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Homework Help: Relative velocities

  1. Mar 15, 2010 #1

    slpnsldr

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    1. The problem statement, all variables and given/known data
    The wind is blowing at 72km/h south relative to the earth, and a plane is traveling 320km/h (28 degrees south of west relative to the wind) directly above a city. give displacement from the city in 2h time


    2. Relevant equations
    Plane is P wind is W earth is E

    VPW=VPE+VWE



    3. The attempt at a solution

    re arrange to find that
    VPE=VPW-VWE
    VPE=248km/h

    The only thing I dont understand is how to determine the new angle of the plane to the earth...
     
    slpnsldr, Mar 15, 2010
  2. jcsd
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  3. Mar 15, 2010 #2

    rl.bhat

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    Homework Helper

    Velocities are vectors. You cannot add or subtract them directly.
     
    rl.bhat, Mar 15, 2010
  4. Mar 15, 2010 #3

    benhou

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    Hi slpnsldr. Your formula was incorrect. It should look like this
    [tex]\vec{_{p}V_{E}}=\vec{_{p}V_{w}}+\vec{_{w}V_{E}}[/tex]

    Notice when you add these relative velocities, the subscript "w" cancel each other out. And that is the only trick to it.
     
    benhou, Mar 15, 2010
  5. Mar 15, 2010 #4

    slpnsldr

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    ok, ive figured out my displacement.. but im still unclear about how to find the planes new angle
    somethign to do with the sine law?
     
    slpnsldr, Mar 15, 2010
  6. Mar 15, 2010 #5

    benhou

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    Oh, that might be hard to explain without a diagram. Maybe you need to include a diagram.
     
    benhou, Mar 15, 2010
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