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Homework Help: Dot product

  1. Jun 30, 2008 #1
    1. The problem statement, all variables and given/known data
    A. If A*B=-(1/2)AB, what do you know about A and B.
    B. If A*B=5Ay-7Ax, find B.

    2. Relevant equations

    3. The attempt at a solution
    Part A I know the angle is 120 degrees in between A and B. Would I know more about the two vectors from that?
    Part B. I don't know where to begin to try and answer that?
  2. jcsd
  3. Jun 30, 2008 #2
    For A, it could be also be 240 degrees.

    For B, think in terms of the components of the vectors. How do you represent a scalar/dot product of two vectors in terms of their components?
  4. Jun 30, 2008 #3
    AxBx+AyBy with that I should be able to find my B?
  5. Jun 30, 2008 #4
  6. Jun 30, 2008 #5
    Do I use what I know in the first part of the problem as well or would this be a totally different scenario??
  7. Jun 30, 2008 #6
    A better question that you must ask yourself should be, does the the answer to the first problem help you in anyway in solving the second, given that you know component-wise multiplication of vectors?
  8. Jun 30, 2008 #7
    So Bx and By would simply be 1 since they are not in the problem right?????
  9. Jul 1, 2008 #8
    Of course not.You have to solve in the context of the upper question.
  10. Jul 1, 2008 #9
    According to the teacher part A and B are unrelated, they are two separate problems, in that case bx and by would be 1 correct?
  11. Jul 1, 2008 #10


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    Science Advisor

    Part B tells you that A*B=5Ay-7Ax. And, of course, A*B= AxBx+ AyBy. Just compare those two equations!
  12. Jul 1, 2008 #11


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    Science Advisor
    Homework Helper

    Hi student 1! :smile:

    (have you noticed the new X2 and X2 tags? :biggrin:)

    Hint: If 5Ay - 7Ax = A.C also, what can you say about B - C? :wink:
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