# Homework Help: Hard Rotational Kinetic energy question (Flywheel)

1. Sep 12, 2014

### orangeincup

1. The problem statement, all variables and given/known data
A motor imposes 5 Nm on a shaft of a 100kg flywheel which is 80cm(diameter)

What is the rotational kinetic energy after 10 mins? assuming the flywheel was at rest and the mass moment of inertia of the shaft is negligble.

2. Relevant equations
Ig=1/2 * mr^2
RKE=1/2*Ig*ω^2
T=Ig*Ac
ω=ω0+ Ac*t

3. The attempt at a solution
Can someone look at this and tell me what I should change? I have no idea if it is right or not.

Solving for moment of inertia: Ig= 1/2 * m*r^2, Ig=1/2*(100kg*9.81m/s^2)*(.8m/2)^2
Ig=78.4(not sure units)

Solving for acceleration:
T=Ig*Ac
5/78.4=Ac
0.063=Ac (not sure units)

Solving for ω:
ω=ω0+Ac*t
ω=0+0.063*(10minutes*60)

Solving for RKE:
RKE=1/2*Ig*ω^2
RKE=56010.5 J

2. Sep 12, 2014

### Staff: Mentor

Why did you multiply by g? You want the mass, not the weight.

Other than that, your method looks OK.

3. Sep 12, 2014

### orangeincup

Opps. Thank you.

4. Sep 12, 2014

### SteamKing

Staff Emeritus
Make sure the information given in the problem matches the information required for your formula.

Is the radius of the flywheel the same as its diameter?

If you are not sure of the units of your answer, calculate them according to the formula you are using.

If inertia = 1/2*m*r^2, then the units will be kg (mass) * m^2 (radius^2), or kg-m^2

Should be 'Oops'. 'Opps' rhymes with 'tops', while 'oops' rhymes with 'loops'.

5. Sep 12, 2014

### Staff: Mentor

It was divided by 2.