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kahensle · Oct 22, 2007, 06:45 PM

A 108 g Frisbee is 28 cm in diameter and has about half its mass spread uniformly in a disk, and the other half concentrated in the rim. With a quarter-turn flick of the wrist, a student sets the Frisbee rotating at 550 rpm.
(a) What is the rotational inertia of the Frisbee?
_________ kg·m2
(b) What is the magnitude of the torque, assumed constant, that the student applies?
_________ N·m

terryg752 · Oct 30, 2007, 04:54 AM

Originally Posted by kahensle

A 108 g Frisbee is 28 cm in diameter and has about half its mass spread uniformly in a disk, and the other half concentrated in the rim. With a quarter-turn flick of the wrist, a student sets the Frisbee rotating at 550 rpm.
(a) What is the rotational inertia of the Frisbee?
_________ kg·m2
(b) What is the magnitude of the torque, assumed constant, that the student applies?
_________ N·m

I am going to give you hints for mass 2m, diameter 2r

You can find tables for moments of inertia in any physics book:

For the rim: mr^2

for the rest of the disc: 1/2 mr^2

Add the two and you get I, total moment of inertia.

Calculate w, the angular velocity

Kinetic Energy attained = 1/2 I w^2

This should equal Work Done = Torque times 1/4 circumference

Now you can calculate Torque!