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Homework Help: Helicopter bolt- find torque

  1. Mar 27, 2009 #1
    1. The problem statement, all variables and given/known data

    Consider a simple helicopter blade, attached to a bolt at its end. The bolt is made to rotate by a powerful motor, and the blade therefore rotates. The blade has a length L= 7.8 m and mass M= 100 kg. The blade is shaped approximately like a long thin, uniform rod.
    What torque must the bolt exert on the blade in order to bring it from rest to full angular speed 320 rev/min in time t= 6.70 s

    2. Relevant equations

    Before, I worked out the force that ther bolt exerts on the blade when the blade is spinning at full speed to be 4.4 x 10^5 N. thus, force= 4.4 x 10^5 N. I thought that i could just find the angular acceleration: wf=wi + at and then find the moment of inertia and use T=Ia but i dont know what the moment of inertia would be. Would it be 1/12mL^2 or 1/3mL^2

    3. The attempt at a solution
    I found the angular acceleration to be 5/rad/s^2.. but i just dont know what the moment of inertia would be becoz i worked them out using the above equations and i keep getting the wrong answer. the right answer is 7.6 x 10^3 N.m. Is T=Ia the right forumla to be using or am i on the wrong track? Any help would be muchly appreciated! thankyou!
  2. jcsd
  3. Mar 27, 2009 #2


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    Hi 9giddjl! :smile:

    (try using the X2 tag just above the Reply box :wink:)

    It's 1/12 mL2 about the centre of a rod,

    and using the parallel axis theorem it's 1/12 mL2 + 1/4 mL2 = 1/3 mL2 about the end :wink:
  4. Mar 27, 2009 #3
    i used 1/2 ML^2 for the moment of inertia and it yielded the correct answer! thankyou for ur help wise teacher!:)
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