Pendula

For David, and anyone else with a serious interest … I recommend starting here:

**LINK**

https://www.gutenberg.org/files/17576/17576-pdf.pdf#page50

… and then maybe check the title page

MichaelG.

SOD,

Did you mean "speed" of gravity. ? My contact with same was always in terms of "acceleration due to ....". I've never even wondered about its speed ! Perhaps it comes into astronomy and is not important for my main interest, mechanical engineering.

Just pick a nit to death, are not all pendulums "compound" since we can not pack mass into a point suspended on a weightless rod as demanded by the definition of a simple pendulum ? Just asking.

.

Indeed … as meticulously described in the book that I referenced

MichaelG.

Without reading the book - why were they messing about with coppers to set the pendulum period?

Even quite modest pendulum clocks I've seen have a screw-ajustable bob, which would presumably be positionable within fine limits. You'd expect Big Ben's to have an ultra-fine thread adjustment with vernier graduation to set it within tenths...

Well, two things. First, adjusting the bob with a screw is surprisingly coarse when you are chasing ~0.5 s/day. Second, you have to stop the pendulum and fiddle with it! It can take days to settle down after that. The Big Ben pendulum weight 203kg, so there's a LOT of weight on any screw thread that has to be slightly lifted and held to allow the rating nut to turn.

You can place a penny on top of the bob, or on a weight tray further up, while the pendulum is swinging, and remove it with tweezers or a soft brush. If you've got an electronic period monitoring system you can see the effect almost immediately.

It would have to be very fine indeed to have the same effect as adding one old penny weighing 9.4 gm to a pendulum weighing 203kg. One penny speeds the clock by 0.4 seconds per day. Second advantage is you can add pennies without stopping the clock

.

I would recommend reading the book

Oh yes … silly me, I did that already

MichaelG.

Chris, the affect of gravity is to cause an acceleration, the speed referred to is the speed of propagation of gravity waves across a distance which was not specifically stated in the post above. So no issue with the use of speed in this case and an understandable misunderstanding in your case.

Martin C

OK, fair answers.

There was me wondering if maybe it'd never occurred to 'em...

I did mean speed, but you're right, for all practical purposes gravity is a force understood in terms of 'acceleration due to'. I don't think gravity having a speed effects mechanical engineering at all, but it's about the nature of gravity, which is unclear, and adds 'what, where, when' knowledge that gets science closer to the 'how and why'. Understanding might have a practical application in future; it often does, as with electrons.

Speaking of electrons, they too are mostly understood in terms of their effects, but it seems they have mass and no dimensions, that is they're a point mass ideal for making a perfect pendulum. Job done, except we also need a weightless rod...

Even though gravity is a weak force, we're all familiar with it because it stops us and stuff from floating away. Everyone is being pulled towards the earth's centre. This begs the question, 'what stops us from being pulled through the floor and into the core?', Most worrying because matter is mostly empty space, there's literally almost nothing in a steel block.

We're saved from being horribly sucked under by Coulomb's Force. The electrons spinning around the atoms making up my feet interact with electrons spinning in the floor, and stop me sinking in. Not many have heard of Coulomb Force but it's considerably more powerful than gravity. Falling 20 metres onto concrete stops the victim dead in his tracks. Mechanical engineers allow for the effect of the force (hardness and softness), and don't normally worry about the details. Matters very much to anyone doing high-technology though, but high-tech is laboratory and R&D territory, rarely tackled in a home workshop.

Dave

When the above mentioned clock was made pennies were pretty valuable given a day's wage could be less than a shilling. I wonder who was providing the coins with such abandon.

The Westmnster clock is regulated as per " ....used at Westminster, where it is easy to put on small weights without disturbing the slow and heavy pendulum at a third of the way from the top.

so presumably on a weight tray. Larger changes are made by as consequence of thunderstorms etc by .. either stopping the scape wheel or letting it trip one beat by lifting a pallet (one of the gravity arms) which alters it by 4 seconds.

The clock has a pendulum of nearly 700lbs of which the bob is approx 4 cwt.The suspension spring is 1/60th inch thick and 3 inches wide with a free part of 5 inches. The pendulum cock and the floor behind is so arranged as to allow a tall man to stand with his head inside the cheeks of the cock so as to look square on at the escapement.

Quotes nd Information taken from Dennison, Clocks Watches and Bells.

regards Martin

As I understand it the addition or subtraction of the small weights was done by the clock keepers to adust the time displayed rather than any permenant alteration of the pendulum period. Basically allowing the clock to be advanced or retarded a little to account for vaguaries of weather etc.

Edited By Martin Kyte on 18/11/2022 18:07:20

Thank you once again Michael, I have downloaded the book and will read it with interest.

I have checked the title and felt a little humble.

David

Hi, where the penny was place in the Juddy's link, is just as I saw it in the program that I watched many years ago, and it had the same two stacks of pennies there as well.

I knew that altering the weight of the bob didn't make any difference, as I also saw a program where they showed large candle holders suspended by chains from the roof in a church. There were several of these and some were bigger and heavier than others, but they all were hanging at the same height, and someone noticed that they were all swinging in total synchrony. I can't remember who it was that was watching them and was puzzled as to why they remained in synchrony, but I think it was someone who realised the fundamental law of pendulums.

Regards Nick.

Are you thinking of Galileo Galilei? Clever bloke. His development of the idea of the helicocentric universe, proposed by Copernicus, was considered heresy, leading to his last few years being spent under house arrest. At least he avoided the torture with which he was threatened. A mere 350 years later, the pope officially pardoned him. So that's alright then...

Hi Kiwi Bloke, I think you are correct, Wikipedia says he noticed a swinging chandelier that was caused by air currents, which is what was said in the programme that I watched. I still can't remember what programme it was though, as it was quite a few years ago, I just remember them showing these chandeliers swinging in time with each other and the explanation why they did.

Regards Nick.

Nick

May I suggest that you dip-into the ‘Rudimentary Treatise’ at page 35 of the book

… it will probably trigger a few memories of what you have seen and heard.

MichaelG.

And on a smaller scale (?) when the bloke used to come and service the works master clock, he too adjusted the rate using weights on the pendulum. Does mean of course you can alter the timing without stopping the clock. Rescued it when conversion to electronics, it was destined for the tip.

Regards Ian.

An interesting point as it is, to a good approximation, the weight and not the mass that is important. One of the few things I remember from A-level physics was the derivation from first principles of the period of a simple pendulum. If you make the assumption that the swing is over a small arc, then the period is independent of the mass of the pendulum and proportional to sqrt(l/g), where l is length of pendulum (pivot point to CoG) and g the local force of gravity.

So, to regulate the clock, adding additional weights at the CoG is pointless but adding weights higher up the pendulum changes its length...

I well remember an experiment at school using a pendulum to show the rotation of the earth, so the suspension of the pendulum must also have to take the all be it small rotational force ? Noel.