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Vector and scalars

  1. Mar 30, 2012 #1
    1. The problem statement, all variables and given/known data
    A plane flies from base camp to Lake A, 280 km away inthe direction 20.0°north of east. After dropping off sup-plies, it flies to Lake B, which is 190 km at 30.0°west of north from Lake A. Graphically determine the distance and direction from Lake B to the base camp.

    2. Relevant equations



    3. The attempt at a solution
    I've drawn a line from base camp to lake A with an angle of 20°, another line from lake A to lake with an angle of 30°.
    I have calculated the components of vector A, I already know the [tex]|A|=280km, |B|=190km[/tex], so:
    [tex]A_{x}=Acos(\frac{pi}{9})=263km[/tex],
    [tex]A_{y}=Asin(\frac{pi}{9})=95.8km[/tex],
    [tex]B_{x}=Bsin(\frac{-pi}{6})=-95km[/tex],
    [tex]B_{y}=Bcos(\frac{-pi}{6})=165km[/tex];
    [tex]R^→=A^→+B^→[/tex];
    [tex]|R|=sqrt((168^2)+(261^2))=310km[/tex],
    [tex]cosσ=\frac{168}{310}→σ=57.2°[/tex],
    [tex]sinσ=\frac{261}{310}→σ=57.4°[/tex];
    I get the exact result because:
    [tex]B_{x}=Bsin(\frac{-pi}{6})=-95km[/tex],
    [tex]B_{y}=Bcos(\frac{-pi}{6})=165km[/tex];
    i don't know why,
    In which quadrant are the vector B and R?
     
    Last edited: Mar 30, 2012
  2. jcsd
  3. Mar 30, 2012 #2

    HallsofIvy

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    "30 degrees West of North" is in the second quadrant (the x-component is negative ("West") and the y component is positive ("North")). Surely that is easy to see.

    Which quadrant R, the return route, is in is a little harder. Assuming your calculations are correct, since the x and y coordinate are both positive, it is in the first quadrant. However, it is hard to tell whether your R is correct because you don't say where you got the "168" and "261" that you use in your calculations. I suspect they are A+ B but, if so, that is incorrect. The location after the second leg is A+ B. The path necessary to fly back to the base camp is -(A+ B). That would be in the third quadrant.
     
  4. Mar 30, 2012 #3
    yes I've got them from A+B=R,
    so R=-(A+B)->|R|=sqrt((-168^2)+(-261^2))=310km is the distance from lake B to base camp;
    and if i calculate σ = arctan Ry/Rx->σ=57.2° south of west.
    OK 30° west of north are 30° beetween y axis and vector B^->, 30°+90°= 120°=2pi/3 in radiant.
     
    Last edited: Mar 30, 2012
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