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Relative Velocity, stumped

  1. Sep 29, 2011 #1
    1. The problem statement, all variables and given/known data

    A Boeing is flying east at 150 km/hr, After 2 hours it is 350 km East and 74 km south of its starting point. What is the magnitude and direction of the wind velocity?

    2. Relevant equations
    Not sure if any

    3. The attempt at a solution

    Well I really had no idea what to do so i started by finding the resultant vector of the distance after 2 hours, which came out to be 357.8 KM at -.00369 degrees...

    My guess is that since I found this, I could make a vector for 150 km/h, and a vector for 357.8 km and the resultant of that would be the Wind velocity vector, to get the angle and then use law of cosines, i would add 180 to the -.00369 degrees and then go forward with law of cosines..

    I also just converted the new displacements given to velocities, using V = d/t to get a resultant of 178.4 km/h with the same angle

    Does this sound right or just absolutely absurd.. I hate relative velocity problems with a passion so if anyone can explain them briefly in general it'd be very much appreciated.
  2. jcsd
  3. Sep 29, 2011 #2
    Suppose I run on a surface that can move under me. If I run at 10km/h east, and the surface moves me southeast as I run, from my reference frame, what does my vector look like?

    In other words, what would the vector of the plane look like without any wind?

    I would start there. Then I would deal with where the plane actually is, and finally find what the wind vector was.
  4. Sep 29, 2011 #3
    Thanks for the advice GrantB, What I did now was I took what it would look like after two hours (300 km) east, and put the vector sum of 357.8 km which is what it actually is and connected them with a vector which would be the Wind vector... The problem now though is i have no angles for this triangle to use law of sines or law of cosines to solve as its not a 90 degree triangle.
  5. Sep 29, 2011 #4
    Yeah I believe you will have to use the Law of Cosines.

    You should be able to find the angle between the windless-plane and the wind-plane.

    Glad I could be of help :]
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