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Homework Help: Dot product and cross product of vectors

  1. Mar 12, 2010 #1
    1. The problem statement, all variables and given/known data

    This isn't so much a problem of calculation, so much as it is I need to know if i did it right, you'll see what I mean.

    vectors v=(2,0,2), u=(-1,1,0), w=(0,-1,1)
    Computer the quantities that make sense (a period denotes the dot product, x denotes cross product):

    (v.u)w

    (v x u) x w + 1

    (v x u) + (v.u)

    (v x u) . w + (v x w) . u + 1


    3. The attempt at a solution


    I said that the first one is the form of scalar * vector which equals a vector

    the second is the product two cross vectors, which is a vector, plus a scalar, which does not make sense.

    The third has the form of a vector plus a scalar, which doesnt make sense

    and the fourth:

    (v x u) . w
    is a vector cross vector, which is a vector, dotted with another which is a scalar

    (v x w) . u + 1
    is a vector cross vector, which is a vector, dotted with a vector, which is a scalar, plus 1, which is a scalar, so youre adding three scalars, which makes sense
     
  2. jcsd
  3. Mar 12, 2010 #2
    I did not see any problem here.
     
  4. Mar 12, 2010 #3
    cool, thanks
     
  5. Mar 13, 2010 #4
    wait, do you mean that you dont see what the "question" is, or you dont see a problem with my answer.

    the question asks to evaluate the ones that make sense. i evaluated the first and last ones on the test because i thought that those were the ones that make sense. i am asking if those are indeed the ones that can be evaluated.
     
  6. Mar 14, 2010 #5
    I see the question, but don't see any error on your part.
    It is easy to check this kind of things using a math package. My favourite is simpy.
    You define v in simpy this way:
    v=Matrix([2,0,2])
    You express (v x u) . w this way:
    (v.cross(u)).dot(w)
    Now you got the idea.

    Ahh a minor error in your part: in the last one you add two scalars, not three.
     
  7. Mar 14, 2010 #6
    well (v x u) . w is a scalar

    (v x w) . u is a scalar

    and 1 is a scalar

    but thanks tho
     
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