# Homework Help: Rotational kinetic energy question

1. Nov 20, 2009

### physicsbro

1. The problem statement, all variables and given/known data
A car is designed to get its energy from a rotating flywheel with a radius of 1.90 m and a mass of 487 kg. Before a trip, the flywheel is attached to an electric motor, which brings the flywheel's rotational speed up to 5150 revolutions per minute. Find the kinetic energy stored in the flywheel.If the flywheel is to supply energy to the car as would a 11.3 hp motor, how long could the car run before the flywheel would have to be brought back up to speed?

2. Relevant equations
KE=(1/2)Iw^2

3. The attempt at a solution

2. Nov 20, 2009

### rock.freak667

so the ke of the flywheel is?

3. Nov 20, 2009

### physicsbro

I don't know.

4. Nov 20, 2009

### rock.freak667

$$E_k=\frac{1}{2} I \omega^2$$

do you know ω? Do you know what I is for a flywheel?

5. Nov 20, 2009

### tiny-tim

Welcome to PF!

Hi physicsbro! Welcome to PF!

(have an omega: ω and try using the X2 tag just above the Reply box )
You need to learn the moment of inertia of the simple shapes …

see eg http://en.wikipedia.org/wiki/List_of_moments_of_inertia" [Broken]

Last edited by a moderator: May 4, 2017
6. Nov 20, 2009

### physicsbro

I'm not sure if this is right, but this is what i got for ω. i thought it would be 10300π/60s or 539.3 rad/s. I don't what I is for a flywheel, i want to say (1/2)mr^2.

7. Nov 20, 2009

### physicsbro

I found the KE to be 1.28E+08 J, any hints for the second part of the question?

8. Nov 20, 2009

### ideasrule

What is 11.3 hp in joules per second?

9. Nov 20, 2009

### physicsbro

Thanks, i got it.