# Homework Help: Angular Momentum of Big Ben

1. Nov 4, 2009

### cdubsean

1. The problem statement, all variables and given/known data

The hour and minute hands of Big Ben in
London are 2.64 m and 4.5 m long and have
masses of 69.4 kg and 94 kg respectively.

1) Calculate the total angular momentum of
the minute hand about the center point.
Treat the hand as long, thin rod. Treat “into
the clock” as the positive direction.
Answer in units of kg · m2/s.

2) Calculate the total angular momentum of the
hour hand about the center point. Treat the
hand as long, thin rod, and “into the clock”
as the positive direction.
Answer in units of kg · m2/s.

2. Relevant equations

Inertia about center point for rod is:

I= (1/3)ML^2

Lz=Iw

3. The attempt at a solution

For Minute Hand:

I= (1/3) (94kg) (4.5m)^2 = 634.5 kg*m^2

THen
Lz = Iw = (634.5 kg*m^2) (1/60 rev/s) (2pi rad/rev) = 66.44469 kg*m^2/s

For the Hour hand I should do the same.

For both of these, the system says I am wrong, any help?

2. Nov 4, 2009

### cdubsean

I have found my mistake.....

The Inertia was right but I needed I did second part wrong.

2pi/3600s = 1.75E-3 rad/s for minute hand
2pi/43200s = 1.45E-4 rad/s for hour hand

then

(624.5kg*m^2)( 1.75E-3 rad/s) = 1.110375 for minute hand and repeat for hour hand.