Torque + Clock Hands?

Question

A tower outside the Houses of Parliament in London has a famous clock commonly referred to as Big Ben, the name of its 13-ton chiming bell. The hour hand of each clock face is 2.7 m long and has a mass of 60.0 kg. Assume the hour hand to be a uniform rod attached at one end. (a) What is the torque on the clock mechanism due to the weight of one of the four hour hands when the clock strikes noon? The axis of rotation is perpendicular to a clock face and through the center of the clock. (b) What is the torque due to the weight of one hour hand about the same axis when the clock tolls 9 AM?


(a) 395 N·m; (b) 395 N·m
(a) 0; (b) 0
(a) 0; (b) 790 N·m
(a) 790 N·m; (b) 395 N·m
(a) 395 N·m; (b) 790 N·m

Answer 1

(a) 0; (b) 790 N·m
When the clock strikes noon the hour hand points straight up, so the torque is zero.
When the clock strikes 9AM, the hour hand is perfectly horizontal.

The force on the hand is m*g
For the computation of torque, place this force 1/2 the length of the hand from the center axis. Now you have

Torque=m*g*l/2

I got 793.8 N*m

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