Kinetic energy stored in the flyweel

[jeneekim]

Homework Statement

A car is designed to get its energy from a ro-
tating flywheel with a radius of 2.19 m and
a mass of 679 kg. Before a trip, the fly-
wheel is attached to an electric motor, which
brings the flywheel’s rotational speed up to
7310 rev/min.
Find the kinetic energy stored in the fly-
wheel.
Answer in units of J.

Homework Equations

KE = 1/2I$$\omega$$²
I = mr²

The Attempt at a Solution

given:
r = 2.19 m
m = 679 kg
$$\omega$$ = 7310 rev/min = 765.50141 rad/s

I = mr²
= (679 kg)(2.19 m)²
= 3256.5519 kgm²

KE = 1/2I$$\omega$$²
= 1/2(3256.5519 kgm²)(765.50141 rad/s)²
= 954157346 J

When I enter this answer, it says that it is incorrect? Could someone please let me know what I'm doing wrong?

Thank you!

 

[jeneekim]

I figured it out! I was using the wrong equation for Inertia...it should be:

I = 1/2mr^2

 

[kerimek]

Your calculations and use of the equations seem to be correct. However, there may be a problem with the units. In the given answer, the units are in Joules (J), but the units for angular velocity (\omega) should be in radians per second (rad/s) instead of revolutions per minute (rev/min). Try converting the given \omega value to rad/s and then redoing the calculations. This should give you the correct answer in Joules.