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Helicopter problem

  1. Oct 23, 2005 #1
    There were two posts reltaed to this question but both of them are so contradictory; the question goes

    a 13,000kg helicopter accelerates upward at 0.41m/s^2 while lifting a 900kg car. to the nearest newton what is the lift force exerted by the air on the rotors?

    now one post would be

    Fnet = (13000)(9.8) + (900)(9.8)

    which in a sense is what we are looking for (but unfortunately, is wrong for some reason)

    and so i don't know what piece of hte puzzple im missing; im sure the 0.41m/s^2 comes in at some point. Is it the force required to accerleate the helicopter? If so what would that be.. im lost.
  2. jcsd
  3. Oct 23, 2005 #2


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    You're right, the acceleration does come in at some point! :)

    HINT: The sum of the forces is mass times acceleration.
  4. Oct 23, 2005 #3
    i cn't tell what im getting wrong becuase its an online question where i plug in the values and get answers.. and thing is i keep getting values off by like. twenty or something. what i did is, fully:

    (13000)(9.8) + (900)(9.8) ------- (1)

    then i obtained

    (13909)(0.41) ------- (2)

    added the two together. am i right?
  5. Oct 23, 2005 #4


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    Ask yourself what is the direction of each of the forces?
  6. Oct 23, 2005 #5
    each of the forces as in?
  7. Oct 23, 2005 #6


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    As in the force of gravity and the force of lift. You were on the right track:

    [tex]F_{lift} - M g = Ma[/tex]

    where M is the combined mass so [itex]F_{lift} = M(g + a)[/itex]. I am not totally surprised that the numbers are off since it makes no sense to ask for the lift to the nearest Newton given data accurate to only two significant digits (shame on the authors for doing that!)

    Using g = 9.8 m/s^2 I get 141,919 N (blindly retaining all the digits after the 4).
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