# Homework Help: Inertia Question

1. Apr 30, 2012

### the_man

1. The problem statement, all variables and given/known data

The full metal wheel of radius 30 cm, 300 kg of mass moment of inertia I = 1/2 (mr ^ 2) operating torque 50nm in time 1 min. Determine the angular acceleration, angular speed of wheel, revolutions per second, revolutions that the wheel makes to achieve this speed, kinetic energy.

2. Relevant equations

ω = 2∏/T, s=1/2(αt^2), Ek=(Iω^2)/2, I = 1/2 (mr ^ 2)

3. The attempt at a solution

First I can get ω, T=60sec, so ω=∏/30...

So I can get all of this using those few formulas, but in the end I get kinetic energy in mJ??

2. Apr 30, 2012

### SammyS

Staff Emeritus
Why not get kinetic energy in Joules ?

3. Apr 30, 2012

### the_man

I can get energy in Jules but then its very small number, but i dont get it why is a small number?

4. Apr 30, 2012

### Staff: Mentor

The torque is acting for 1 minute; The given time of 1 minute is not the period of oscillation or rotation...

5. Apr 30, 2012

### the_man

Okay. I got it! Now its Ek=7,5kJ

6. Apr 30, 2012

### Staff: Mentor

Perhaps you should elaborate on your calculations; To me, 7.5 kJ doesn't look right.

7. Apr 30, 2012

### the_man

M=F*r
F=m*α
α=ω/T
Ek=(Iω^2)/2

8. Apr 30, 2012

### Staff: Mentor

I see a collection of formulas, at least one of which is rather dubious, but no calculations. Can you show your numerical work and explain what the steps are that your taking?