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Vectors with an airplane and blowing wind; Also another w/ no idea how to describe

  1. Feb 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Assume all speeds are constant.

    (a) A plane flies at speed vstill = 203 km/h in still air. Now, there is a wind blowing at speed vw = 77.6 km/h, with its direction at 30.4 degrees to the east of north. If the pilot wishes to fly due north with the wind blowing, find:
    - θ, the angle between the direction the plane flies and due north.


    - vp, the speed at which the plane will fly relative to the ground.



    (b) A man walks up a stalled escalator in time tw = 89.3 s. If he stands on the moving escalator, he reaches to top in time tm = 26.4 s. Find twm, the time it would take him to reach the top if he walked up the moving escalator?

    2. Relevant equations
    The only one I think that is possible is the Law of Cosines.


    3. The attempt at a solution
    I have one vector going directly North because that is what we began with. Then there is a Northeast wind blowing, which is 30.4°. I put this vector's tail at the tail of the first vector going North. The resultant vector is then 141.8 if we use the Law of Cosines. As for the angle that the plane must fly, I'm at a loss, and have no idea. You don't necessarily have to tell me the answer, but where do I begin?

    For the second question, I just have no idea at all.
     
    Last edited: Feb 14, 2012
  2. jcsd
  3. Feb 14, 2012 #2
    Re: Vectors with an airplane and blowing wind; Also another w/ no idea how to describ

    I know I shouldn't be double-posting, but I just wanted to bump it so that people know I edited the post. I saw a mistake, and now it better reflects where I'm stuck at.
     
  4. Feb 15, 2012 #3
    Re: Vectors with an airplane and blowing wind; Also another w/ no idea how to describ

    Hint for elevator problem:

    Write expressions for the velocity of man walking up steps to top and also for elevator taking him to top. What is velocity if he walks up moving steps?
     
  5. Feb 15, 2012 #4

    HallsofIvy

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    Re: Vectors with an airplane and blowing wind; Also another w/ no idea how to describ

    Draw a picture with a vertical line representing the desired northward flight. From the bottom of that line, draw line segment at 30.4 degrees to it, marking its length as 77.6. Finally, use compasses set to draw, from the tip of that segment, a circle with radius 203.
    However, there is no "SSA" rule for congruent triangles. Unless that last segment, representing the flight of the airplane, is perpendicular to the first, there will be two points of intersection giving two solutions.



    Let the length of the escalator be L. Then his walking speed is L/89.3 and the speed of the escalator itself is L/26.4. Combining them will give him a speed of L/89.3+ L/26.4.

     
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