# Moment of Inertia/Torque Question

1. Jul 10, 2007

### kojack21

The combination of an applied force and a frictional force produces a constant torque of 34 N.m on a wheel rotating about a fixed axis. The applied force acts for 9.1s, during which time the angular speed of the wheel increases from 0 to 16 rad/s.

a)What is the moment of inertia of the wheel?

If the applied force is removed, the wheel comes to a stop in 73s.

b)What is the frictional torque?

c)How many revolutions does the wheel make during the entire 82s interval?

2. Jul 10, 2007

### mjsd

first you may want to identify some potentially useful formulas or concepts

3. Jul 10, 2007

### kojack21

Well torque= moment of inertia X angular acceleration....but how do u get angular acceleration if you dont know the radius??

4. Jul 10, 2007

### kojack21

I figured out moment of inertia, but is it the same value to use when you are trying to get frictional torque??

5. Jul 10, 2007

### Momentum09

This problem looks so familiar. This might be random, but you don't happen to be in Dr. Turner's class?

6. Jul 10, 2007

### kojack21

yeah...this homework has really been giving me problems. i just want to finish. any help would be nice. its always good to have extra help from other people in the class

7. Jul 10, 2007

### Momentum09

lol what a coincidence. I got part a and b, but c I'm just lost. Wait till you look at HW 24...it's a killer I just wanna shoot myself in the head.
For part A, use that equation Torque = mr^2 x acceleration.
You know the torque...for acceleration just take 16/9.1...plug those values in...and you are left with mr^2, which is the moment of inertia.
For part B, apply the kinematics equation...wf= w0 + at, find the acceleration, and plug it into Torque = mr^2 x a.

8. Jul 11, 2007

### kojack21

but for this question u aren't given a radius. i need q15 on hw 23 (part b)...for part a i did torque= I x angular acceleration and i got it right like you said....

for part b...how do i do it? i get what you are saying...but it says the wheel comes to REST after 73 seconds.. so if u do the kinematics equation wf=wo + at, wouldn't wf be zero?? how do u calc that??

sorry i know you're busy w/ hw 24 i just really wanna do this one before the 2am cutoff

9. Jul 11, 2007

### Momentum09

it's okay,
set wf = 0 and w0 = 16, and t = 73. solve for a, which sould be a negative #.

10. Jul 11, 2007

### kojack21

Thanks!!! if i get q16 ill let u know...i can't seem to figure it out either...i hate to bug u but how did u figure out question 18 about the velocity of the wheel?? i got 17 but i dont know how to do number 18. please let me know...i haven't answered very many questions on this hw

11. Jul 11, 2007

### Momentum09

AIM me...Sherry22