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Homework Help: Air Drag Doubts

  1. Jan 9, 2006 #1
    Hi,

    I have second doubts on air drag problem.

    Let's say an object is moving with speed v_obj = 15 m/s and that the air is moving with speed v_wind = -2 m/s. What is the the force of air drag? If There would be no wind. The force would be 1/2*drag coeff.*density of air*area*v_obj^2. But with the wind blowing, should I say that there are 2 forces acting, force of wind which is 1/2*drag coeff.*density of air*area*v_wind^2 + force due to movement of the object in the air 1/2*drag coeff.*density of air*area*v_obj^2. Soo the solution is 1/2*drag coeff.*density of air*area*(v_obj+v_wind)^2. Soo the speed we use is actually 15 m/s - 2m/s. Which will give the same resoult as we would be moving trough still air with speed of 13 m/s? That doesnt seem logical to me.
     
  2. jcsd
  3. Jan 9, 2006 #2

    Astronuc

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    Drag on an object is dependent on the relative fluid velocity with respect to the object, so if a plane is moving at 15 m/s (wrt to coordinate system - e.g. ground) and the air is moving at -2 m/s (wrt same coordinate system as plane) then the relative velocity is 17 m/s, and this would be the same as plane at 17 m/s in still air.

    The coordinate reference is key.
     
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