Helicopter blade energy physics problem

[Kris1120]

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.

Homework Equations

3. a. I=mr^2
=(240kg)(6.7)^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?

 

[Hootenanny]

I=mr^2

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.