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Homework Help: Relative Velocity problem -- Help please

  1. Sep 17, 2014 #1
    A light plane is headed due south with a speed relative to still
    air of 185 km/h. After 1 h, the pilot notices that they have
    covered only 135 km and their direction is not south but 75˚ south
    of east. What is the wind velocity?

    I know I have to set up a triangle and solve from but there, but I dont even know how to draw it. Help please!
  2. jcsd
  3. Sep 17, 2014 #2
    I just can't get the question. The plane had been moving due south with a relative speed of 185km/h. Then they decided to turn 75 degrees toward south of east, distancing 135 km in an hour. The question is to figure out what the wind velocity was. Is it the question?
  4. Sep 17, 2014 #3

    Simon Bridge

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    The aircraft has an airspeed and bearing (given) but the plane has been blown by the wind so that the resulting motion over the ground is something else (also given). OP is being asked to find the direction and speed of the wind wrt the ground.

    ... you have to use the definition of the relative velocity.
    You are given the velocities plane wrt ground and plane wrt air, and you want to find the velocity air wrt ground.
    How are these velocities normally related to each other?
  5. Sep 17, 2014 #4


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    No. Its been heading/pointing south not moving south. Its movement over the ground has been affected by unexpected wind.
  6. Sep 17, 2014 #5
    ...I still dont get it :(
  7. Sep 17, 2014 #6

    Simon Bridge

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    Where did we lose you?
    Do you know how to add vectors?

    A typical example would be:
    Aircraft can fly at 60kmph wrt the air.
    The wind is blowing at 30kmph due east.
    The pilot wants to fly due south over the ground.
    Which direction does he have to point the plane?

    How would you normally do that problem?
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