MGB & GT Forum

Numedous?

There are two popular metric systems: MKS and CGS. Most scientific people these days use MKS (meter kilogram second) as opposed to CGS (Centimeter Gram Seconds). The units in CGS are unfamiliar to most people. For instance, force is measured in dynes.

It is hard to type formulas in this forum and I got the dimensional analysis wrong.

A Newton is defined as the force required to accelerate 1 Kg by 1 m / s^2. Or, a (Kg*m)/s^2. A Pascal is N/m^2 So, a Pascal is (Kg*m/s^2) * 1/m^2 or Kg/(s^2*m) . Not particularly helpful was that! Hopefully I got it right this time. I guess I concede that it was a good idea for the System Internationale people to name the units. It is easier to speak of Amperes than Coulomb's per sec.


That is a hack to allow people to think they can convert from Kg to pounds. If you try to do physics formulas using the pound as a mass, it just doesn't work. You need to convert from force to mass. On earth, this is done by dividing by the acceleration due to gravity -- 32 ft/s^2. The name of that unit is the "slug".

For instance, F = ma (actually, it is F ≡ ma, a force is defined as that which causes a mass to accelerate). Anyway, suppose you try to use pound-mass. Let's say 1 pound accelerating at 1 G (32 ft/s^2). If you try to use the equation to find the force, F = 1 lb x 32ft/s^2, you arrive at a force of 32. The units are unclear because if lb is mass in the equation, it can't also be force. On the other hand, if you use the imperial unit for mass (the slug) you discover that a force of 1 pound will accelerate a mass of 1/32 slug at 32 ft/s^2. Whew! Did I get that right? They only made us use imperial just long enough to see how difficult it was for scientific calculations.

Anyway, we're now waaay off course. And the head is still not torqued!

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

While we've wandered off topic, what I want to know, regardless of units is:
If a chicken and a half can lay an egg and a half in a day and a half, how long does it take a grasshopper with a wooden leg to kick the seeds out of a dill pickle?

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The important thing is not to get ahead, but to get along.

Nah, he has already forgotten about this post

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

WOW: all, this HIGH drama about Imperial vs Metric and about 80% of the forum members, still can't find TDC or figure out why they don't have spark at the plugs.
Just an observation. Carry On.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

Chemistry checking in here with a not insignificant number of CGS units in use.

Centimeters and grams just make more sense for a lot of what we do, and many fundamental relationships, such as the aproximate relationship between AMUs and Avogradro's number, works out easiest in grams.

Of note too, the wavenumber, which is a CGS unit, is still in common use in certain applications. IR and Raman spectroscopy are the two big ones-sure anyone who understands the relationship could tell you the wavelength of an OH stretch in meters or centimeters or nanometers, but they'd need a minute to figure out. OTOH, wake a lot of chemists up in the middle of the night and ask them that question and they're probably spout off "3400" without even thinking and without specifying the units(which, BTW, are in wavenumbers).

Go ahead and measure the speed of light. I think you will find that it is a theoretical assumption, not something measured with a ruler.

Fact remains that no measurement is based on anything otherwise derivable in immutable units. All are assumptions, deductions, conventions or whatever, so one assumption cannot define another. They are all meaningless until we give them meaning. A cubit is the length of an arm from elbow to fingertip. Whose arm do we use. Barley grains differ with the weather. The speed of light is not measurable conventionally so has no meaning.

As for technical measurements, I use thousandths of an inch as measured by my micrometer. I have yet to turn a piece on a lathe using 64ths of an inch, and at that point, measurements are all decimal, just as with meters. Neither is more or less accurate because all are someone's convention.

Aah - the chemists! I was talking about the hard sciences (i.e., physics).

Yes - CGS is still in use. But chemists don't do a lot of calculations involving motion and so forth. I think MKS is more used for those sorts of things. I must confess I've always liked the sound of the word "dyne" over "Newton". (dyne is CGS).

The meter used to be defined as a multiple of the wavelength of one of the emission lines of krypton (wow, now we've got Superman into this discussion). The speed of light thing is really based on the definition of the second - which is based on the frequency of some emission line of cesium, I think.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

DONT CARE, about SUPERMAN, its easy to HIDE the Suoer suit under his double-breasted, JC Penny suit, I want to know where "Dianna Princes" underwear goes when she spins around and turns into Wonderwoman.Now, that is puzzling.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

I'm glad that you take time to reflect and ponder the mysteries of life!

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

It indeed has nothing to do with value. It is only a widely supported agreement to use the so-called "SI" system worldwide for everything scientific.
The SI system is the one based on the metre and the kg mass as basics, with the force in Newton as derived value.This is called a "static" system of standards. If you take the metre and the kg force as basis and mass as derived value you get the earlier "dynamic" system. The main advantage to science is that mass is the same everywhere, while the weight or downward force Newton once started his experiment with the falling apple varies accross the globe, since the gravitational constant g varies (Newtons 2nd law is F=m x a (or called g here)). It is also the reason this platinum stick in Paris which varies in length by temperature changes as the standard of the metre since Napoleon said so, is now redefined in a better way.

Next to these larger static and dynamic systems in the metric environment two small static and dynamic systems exist that are less well known: the small static one has cm, g mass and the force as derived value in dyne, the dynamic one has cm, g force and mass as derived value

All the above stands completely separate for any debate about Imperial versus Metric or any other system of parameter values based on human or non-human dimensions (the Dutch knew something similar to the inch by the length of a thumb "duim" and elbow to hand was once called an "el" for measuring cloth). 1/2" waterpipe fittings are still called "half-duims" by those working on inhouse plumbing, have no fear. In the word feet in Imperial something similar can be found. Still once the metre got its definite place in the SI system, all its derivates (mms, kms, cms) became the more widely used ones, all the more because the fractions used in the imperial system (5/16" studs in the cylinder heads) are not so practical and having decimal and "one step in a time" relations inbetween does have its benefits (grab a spanner on 12mm and if too narrow take the next one in 13mm. With 3/8" as too narrow one has to remember 3/8" is also 6/16" and therefor 5/16" is smaller than 3/8": either 7/16" or 1/2" which is 8/16" are the ones to grab.

Other OT areas we can go into are pressure in N/m2, called Pascal nowadays and magnetic flux in Wb/m2 now renamed Tesla. These are just more recent naming conventions to honour more people from the scientific society than was the case when I still studied at the UNI in the 1960's. For pressures lots of us rather use the bar or the atmosfere value over Pascal when pumping up our MGB tires or just use the psi scale (pounds/square inch or lbs)

OK, no use top prolong this "OT or not OT" debate and return to the job at hand for torqueing the cylinder head studs

PS Kg force in Metric dynamic used to be shorted to kgf and so was gramforce to gf; with the Imperial pound force lbf this can be misunderstood a meaning lb x feet for the momentum value on torque wrenches. On the static SI system Nm, just means N x metre for momentum here
Headnuts in Newton Metre were mentioned earlier by me in the range 67-74,5 Nm as max and the cylinder head smaller ones are 33 Nm as max

PS2 I find Australia interesting, since over there they redefined their Imperial heritage from Britain in Metric units that are close enough in size
The inch stays what it was for them, but is less and less used because of this redefinition of sizes in practical day to day mechanical activities




Edited 5 time(s). Last edit at 2024-04-10 05:23 AM by Donthuis.

Yes these soft keyboards are a PITA late at night.
We now together covered a lot of ground on standards, see my other post if you want, I included the dyne in it

PS Before you know Fahrenheit versus Celsius will also plague us once more and all these various distances in nautical and landmile speeds or different gallons on both sides of the Atlantic are some spectacle to watch with amazement. And to know that Britain once had a very peculiar money scheme with guineas, pounds, shillings and dimes (or pennies?) in a non-decimal interrelation. They've gone decimal by now fortunately




Edited 1 time(s). Last edit at 2024-04-10 04:05 AM by Donthuis.

Heck with all this nonsense. I take 1/2 a quart, 16 ounces or what ever fills it!

Actually part of the big picture of all of this is that the speed of light, by international convention, is defined as exactly 299,792,458 m/s. It doesn't matter how your measure it-this is the answer.

I'll also mention that I've measured the speed of light. I took an advanced experimental physics course in college where this was one of the things we did. We did it by setting up a laser, photo sensor, and oscilloscope on a cart at one end of the hallway, then a mirror at the other end. The was set to fire periodic pulses and sent a signal to one channel of the scope every time it did. The photo sensor was connected to the other channel of the scope. There was a measurable delay between the two pulses, something easily seen on the scope. It was small but certainly there using the ~200 ft. hallway of the building we were in. This same basic experiment has been done using the mirrors several Apollo missions left on the moon. So yes, it is measurable, and we did use a ruler along with some other tools to do it.

LOL. I have to use both in my world, as there is a lot of stuff that uses both. Wheels are a good example of a combination of both, engines as well. I broke a ring the other day, not something I do often but it happen. So I call my buddy at Total seal, as they have one of the largest independent ring inventories in the world and they work on dimension not application, which seems to be the norm in custom. So I know the data he wants before I call, so I tell him bore is 2.840", ring thickness is right at .060", so 1.5mm, and the radial depth of the ring is .127". I ordered enough custom stuff in my life to know how to order it, and that is exactly the method use by them, so one metric measument, and two imperial measurements. Same deal with custom pistons, pistons as for bore size are mostly speced in imperial, rings in a combination of both, pin diameter could be either one, like i would often times use a 19mm pin (.748" instead of the stock MGB size of .812". All my precision tools measure in imperial,and I just convert if needed.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

The hilarious part of this is that, after converting tire sizes to metric, we're now left with a combination on almost all tires. I.e., the width is in mm but the inner diameter (where the wheel goes) is in inches.


In English, I believe that is the cubit! Weren't the measurements for Noah's ark given in cubits?


That is generally what happened. In the U.S. (and I presume Canada), the inch is defined as 2.54 cm. Other measurements are defined in a like way. I was once told that this led to a discrepancy between the original British inch and the newly defined inch, but the error was in the hundreds of thousandths column so almost no one noticed the change. I guess we got lucky that a two decimal value was such a close match! Wouldn't you hate it if you had to remember some weird number like 2.539162 ?

Would that all mechanics and machinists were as conversant and bi-metric as Hap!

One thing the metric system did for us was to cause definitions to be stated in terms of reproducible physical phenomena. Previously there were standard bars (for length) and standard weights and so forth. These would be reproduced as accurately as possible - but just copying the original held the possibility of altering it. I.e., if you measure the bar's length with calipers you might distort it. Now, anyone with the proper equipment can determine what a unit is without looking at any other example.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com