View Full Version : Rotational inertia of the Frisbee
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
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!