# The Naked Scientists Forum

### Author Topic: Why put pennies on Big Ben's pendulum?  (Read 4582 times)

#### @NickDay2

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##### Why put pennies on Big Ben's pendulum?
« on: 09/05/2011 19:30:02 »

If the period of a pendulum is dictated by its length, not the weight of the bob, why do they put pennies on Big Ben's?

What do you think?
« Last Edit: 09/05/2011 19:30:02 by _system »

#### MikeS

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##### Why put pennies on Big Ben's pendulum?
« Reply #1 on: 09/05/2011 20:01:56 »
In all honesty I don't know but I suspect it could be something to do with the the inefficiently of the escapement.

#### Geezer

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##### Why put pennies on Big Ben's pendulum?
« Reply #2 on: 09/05/2011 20:48:14 »
Adding pennies to the bob effectively shortens the length of the pendulum. The center of mass of the bob with the pennies is slightly closer to the fulcrum, so the period will be reduced, very slightly.

#### yor_on

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##### Why put pennies on Big Ben's pendulum?
« Reply #3 on: 09/05/2011 20:52:12 »
That one is very clever.

"The pendulum is installed within an enclosed windproof box sunk beneath the clockroom. It is 3.9m long, weighs 300 kg and beats every 2 seconds. The clockwork mechanism in a room below weighs 5 tons. On top of the pendulum is a small stack of old penny coins; these are to adjust the time of the clock. Adding a coin has the effect of minutely lifting the position of the pendulum's centre of mass, reducing the effective length of the pendulum rod and hence increasing the rate at which the pendulum swings. Adding or removing a penny will change the clock's speed by 0.4 seconds per day."

From Big Ben.

#### MikeS

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##### Why put pennies on Big Ben's pendulum?
« Reply #4 on: 10/05/2011 06:41:19 »
Obvious really,I should have thought of that.

#### Geezer

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##### Why put pennies on Big Ben's pendulum?
« Reply #5 on: 10/05/2011 17:04:26 »
It wasn't that obvious to me. I had to think about it for about five minutes to figure it out

#### imatfaal

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##### Why put pennies on Big Ben's pendulum?
« Reply #6 on: 10/05/2011 17:29:11 »
I'll admit that even after I had read Geezer's answer and Yor-on's confirmation I still didn't believe it could make enough difference on a 300 kg pendulum; but I did the maths.  On an ideal pendulum of the size in Yoron's post a 10gramme coin about quarter/half way up would make a change of the right magnitude.

#### The Naked Scientists Forum

##### Why put pennies on Big Ben's pendulum?
« Reply #6 on: 10/05/2011 17:29:11 »