# Rotational Kinetic Energy of Big Ben

1. Dec 3, 2012

### Bashyboy

The problem I am working on is:

"Big Ben, the Parliament tower clock in London, has an hour hand 2.70 m long with a mass of 300 kg, and a minute hand 4.20 m long with a mass of 100 kg (see figure below). Calculate the total rotational kinetic energy of the two hands about the axis of rotation. (You may model the hands as long, thin rods rotated about one end. Assume the hour and minute hands are rotating at a constant rate of one revolution per 12 hours and 60 minutes, respectively.)"

(Converted) Angular Speed of Clock Hands:

Hour Hand $1.45⋅10^{−4} rad/s$
Minute Hand $1.75⋅10^{−3} rad/s$

Rotational Inertia:

Hour Hand $I=1/3(300 kg)(2.70 m)^2=729 kg⋅m^2$
Minute Hand $I=1/3(100 kg)(4.20 m)^2=243 kg⋅m^2$

Rotational Kinetic Energy:

$K_{rot}=1/2(729 kg⋅m^2)(1.45⋅10^{−4} rad/s)^2+1/2(243 kg⋅m^2)(1.75⋅10^{−3} rad/s)^2$

When I calculate this, it comes out incorrect, what has happened?

Last edited: Dec 4, 2012
2. Dec 3, 2012

Check this.

3. Dec 4, 2012

### Bashyboy

It worked with the corrected value! Thank you so much!

The rotational inertia of the minute hand is $I=588~kg\cdot m^2$