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Difficulty solving problem

  1. Dec 10, 2006 #1
    1. The problem statement, all variables and given/known data
    going through my a level text i found a prob i cannot solve

    2. Relevant equations
    1) a helicopter of total mass 1000kg is able to remain in a stationary position by imparting a uniform downward velocity to a cylinder of air below it of effective diametre 6m. Assuming the density of air to be 1.2 kg/m^3, calculate the downward velocity given to the air

    3. The attempt at a solution
    m=1.2*28.27= 33.92 kg
    stuck i know the ans is 17.2 m/s but i dont know how to get it
  2. jcsd
  3. Dec 11, 2006 #2
    what is the formula to calculate the force on a column of air of known density and cross sectional area??
    once you figure out that formula ask yourself

    what is the downward force on the helicopter?

    if the helicopter is to stay in one place what forces must be equal in magnitude and opposite in direction?
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