How Do You Calculate the Angular Momentum of Big Ben's Clock Hands?

[cdubsean]

Homework Statement

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.

Homework Equations

Inertia about center point for rod is:

I= (1/3)ML^2

Lz=Iw

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?

 

[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.

 

[kerimek]

Your calculations are correct, but the units are not. The correct units for angular momentum are kg*m^2/s, not kg*m/s. Therefore, your final answers for both the minute hand and hour hand should be 66.44469 kg*m^2/s. Make sure to always check your units to ensure they are consistent and correct.