# Homework Help: Rotation, Power, Inertia

1. Apr 6, 2004

### PhysicsPhun

Delivery trucks that operate by making use of energy stored in a rotating flywheel have been used in Europe. The trucks are charged by using an electric motor to get the flywheel up to its top speed of 198π rad/s. One such flywheel is a solid, homogeneous cylinder with a mass of 530 kg and a radius of 1.0 m. If the truck operates with an average power requirement of 8.5 kW, how long can it operate between chargings?

Just a push in the right direction, i've written out so many formulas relating to rotation and Power/Work. And i haven't found a starting point.

2. Apr 6, 2004

### Staff: Mentor

Start by figuring how much rotational KE is stored in the flywheel when it rotates at top speed.

3. Apr 6, 2004

### PhysicsPhun

(edit again)
I solved for I with I=(1/2)mr^2
I plugged I and w into K= Iw^2 and solved for K.
Whew..

I'll keep workin from here, thanks.

Last edited: Apr 6, 2004
4. Apr 6, 2004

### Staff: Mentor

Rotational KE

I'm not sure what that equation is supposed to be. Here's another hint: The rotational inertia of a cylinder about its center is I = 1/2 M R2. You'll need it.

5. Apr 6, 2004

### PhysicsPhun

I=(1/2)mr^2
I=265
(edited)

I'm thinkin my next step is to find alpha or torque, i don't see how to do that but there is
a_r=w^2r
What is a_r?

Last edited: Apr 6, 2004
6. Apr 6, 2004

### PhysicsPhun

Oh, I didn't know what a fly wheel was really. : )

7. Apr 6, 2004

### ShawnD

I get the "I" as being something completely different.

$$I = \frac{1}{2}mr^2$$

$$I = \frac{1}{2}(530)(1)^2$$

$$I = 265$$

Once you get the energy

$$E = Pt$$

8. Apr 6, 2004

### PhysicsPhun

Ya shawn, you're right.

9. Apr 6, 2004

### PhysicsPhun

Am so out of it. disregard this reply :)

10. Apr 6, 2004

### ShawnD

Power is 8500. Time is what you are supposed to find.

11. Apr 6, 2004

### PhysicsPhun

Got it, Thanks Shawn and Doc Al.

Brain moving slow today.