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Homework Help: Finding the velocity of the wind

  1. May 21, 2012 #1
    1. The problem statement, all variables and given/known data

    An airplane travels N40∘E at an airspeed of 1000km/h. Measurement on the ground shows that the plane is travelling N45∘E at a speed of 1050 km/h. Calculate the velocity of the wind.

    2. Relevant equations

    Vg = Va + Vw

    3. The attempt at a solution

    so i know we're suppose to use the formula Vg = Va + Vw

    we know Vg is 1050km/h N45∘E
    and Va is 1000km/h N40∘E
    we don't know Vw, so we subtract it to the other side of the equation and we have a subtraction of vectors

    I drew my vector diagram, and applied to cos law to get the veloctiy of the wind,

    a= sqrt[1050^2 + 1000^2 -2(1000)(1050)cos∘]
    and got 102.4 km/h as my wind velocity,

    can someone confirm if i'm right? :s
  2. jcsd
  3. May 21, 2012 #2


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    Staff: Mentor

    A good way that you can confirm the answer yourself is to use component-wise subtraction, and convert that back into polar coordinates. Are you familiar with converting back and forth between polar and rectangular coordinates?
  4. May 21, 2012 #3
    no.... :l
  5. May 21, 2012 #4


    User Avatar

    Staff: Mentor

    Here is a referenc for you then (partway down this page):


    Vectors are usually added and subtracted in rectangular coordinates, so a natural way to do your problem is to convert the vectors you are given into their x and y components in rectangular coordinates, do the subtraction, and then convert the answer back into the polar notation of the problem.
  6. May 21, 2012 #5
    I worked it out. It is, indeed, 102.425km/h -13° from the positive x axis, or using your notation South 76.69° East.
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