold Member

    If you're typing equations in ASCII, you should use the pipe symbol (|) to denote absolute values, norms, and stuff:

    ||v x w|| = ||v|| ||w|| |sin theta|
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