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Homework Help: Vector displacement problem, Help please!

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data

    The route followed by a hiker consists of three displacement vectors A, B and C. Vector A is along a measured trail and is 2500 m in a direction 25.8° north of east. Vector B is not along a measured trail, but the hiker uses a compass and knows that the direction is 43.1° east of south. Similarly, the direction of vector C is 32.2° north of west. The hiker ends up back where she started, so the resultant displacement is zero, or A + B + C = 0. Calculate the magnitude of vector B.

    2. Relevant equations

    This is for my physics course. The teacher expects us to know vectors already, and I have never had them. I have tried for over 3 hours to solve this question. No luck.

    3. The attempt at a solution

    I realize that the vectors form a triangle since the displacement is 0. So I tried to assign the correct values to the interior of the triangle, and then use the law of sines to find the magnitude of vector b.

    I bet there is a better way to solve this but I have no idea.

    Please help me guys!

  2. jcsd
  3. Sep 7, 2009 #2
    That's the correct way to do it.
  4. Sep 7, 2009 #3
    I guess I am just getting the incorrect angles.
  5. Sep 7, 2009 #4


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    Homework Helper

    malik123, show your work so we can see what you've done. We can look for your error.
  6. Sep 7, 2009 #5
    Amazingly quick response. Ok! one sec
  7. Sep 7, 2009 #6
    Ok the angles I got were:

    58 degrees, opposite to the B side.
    14.7 opposite to 2500.
    107.3 opposite to the x axis.

    I found these using alternate angles. I think I most certainly did it wrong, cause my answer is not correct.

    Then I used the law of sines,

    Sin 14.7/2500 = sin58/ B

    B = 8354.9

    Which is wrong!!

    Just so you don't get the wrong idea. I am not trying to come get a free answer. I just need a little help or direction, and then I can try once again to solve the problem!

    Last edited: Sep 7, 2009
  8. Sep 7, 2009 #7
    Show how u used Laws of Sine cos. I found the angles to be the same as yrs.:wink:
  9. Sep 7, 2009 #8
    I just did. So what did I do wrong?
  10. Sep 7, 2009 #9
    Lol, I was typing my comment while u were typing yr solution. K, anw, tht's the same answer I got.
  11. Sep 7, 2009 #10
    Thanks for the help.

    For some reason though, the online answer checker wont accept this answer as correct. =(
  12. Sep 7, 2009 #11
    Hey do you go to UGA or use a website called loncapa?
  13. Sep 8, 2009 #12
  14. Sep 8, 2009 #13
    Your interior angles are wrong, 58 is right though. Draw a picture of where A and B meet. Draw a horizantal line at the vertex and label what you know.
  15. Sep 8, 2009 #14
    I tried again and I keep getting the same angles.

    I don't know what I am doing wrong!
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