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Author Topic: Mass (kilogram) vs. other base units - see why "kg" just doesn't make any sense!  (Read 405 times)
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November 03, 2018, 09:43:32 PM
Merited by suchmoon (4), Foxpup (3), o_e_l_e_o (2)
 #1

Probably one of the best Twitter threads I've ever read:

https://twitter.com/MaxFagin/status/1058062597121929216

After reading it all, you'll find it pretty strange that we've been using a unit of mass which is actually not perfect. For example:

-one second = the time it takes an electron in a cesium-133 atom to oscillate 9,192,631,770 times (a constant),
-one meter =  the distance light travels in a vacuum in 1/299,792,458 of a second (another perfect constant), etc.

BUT the one kilogram is surprisingly equal to the mass of a physical artifact stored in a vault in France (called the International Prototype Kilogram, the IPK); I'm finding it hard to believe:



This thing is not constantly remaining the same! See the reason here. The mass of 1 kg is literally losing its mass over time!

Kilogram never made any *good* sense but we've been using it for hundreds of years, just like fiat never made any sense but is still being used even after all this time. Roll Eyes
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November 03, 2018, 11:37:43 PM
Last edit: November 04, 2018, 12:11:04 AM by bluefirecorp_
Merited by Vod (2)
 #2

what's 1 litter of water weigh? 1 kg  Roll Eyes

Oh look at that, easy to convert volume to mass measurement.

What's a gallon weigh in lbs? who the fuck knows.

---

There's a reason why we have both C and K, but they're both the same "scale".
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November 04, 2018, 08:19:59 AM
 #3

I guess a perfect standard might be that a kg is the mass of 1000 moles of hydrogen. I think the problem is that we have no practical way to count atoms.
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November 04, 2018, 10:32:17 AM
Merited by Foxpup (3)
 #4

what's 1 litter of water weigh? 1 kg  Roll Eyes

Oh look at that, easy to convert volume to mass measurement.

What's a gallon weigh in lbs? who the fuck knows.

1 liter of water weighs approximately, but not exactly 1kg. Still, this has never been used as a definition of a kilogram, in the same way that the time it takes for a clock to tick has never been used as the definition of a second. This isn't an argument against SI units, however. Everyone knows that pounds, ounces, feet, yards, etc, are a silly system when compared to SI units.


I guess a perfect standard might be that a kg is the mass of 1000 moles of hydrogen. I think the problem is that we have no practical way to count atoms.

Given that the Avogadro constant is 6.022140857(74)×1023 mol−1 (i.e. +/- 0.000000074), this method would also be imprecise. Also, the Avogadro constant is expected to be redefined again next year, meaning the molar mass constant will no longer equal 1g/mol.

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November 04, 2018, 11:13:11 AM
Merited by suchmoon (4), paxmao (1)
 #5

what's 1 litter of water weigh? 1 kg  Roll Eyes

Oh look at that, easy to convert volume to mass measurement.

What's a gallon weigh in lbs? who the fuck knows.

1 liter of water weighs approximately, but not exactly 1kg.
By contrast, 1 imperial gallon of water at 17°C does weigh exactly 10 avoirdupois pounds, since that's the definition of a gallon outside of the US, whose gallons are inexplicably smaller than everyone else's.
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November 04, 2018, 12:00:58 PM
Merited by vapourminer (1), Raja_MBZ (1), acarli (1)
 #6

By contrast, 1 imperial gallon of water at 17°C does weigh exactly 10 avoirdupois pounds, since that's the definition of a gallon outside of the US, whose gallons are inexplicably smaller than everyone else's.

In old English Units, a gallon was defined as the volume of wine contained within a cylindrical vessel 7 inches in diameter and 6 inches deep: https://books.google.nl/books?id=kJ_sTP6dw2AC&pg=PA8&lpg=PA8

pi*3.52*6 = 230.91, which was rounded to 231 cubic inches, or 3.785 liters. This definition is still used in the US, with the smaller volumes of quarts, pints and ounces being derived from this value.

The British Weights and Measures Act of 1824 changed the definition of a gallon to, as you say, the volume of 10 pounds of distilled water at a temperature of 17°C and a pressure 102kPa, which works out at 277.42 cubic inches or 4.546 liters. Again, the smaller volumes were derived from this one.

Since the US had already declared independence before 1824, they stuck with the old system and did not take on the new system that was accepted throughout the rest of the Commonwealth, which explains why their fluid measurements are all smaller than the rest of the world.

And then, of course, all reasonable people abandoned these systems entirely and starting using the metric system.

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November 05, 2018, 12:00:46 PM
Last edit: November 05, 2018, 12:27:36 PM by paxmao
Merited by vapourminer (1), Don Pedro Dinero (1)
 #7

From Wikipedia: "A new standard, defined in terms of invariant constants of nature, is expected to take effect in 2019.[3] ", in any case you can always recur to the original definition "the mass of 1 cubic decimeter of pure water at its freezing point."

This definition was created during the "Ilustration" period - a period of rational and practical thinking - and it is surprising that it is still making, in my view, perfect sense. It is manageable for "daily use" and corresponds to a decimal measure of water that is something that is frequently used in Engineering, Physics and Chemistry. It is quite simple to know that 1m3 of water is 1000 Kg for example. It makes things simpler for decimal system users.


There's a reason why we have both C and K, but they're both the same "scale".

I can elaborate on that one -  a practical reason: Celsius are very intuitive and easy to understand for "daily use", however when you have to use the ideal gases equations you would need to refer to absolute temperatures. For example:

You have 1 liter of a perfect gas at 1 atm and 10C. What would be the volume at the same pressure if the temperature rises to 20C

Now, Charles Laws tell you that the volume would double if the temperature doubles. BUT that is not true if you use Celsius, you need to refer to absolute temperatures above the absolute zero, that is Kelvins. So the real increase in volume is (approximately)

10 C = 283 K
20 C = 293 K

increase (%) = 293 / 283 = 3% only.
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November 06, 2018, 10:57:44 AM
 #8


As I recall from waaaaay back, 'mass' is kind of a bullshit unit.  Not a 'fundamental' unit in most systems which make much sense.  The reason for this is that it is dependent on gravity so the chunk in France happens to produce a certain more fundamental artifact (a 'force') of a given magnitude by virtue only of it's location relative to other bodies.  Namely the center of the earth.  F=ma where a is 'acceleration due to gravity'.

I do remember one fucked up engineering system which used both mass and force as fundamental units IIRC, and it was a bitch to work with.

I would guess that 'they' left the definition of mass as the lump in France because nobody gave a shit and it didn't matter.  It could easily be translated to the equivalent of a certain number of mols of a any specific isotope...with respect to a certain inertial frame of reference...

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November 06, 2018, 11:16:51 AM
Last edit: November 06, 2018, 11:30:26 AM by paxmao
Merited by suchmoon (4)
 #9


As I recall from waaaaay back, 'mass' is kind of a bullshit unit.  Not a 'fundamental' unit in most systems which make much sense.  The reason for this is that it is dependent on gravity so the chunk in France happens to produce a certain more fundamental artifact (a 'force') of a given magnitude by virtue only of it's location relative to other bodies.  Namely the center of the earth.  F=ma where a is 'acceleration due to gravity'.

I do remember one fucked up engineering system which used both mass and force as fundamental units IIRC, and it was a bitch to work with.

I would guess that 'they' left the definition of mass as the lump in France because nobody gave a shit and it didn't matter.  It could easily be translated to the equivalent of a certain number of mols of a any specific isotope...with respect to a certain inertial frame of reference...



I find quite a few inaccuracies here. Mass does not depend on acceleration (e.g. does not depend on gravity). You probably mean that as it is defined today, with the alloy chunk, you need to know the local gravity to evaluate the mass. That is not much of a problem really.

A Force is something merely theoretical ("a bullshit unit" in your book). If you want to be "realistic" you would always be speaking of pressures, which are fully "real" at macroscopic level.

They defined the weight as that chunck because the trendy thing at the time was to be practical. So the meter was the length of a certain chunk and the mass was that of this other chuck, etc...

They did care a lot a bout those things at the time, that's why they bothered to unify the measuring units. In fact they did not just say... "here is it", the guys actually bothered to assemble several scientific travels to properly measure a meridian of the Earth. Also, the "mole" was something that was established a bit later I think, but in any case it was not an old enough thing to use as standard.

The tendency now is to be super-precise, so everything is defined in terms of the most stable measures available, normally particles wavelengths, speed of light, isotopes, and the like.





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November 06, 2018, 12:00:20 PM
Merited by paxmao (1)
 #10


As I recall from waaaaay back, 'mass' is kind of a bullshit unit.  Not a 'fundamental' unit in most systems which make much sense.  The reason for this is that it is dependent on gravity so the chunk in France happens to produce a certain more fundamental artifact (a 'force') of a given magnitude by virtue only of it's location relative to other bodies.  Namely the center of the earth.  F=ma where a is 'acceleration due to gravity'.
...

I find quite a few inaccuracies here. Mass does not depend on acceleration (e.g. does not depend on gravity). You probably mean that as it is defined today, with the alloy chunk, you need to know the local gravity to evaluate the mass. That is not much of a problem really.

A Force is something merely theoretical ("a bullshit unit" in your book). If you want to be "realistic" you would always be speaking of pressures, which are fully "real" at macroscopic level.

...

A quick look at wikipedia shows that under the SI system (which is, as usual, the most well thought out one) mass is a fundamental unit so I remembered quite wrongly and didn't take much time to think about it either.  Thx for the correction.

I would say that F=ma holds and thus m=F/a.  Not quite accurate to 'does not depend on'.  Depends on the context, and the properties exhibited by the relic in France is a context where it does.  That is to say, the force it exerts on the floor of it's enclosure.

As I recall, again from waaaay back, we habitually broke everything down into fundamental units then compared both sides as a sanity check, and to convert into more convenient derived units.  Doing so with F (kg meters per second squared) and acceleration due to gravity (meters per second squared) yields the correct fundamental unit (kg or mass).

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November 07, 2018, 12:02:16 PM
Merited by vapourminer (1)
 #11

I would say that F=ma holds and thus m=F/a.  Not quite accurate to 'does not depend on'...

That would be true if you want to interpret the mass a mathematical function of F and a, then it will depend on F and a. You could as well say that since E=mc^2 then m=E/c^2 and thus the mass depends on the speed of light and the Energy (but then not on a and F).

However in physics these have to be interpreted as relations, not dependencies. To clarify: If I take a kilogram of mass to the moon "a" and thus "F" have changed, but the mass is the same. And if you take it somewhere else, the mass would still remain constant. Mass is a property of matter (at least in non-quantum physics I believe).

As I recall, again from waaaay back, we habitually broke everything down into fundamental units ...

Absolutely, units are a matter of convenience. If you are working in the aerospace industry in USA, Farenheit, square inches and other non-SI would be the only things you'd find in the diagrams and tables.n

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November 16, 2018, 05:21:23 PM
 #12



Xkcd hits the head on the nail on this thread. Ahaha.


But seriously; https://www.sciencenews.org/article/official-redefining-kilogram-units-measurement
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November 16, 2018, 09:08:44 PM
Merited by Raja_MBZ (1)
 #13

http://www.npl.co.uk/news/international-system-of-units-overhauled-in-historic-vote

Quote
16 November 2018, Paris, France: Today, in a landmark decision, representatives from 60 countries voted to redefine the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole, for ever.
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November 16, 2018, 10:02:21 PM
Merited by Foxpup (3), dbshck (2)
 #14

We've had a pretty good handle on what a kilogram is for a while now. The physical representation isn't a standard, its just some guys that were bored that thought it'd be cool to have a metal cylinder, or more recently a less reactive silicon sphere. The issue of the official kilogram losing mass isn't that big of a deal, they started with non reactive materials for a reason, in a few billion years we may need to revisit it to change the 25th value after the decimal place. All of the changes that have been made are just for the sake of people with OCD.

Technically speaking, you can't convert between SI units and Imperial, you will always gain an additional uncertainty factor thats a pain in the junk to calculate. Though most people asking for directions don't care about the .0005 inches that we gained when converting, so it doesn't effect too many people.

The Imperial system is an absolute mess, the US was on track to change over to Metric in the 1970s, but lobbyists from Screw manufacturers said it would cost them too much to change out their machinery. So here we have a system that isn't base 10, its got all sorts of weird imprecise conversions that make engineering difficult.
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November 17, 2018, 10:22:12 AM
 #15

what's 1 litter of water weigh? 1 kg  Roll Eyes

Oh look at that, easy to convert volume to mass measurement.

What's a gallon weigh in lbs? who the fuck knows.

1 liter of water weighs approximately, but not exactly 1kg. Still, this has never been used as a definition of a kilogram, in the same way that the time it takes for a clock to tick has never been used as the definition of a second. This isn't an argument against SI units, however. Everyone knows that pounds, ounces, feet, yards, etc, are a silly system when compared to SI units.


I guess a perfect standard might be that a kg is the mass of 1000 moles of hydrogen. I think the problem is that we have no practical way to count atoms.

Given that the Avogadro constant is 6.022140857(74)×1023 mol−1 (i.e. +/- 0.000000074), this method would also be imprecise. Also, the Avogadro constant is expected to be redefined again next year, meaning the molar mass constant will no longer equal 1g/mol.



actually pretty sure in ancient greece where kg was invented, these were the instruments used for the calculations
1x1x1metres (1 metre cube) can contain 1000 litres of water and weighs 1000 kgs
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November 17, 2018, 10:23:08 AM
 #16

the old kilogram is dead, long live the new kilogram
https://www.youtube.com/watch?v=c_e1wITe_ig&t=447s
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November 18, 2018, 11:05:21 AM
 #17

actually pretty sure in ancient greece where kg was invented, these were the instruments used for the calculations
1x1x1metres (1 metre cube) can contain 1000 litres of water and weighs 1000 kgs

You are pretty sure about that? You should check before deciding you are absolutely sure. Wink
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November 20, 2018, 10:35:43 PM
 #18


actually pretty sure in ancient greece where kg was invented, these were the instruments used for the calculations
1x1x1metres (1 metre cube) can contain 1000 litres of water and weighs 1000 kgs


If anyone can provide a reliable source of that being true I will open my meritbag.
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November 21, 2018, 04:03:52 AM
Merited by paxmao (1)
 #19

The tendency now is to be super-precise, so everything is defined in terms of the most stable measures available, normally particles wavelengths, the speed of light, isotopes, and the like.
After the latest trend in measuring gravity waves, the theory of "constant" speed of light in a vacuum. Space is not a true vacuum, but the most practical approximation. There is still mass in space (most Hydrogen). There are so many uncertainties. Still, our most advanced scientific theories are still a myth. We can infer a lot with observation. We can take those observations and make models that make us feel good. We are making progress, but slow progress. Talking about the theoretical vs. practical reminds me of this joke I heard before.

Three men enter a doorway to a room with a naked lady in a bed on the other side. Of these men, there is a theoretical physicist, mathematician, and a Mechanical Engineer. They hear a voice that says, "gentlemen, you may walk towards the woman on the bed. Every 3 seconds you must only travel half of the distance between you and the lady. The Engineer immediately lunges forward, walks halfway and pulls down his pants. The mathematician and the theoretical physicist start to walk away discouraged. They yell back to the Engineer .. "You will never get there !!!".. The Engineer smiles and says.. "I'll get close enough that it will matter".. lol.



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November 21, 2018, 04:13:36 AM
 #20

Three men enter a doorway to a room with a naked lady in a bed on the other side. Of these men, there is a theoretical physicist, mathematician, and a Mechanical Engineer. They hear a voice that says, "gentlemen, you may walk towards the woman on the bed. Every 3 seconds you must only travel half of the distance between you and the lady. The Engineer immediately lunges forward, walks halfway and pulls down his pants. The mathematician and the theoretical physicist start to walk away discouraged. They yell back to the Engineer .. "You will never get there !!!".. The Engineer smiles and says.. "I'll get close enough that it will matter".. lol.

Ah, the age old question, is it gay if its a four way.

That aside, you are absolutely right, I highly doubt we will ever perfect physics, however we are at the point now where with our current models and projections, we can do practically anything anyone would possibly want to do. We might not be able to traverse inside of a black hole yet, but even with proper understanding of the universe, we'd still be ages behind in practical application due to material science constraints. Its been a while since we really had a massive breakthrough, but as we have more and more breakthroughs, we are just adding one additional digit of certainty to our calculations. Though, don't get me wrong, there is still a massive amount of value in increasing our understanding.

There are a ton of ways to define the Kilogram, and for all practical purposes, we can take the least accurate method, and we'd still be able to traverse space with it. Its kind of like, whats the point of calculating pi to the 400th digit, when we just round it to 3.1416 for all practical applications.

(hides from the mob of mathmagicians with pitchforks)
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