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25 August 2019 21:51

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Asked by: Nitro
Subject: MacPhail's pendulum
Question: Hi again all,

Pull up a cosy chair, pour yourselves a beer and get comfy.....


Ravi, Blaze Minor, and you scruffy lot in the back; sit up straight and pay attention. That includes you Harry K! And Sandy Junior; I hope that is not gum you are chewing!

Luis, you’re a good didactic argumentalist but you seem to need explanations from others in the school with better maths on Newtonian physics than I have. Blaze? Momentus? Anyone?

OK Luis, as I am unable to move far from the computer at the moment I will play didactic games for now.............

Further to the points you first made:-

“1 - The initial pendulum-motion is not purely linear”
No kidding! This could possibly be because it’s a bloody pendulum!

“2 - The Curvature in the motion produces a mild “Tilting-Torque” when combined with Gravity’s force, and so it causes precession in the enclosed gyro (and we are aware how precession can induce an orbiting arc)”
Yes! But the point is:- do you really think that such mass displacement caused by the “mild tilting force” complies with the third law? i. bloody well e.:- Where is the opposite mass displacement required to comply with the third law?
And don’t be like Harry K (The door casing = the Earth, where the string is fixed. Unfortunately to small to notice.) and go for the Mass of the world is moving a tiny bit, too small to notice crap. If you did the proper observation homework before throwing up such nonsense you would have realised that the world is being moved (that tiny bit) in the same direction as the pendulum’s displacement. A pure anomaly outside Newton, unless you think that you can push something with a piece of string? So no cigar for you either Harry.

“3 - The curvature in the pendulum-motion cannot exist without the “EXTERNALLY attached Swing-Pivot” which introduces the necessary “Centripetal-Acceleration” to prevent the pendulum-motion from being Truly Linear”
Yes! But see answers to 1. and 2. above and a further simple description to help you, below. And, do you think that Newton’s laws do not apply to orbiting bodies because, last time I looked, they don’t go straight either? Or does Newton only not apply if planets are fixed by a string to a hook to a door frame (casing) etc., etc.?

My understanding is that the fundamental maths and observations of pendulum law were laid down by Galileo way before Newton, which is presumably why there is only passing reference to a pendulum in the Principia. Newton was a plagiarist but he would not risk stealing ideas that were in common knowledge by his time.

“MacPhail’s pendulum” is the simplest form of mass displacement device that I can think of and thus the reason for me using it to try and get you lot to understand first principles. Though if you had paid attention and worked harder on your homework we wouldn’t have to go over first principles again.

Putting standard pendulum motion as simply as I can:-

Once set in motion a pendulum will not alter from its straight “swing “ path (see Newton, Galileo, Foucault etc.) unless a force is applied to it to change its swing path. So; that part of a pendulum’s motion is a, surely simple to understand, variant of the first law.

The second law gives its accelerations and decelerations (while Galileo's equations gives that its period depends on its “string” length and, if you want to be very pedantic or are designing an accurate grandfather clock, the strength of gravity).

The third law gives that the action of the weight (or mass if preferred) and the opposing reaction (which are proportional to the relative masses of the pendulum and, via a hook, the Earth) on the top pivot at each point of its swing.

A pendulum’s motions are thus all part and parcel of those prescribed by Newton’s laws of motion. The laws of motion do not cease to apply because an object is swinging instead of traveling in a straight line in space (if that is possible) any more than they cease in the case of an orbiting body; indeed Newton’s three laws of motion can be found used all over academia to describe the pendulum’s or the orbiting body’s motions, as well as straight line motion, by much smarter people than me.

HOWEVEREVEREVER; Newton’s laws of motion most certainly do not explain – no matter how you try and “square the circle” with the maths, Luis – why the pendulum shown in the video (or why my “one shot” machine shown in earlier videos) displaces mass from an internal mechanism. You would have to completely sod up at least the first and third laws of motion to explain it. Your assertion, that this kind of mass displacement cannot be achieved without “attachment” to the Earth is wrong because it can. It is possible, though unnecessary, to build an experimental model that, as in the case of John Harrison’s fabulous clock, can easily replace gravity with a/some spring/s with the unwanted torques countered by contrarotation. Thus the same type of mass displacement could easily be demonstrated, even in weightless orbit. A nice example of this type of clock mechanism can be seen here:-


Perhaps you would care to put up a demonstration video Luis, of the cylinder device you described which, you say, will give the same mass displacement effect without a gyro. I think you’ll find your idea to be a total non starter unless Newton’s laws cease to apply just because a mass is being shifted in a cylinder. Love to see a video of that one, or any of your experiments you care to show us.

You may want to look again, with cynicism by all means but without your preconception hat on, at:-


It shows all that is needed to see something that needs new maths to explain what I have said before shows: “Loads of the equal but not all of the opposite”. If you don’t agree you will have to wait and hope I will last long enough and retain my faculties and interest to finish the fast repeater.


Luis, I am knackered and cannot be arsed to itemise and properly, politely, answer all of your later, scatter gun, dismissive questions – but here is my, half arsed, best shot:-

“My personal preference at the Science Museum in London was Babbage’s Difference Engine.”
Perhaps you should have stood and studied the Foucault pendulum in the foyer, instead of Babbage’s useless difference engine, it may have helped you with the present subject. You must have visited very much later than I as, to my memory, there was only an incomplete, boring, display of a heap of gear bits that looked like a stripped down bus gearbox next to an even more boring panel of words. I have an old, beautiful, Olivetti mechanical adding machine that is as fiendishly complex as Babbage’s but it was useful, works and is portable. Cannot do Infinitesimals, of course, but then nor could the bits Babbage left incomplete. No way was Babbage’s machine a forerunner of the computer as is often claimed. A Lyonnais, Jacquard, weaving machine from 1725 is a true forerunner of the computer though, complete with punched card program. Hollerith and IBM were much later copiers.

“You seem to agree that your pendulum never travels in a straight line but it’s not clear whether you agree or disagree that your experiment has nothing to do with the First Law. Which is it?”
I am sorry I thought that even a pedantic rote chanter would understand that the straight line swing referred to in the video was “the straight swing line” of a pendulum. I apologise for crediting you with better understanding. The experiment shown in the video, I am sure with sufficient clarity for most people, has everything to do with the first law and the second law and the third law but also with something else.

“Also, I am not able to determine what portion(s) of my explanation you don’t agree with, or do you disagree with every single statement I presented? I also can’t help wondering, do you agree with all statement by others on this subject?”
That sounds like a “when did you stop beating your wife” question. Also, I would probably try and answer this if I were less weary and I also could be bothered.

“I suppose we can… agree to disagree, as it will all become clear eventually.”

No! I suppose we cannot! I can only agree that you are wrong with your cylinder nonsense. “It”, is already very clear.

“Oh yes, the name Nitro MacMad evokes images of a fully bearded Scotsman performing Caber Toss in the Scottish Highland Games.”
I have never lived in Scotland but I have been known to toss the occasional caber, and I do sometimes visit the family’s croft in Mull.

“By the way, I have always enjoyed traveling through your fair land.”
I doubt you have travelled through my fair land as I live in the Channel Islands.

Like I said before Luis; good but no cigar. Just as well that you don’t smoke.

And as you seem attached at the hip to your baby of your own interpretations of Newton’s laws; a little less, please, of your piss taking hypocrisy like:- “My apology, as I did not realize how attached you are to your own interpretations, or as you call them your baby…”

Right you lot, that’s it! There’s the bell. Wait for it, wait for it, Blaze minor! Do your homework tonight carefully, all of you. Tonight’s homework is called “Observing is different to just looking”. I just hope your questions to the answers are better in the next exam. OK, Class dismiss! No running in the corridor, Sandy junior!

Kind regards
Date: 19 February 2013
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