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Dot product

  • Thread starter student 1
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  • #1
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Homework Statement


A. If A*B=-(1/2)AB, what do you know about A and B.
B. If A*B=5Ay-7Ax, find B.


Homework Equations





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?
 

Answers and Replies

  • #2
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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?
 
  • #3
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AxBx+AyBy with that I should be able to find my B?
 
  • #4
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Yes.
 
  • #5
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Do I use what I know in the first part of the problem as well or would this be a totally different scenario??
 
  • #6
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Do I use what I know in the first part of the problem as well or would this be a totally different scenario??
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?
 
  • #7
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So Bx and By would simply be 1 since they are not in the problem right?????
 
  • #8
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Of course not.You have to solve in the context of the upper question.
 
  • #9
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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?
 
  • #10
HallsofIvy
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Part B tells you that A*B=5Ay-7Ax. And, of course, A*B= AxBx+ AyBy. Just compare those two equations!
 
  • #11
tiny-tim
Science Advisor
Homework Helper
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B. If A*B=5Ay-7Ax, find B.
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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