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Question on Energy conservation - Car pushing on air column

  1. May 5, 2014 #1
    Question on Energy conservation -- Car pushing on air column

    1. The problem statement, all variables and given/known data
    An automobile moving through the air causes the air velocity to decrease and fill a larger flow channel. An automobile has an effective flow channel area of 3 m^2. THe car is traveling at 90 k/h on a day when the barometric pressure is 75 cm of mercury and the temperature is 30 C. Behind the car, the air veloctiy is measured to be 82 km/h, and the temperature is 30 C. Determine the power required to move this car through the air and the effective flow channel behind the car?

    2. Relevant equations

    Here we need to find the mass flow rate through (A1V1)/specific volume and calculate the change in kinetic energy of the wind

    3. The attempt at a solution
    The only part I dont understand is how the power required by the car is equal to energy change of the wind and how the velocity of the wind initially is 90kmph?
  2. jcsd
  3. May 5, 2014 #2


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    2017 Award

    Staff: Mentor

    Neglecting other losses, this is just energy conservation.
    As seen by the car, what else do you expect?
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