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Homework Help: Is the first question worked right and any hints on the second please

  1. May 15, 2010 #1
    1. The rotating blade of a hovering helicopter sweeps out an area of radius 4m imparting a downward velocity of 12ms^-1 to the air displaced. Find the mass of the helicopter.
    (g=10ms^-2, density of air= 1.3 kgm^-3)

    3. The attempt at a solution
    F=d/dt(mv)
    F= (density of air*volume)*12
    = 1.3*pie*16*12*12
    =9409N
    mass of helicopter= 940kg (the answers matched, but is it worked correct?)

    2. The speed of rotation of the blades of the helicopter in question 1 is increased so that the air now hasa downward velocity of 13ms^-1. Find the upward acceleration of the helicopter.
    attempt at the solution
    F=mv
    11043N=1.3(pie*16*13*13
    F=ma
    11043N=940*a

    11043/940=a

    Actual answer= 1.7ms^-2
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. May 15, 2010 #2

    phyzguy

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    Science Advisor

    The first part looks right, but in the second part remember that the 12 m/s air velocity just held the helicopter stationary. So to calculate the acceleration, you need to calculate the difference between the force that held the helicopter stationary (12 m/s air) and the increased air velocity (13 m/s air.). You calculated the entire force from the 13 m/s air and forgot to subtract off that part of the force which is compensating gravity.
     
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