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Homework Help: Helicopter blade energy physics problem

  1. Jun 25, 2008 #1
    1. A helicopter has two blades , each of which has a mass of 240 kg and can be approximated as a thin rod of length 6.7 m. The blades are rotating at an angular speed of 45 rad/s. (a) What is the total moment of inertia of the two blades about the axis of rotation? (b) Determine the rotational kinetic energy of the spinning blades.

    2. Relevant equations

    3. a. I=mr^2
    =10773.6 kg*m^2

    b. KEr=(1/2) I w(meaning angular velocity)^2
    = .5 (10773.6kg*m^2) *45 rad/s ^2
    = 1.09E7 J

    The computer program we use for homework said they were both wrong! What did I do wrong?
  2. jcsd
  3. Jun 25, 2008 #2


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    Staff Emeritus
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    Gold Member

    Re: Help!

    That is the moment of inertia of a point particle of mass m at a distance r from the axis of rotation, not a thin rod.
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