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Homework Help: Analytical mechanics/vectorshelp

  1. Sep 26, 2005 #1
    VECTORS..help plz

    hi there
    i had these 2 questions that i wanted someone to plz double chek for me..

    Q1) Given vectors A= c i + c j + 3k and B= ci + j - 2k where c is any constant.
    a)Find a value of 'c' such that A is parallel to B?..
    b)Find a value of 'c' such that A and B have the same length?

    a) For this particular question I said that if A is parallel to B then the cross product shud be 0.And thereby solving for 'c', I got values -3/2;0 and 1.However after substituting each of these values of 'c' separately in the cross product A x B, none of the equations reduce to zero..So i concluded there is no such value of 'c' that makes A parallel to B...Is this right?

    b)For the lengths to be same i equated their magnitudes to find 'c'..however after doing this i got an imaginary value of 'c'= +-2i.i concluded saying that this is a complex number says only abt direction and not magnitude...So no such value of 'c' exists that makes the lengths A and B equal...!!Am i right??

    Please help me!.
    Last edited: Sep 26, 2005
  2. jcsd
  3. Sep 26, 2005 #2

    For part a, try the dot product being set equal to 1.
    For part b, I guess your answer is correct because I came up with 2i also.
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