## Kilogram as a unit of weight

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## Do you die a little whenever someone uses kg as a unit of weight?

Yes
23
27%
No
62
73%

Arariel
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### Kilogram as a unit of weight

I hope I'm not the only one horribly annoyed whenever someone uses kilograms as a unit of weight.
Although I do keep forgetting I don't multiply by 9.8 if it's in pounds.

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sikyon
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### Re: Kilogram as a unit of weight

I don't mind at all since it's easy to understand.

Interactive Civilian
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### Re: Kilogram as a unit of weight

It depends. Here on Earth, no, it isn't a problem. Weighing and Massing are essentially talking about the same property, at least for human weight. But sometimes in Science Fiction, I see it being used when talking about things in different gravity environments, and when it is interchanged for pounds then, it drives me batty. If you "weigh" 60kg here on Earth, you will still be 60kg (mass) on the Moon, not 10kg. You may effectively feel like what 10kg feels like on Earth, but you still have the inertia of 60kg of mass to deal with.

When I see little things like that in an otherwise really good sci-fi book (Kim Stanley Robinson does this a few times in the Mars Trilogy, which I love in all other aspects), it really annoys me. How could they get so much of the other stuff right, and then flub that.
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idobox
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### Re: Kilogram as a unit of weight

I really don't care people using mass units for weight on normal Earth conditions, as long as they are metric, because I never remember how much apound or ounce is.

Speaking in terms of mass for space exploration is understandable when your audience is not well informed. It is much easier to explain that you weight lesson the moon, than to explain the difference between inertia and gravity.

What really, really bothers me, is the use of kg or pound in very technical units where they shouldn't be. Like people using kg/cm² for pressure, or kg.m for torque.
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Mapar
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### Re: Kilogram as a unit of weight

I remember that in my first physics lesson ever, the teacher asked someone: 'What is your weight?' Of course, the student replied something like '52 kg', and then he said -with a huge smile: 'Well, no, it's actually 500 newton.' Cue puzzled faces across the room. It was hilarious.
Hi.

eternauta3k
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### Re: Kilogram as a unit of weight

Can't you just mentally draw a little arrow over the kg? Tada, kilograms-force
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Velifer
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### Re: Kilogram as a unit of weight

Yes, because a pint's a pound the world around!
We measure weight in pounds! Anything else is a direct attack on my culture!
Aaaaack! One World Government! Conspiracy!
Just an example: wine (the swill of the French!) must be sold in metric units here. Beer must be sold by the ounce.

But srsly--gravity near the surface of the earth, where I deal with weight most of the time, is constant enough that weight in Kg is useful and doesn't make me twitch.
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Foremorrow
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### Re: Kilogram as a unit of weight

Not at all. It's simply too popular that people assume mass equates to weight so it's not worth to let it bother me.

Does anyone know where to get a chart for how to convert our weight in Kg (according to a weight scale) into Netwons depending on where on earth we are and depending on where the moon is?

Also, for those of you who just cannot stand to have Kg understood as a measurement of weight, see the attached file.
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Screen shot 2010-12-17 at 16.41.47.50.png (46.47 KiB) Viewed 4348 times
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Technical Ben
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### Re: Kilogram as a unit of weight

Pound = measurement of mass.
Kilogram = measurement of mass.
They are both wrong then? I also hate it when people say "I want to loose weight". When in fact, they want to reduce the circumference of their waist, or lessen the bulges in their bum.
How many size zero super models would care if their bones were made out of lead?
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thoughtfully
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### Re: Kilogram as a unit of weight

Technical Ben wrote:How many size zero super models would care if their bones were made out of lead?

I expect they'd be quite horrified at all the bulging muscles they'd develop

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idobox
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### Re: Kilogram as a unit of weight

eternauta3k wrote:Can't you just mentally draw a little arrow over the kg? Tada, kilograms-force

I understand how they arrived there, but it's not a reason to use a unit of mass as a unit of force.
One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.

Velifer wrote:We measure weight in pounds! Anything else is a direct attack on my culture!

The lack of millipounds and kilopounds makes it a unit fit only to weight grain in 18th century England
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### Re: Kilogram as a unit of weight

I have no problem expressing weight in kilograms.

I regularly express mass in units of energy or length in units of time. Even expressing time and distance in terms of inverse energy, or describing mass in terms of inverse time, are commonly done. Units are not as rigid as some people here seem to think. Using kilograms for weight is completely unambiguous and is very convenient. That's reason enough to do it.
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Ulc
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### Re: Kilogram as a unit of weight

Nope, on the other hand I'm in the camp that believes that there is a special hell for people that uses non-SI units in relation to science. And yes, that means that if you use pounds for science, you'll burn forever!

Along with the people that round pi to three.

(Incidently, I would really prefer that we used Molal over molar, but that's mostly a religious war between the pragmatic, and the exact)
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### Re: Kilogram as a unit of weight

Technical Ben wrote:Pound = measurement of mass.
Kilogram = measurement of mass.
They are both wrong then? I also hate it when people say "I want to loose weight". When in fact, they want to reduce the circumference of their waist, or lessen the bulges in their bum.
How many size zero super models would care if their bones were made out of lead?

No, a pound is a measurement of weight. You could define a pound mass as "the amount of mass that weighs one pound on earth", and that would be a mass. I tend to agree though that for all intents and purposes, if you tell me your weight in Kilograms or mass in pounds I'm prefectly capable of doing the requisite "fill in the blanks" to figure out that you meant "Newtons this many kilgrams would exert in earth's gravity" and "mass of an object that weighs one pount on earth".

Which is actually also the reason I lose or gain weight, as opposed to reduce or expand my waist, losing/gaining weight is much simpler to say, and since everyone is aware of the convention I'm indicating, it's fine.

Also I will note that I use pi=3 all the time. For example, if I've only got one significant digit, then that's the only relevant digit anyways. Or if I just want to do an order of magnitude calculation, or if I'm checking the ballpark of something and don't want to break out the calculator, then pi=3 and pi^2 = 10.
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thoughtfully
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### Re: Kilogram as a unit of weight

If anyone's interested, the unit that is pounds/little-g is called a slug. Presumably since getting pounded and getting slugged are similar experiences

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gorcee
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### Re: Kilogram as a unit of weight

thoughtfully wrote:If anyone's interested, the unit that is pounds/little-g is called a slug. Presumably since getting pounded and getting slugged are similar experiences

Yeah, it's a little-known and little-used unit, yet the majority of my Aero courses in college dealt with air density at STP equals 0.002377 slugs/ft^3. That number will be forever ingrained in my head.

gmalivuk
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### Re: Kilogram as a unit of weight

idobox wrote:One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.
So 1L *weighs* 1kg, but a kg isn't a unit of weight? How's that work, exactly?

Velifer wrote:We measure weight in pounds! Anything else is a direct attack on my culture!
The lack of millipounds and kilopounds makes it a unit fit only to weight grain in 18th century England
What lack? Those may not be common units or anything, but I wouldn't say they don't exist, on account of you just said both words and everyone reading knew exactly what you meant.
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### Re: Kilogram as a unit of weight

Ulc wrote:Nope, on the other hand I'm in the camp that believes that there is a special hell for people that uses non-SI units in relation to science.

I don't think there's a single scientist in the world who only ever uses SI units. All of them regularly use SI units. In many cases SI units are simply very inconvenient and other units simplify everything a lot. There's no reason not to use those then.

Units are not as rigid as people often make them out to be. Like I said earlier, it's perfectly normal to express mass in terms of energy, or distance in terms of time. Expressing weight in terms of mass is no different. And very convenient I might add. I fully endorse it.
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gmalivuk
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### Re: Kilogram as a unit of weight

Diadem wrote:I don't think there's a single scientist in the world who only ever uses SI units.
For example, atmospheres of pressure, g's of acceleration, degrees Celsius/Centigrade, light-years or parsecs or angstroms of distance, electron-volts or calories or watt-hours or (kilo/mega/giga)tons (of TNT) of energy, minutes or hours or days or years of time, and so on. None of these are SI units, and yet I'm sure all of you have used some of them (and dozens more), even when talking about science.
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Technical Ben
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### Re: Kilogram as a unit of weight

So, do kilograms measure weight?
By asking "how much do you weigh" are we really asking "what is you mass, don't bother to tell me your "weight" as I know you are on earth". So we can do the math in our head, no need for: $F_g = m g \, ,$
This also add to my bugbear. I need to go around telling everyone to loose more mass.

Hmm. This does not mean we could not have a moon Kilo, or a Mars Pound does it? If I take a litre of water to the moon... I can then term it a Moon Kilo?
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jmorgan3
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### Re: Kilogram as a unit of weight

gmalivuk wrote:
Velifer wrote:We measure weight in pounds! Anything else is a direct attack on my culture!
The lack of millipounds and kilopounds makes it a unit fit only to weight grain in 18th century England
What lack? Those may not be common units or anything, but I wouldn't say they don't exist, on account of you just said both words and everyone reading knew exactly what you meant.

Kilopounds is a pretty common unit in engineering statics/structures problems, along with ksi (kilopounds/in^2).
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idobox
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### Re: Kilogram as a unit of weight

gmalivuk wrote:
idobox wrote:One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.
So 1L *weighs* 1kg, but a kg isn't a unit of weight? How's that work, exactly?

As I said, I do not have any problem with people using kg as unit of weight under normal Earth conditions.
By the way, is there a verb for "to have a mass of"?

gmalivuk wrote:
Velifer wrote:We measure weight in pounds! Anything else is a direct attack on my culture!
The lack of millipounds and kilopounds makes it a unit fit only to weight grain in 18th century England
What lack? Those may not be common units or anything, but I wouldn't say they don't exist, on account of you just said both words and everyone reading knew exactly what you meant.[/quote]
It was an attempt at humour. Apparetnly, it was not as good and as understandable as I intendedit to be.
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### Re: Kilogram as a unit of weight

idobox wrote:
gmalivuk wrote:
idobox wrote:One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.
So 1L *weighs* 1kg, but a kg isn't a unit of weight? How's that work, exactly?

As I said, I do not have any problem with people using kg as unit of weight under normal Earth conditions.
By the way, is there a verb for "to have a mass of"?

I've heard (and use) "to mass" in exactly the same manner as "to weigh" but referring to mass e.g. "Did you mass the measuring cylinder?" "Yeah, it massed 27g" (this cropped up in a physics practical our teacher thought would be worthwhile (it wasn't) to calculate the density of water).
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Kirby
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### Re: Kilogram as a unit of weight

On my physics final I kept trying to multiply some weight in pounds by g (32 feet per second per second, of course) to figure out work done or something (of course after multiplying by the distance traveled). I even managed to convince myself that the units worked out. When I realized my mistake, I wanted to shoot myself.

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### Re: Kilogram as a unit of weight

That's odd. Even here IN AMERICA, I've never taken a physics class that bothered with standard/English/imperial/customary/whatever units.
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### Re: Kilogram as a unit of weight

Sir_Elderberry wrote:That's odd. Even here IN AMERICA, I've never taken a physics class that bothered with standard/English/imperial/customary/whatever units.

That's pretty much the case here (I'm in America). We always use SI units in physics. They decided to throw in pounds, just once, on the final. Argh.

akashra
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### Re: Kilogram as a unit of weight

First day of Semester 2 uni my maths lecturer claimed that weight was constant, not really thinking about it.

And from there, the fight was on
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### Re: Kilogram as a unit of weight

Sir_Elderberry wrote:That's odd. Even here IN AMERICA, I've never taken a physics class that bothered with standard/English/imperial/customary/whatever units.

Ahh.. well you see.. he was taking an engineering class. Silly engineers.. having to deal with the real world (in America)!

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Rackum
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### Re: Kilogram as a unit of weight

idobox wrote:
gmalivuk wrote:
idobox wrote:One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.
So 1L *weighs* 1kg, but a kg isn't a unit of weight? How's that work, exactly?

As I said, I do not have any problem with people using kg as unit of weight under normal Earth conditions.
By the way, is there a verb for "to have a mass of"?

Ummmm, how about "has a mass of" ... As in: "1L of water has a mass of 1kg." Seems to me that it works pretty easy and is only a few extra syllables.

Tass
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### Re: Kilogram as a unit of weight

Rackum wrote:
idobox wrote:
gmalivuk wrote:
idobox wrote:One liter of water weights 1kg (I think it's the oldest definition of the kg), yet I don't start measuring weight in liters.
So 1L *weighs* 1kg, but a kg isn't a unit of weight? How's that work, exactly?

As I said, I do not have any problem with people using kg as unit of weight under normal Earth conditions.
By the way, is there a verb for "to have a mass of"?

Ummmm, how about "has a mass of" ... As in: "1L of water has a mass of 1kg." Seems to me that it works pretty easy and is only a few extra syllables.

1l of water masses 1kg?

Why do we write those ones, anyway? They are not mathematically necessary. It is okay to say "it has a speed of c", but not "it has a speed of m/s" - people would then ask "how many m/s?".

(Yeah okay in this case it probably signifies an exactness of the figure, but we also say "one ampere equals one volt per oh" rather than "volt equals ampere per ohm")

StNowhere
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### Re: Kilogram as a unit of weight

Ulc wrote:Along with the people that round pi to three.

Came a little late on this thread, but had to comment on this. I firmly believe that this warrants the ultra-special Hell, with Celine Dion's "My Heart Will Go On" playing in an infinite loop while being forced to read and write reports detailing the socioeconomic, geopolitical and philosophical implications of every *chan post in existence. Your reward for actually producing such a report (already impossible): having the Celine Dion exchanged for Kenny G.

Tass
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### Re: Kilogram as a unit of weight

StNowhere wrote:
Ulc wrote:Along with the people that round pi to three.

Came a little late on this thread, but had to comment on this. I firmly believe that this warrants the ultra-special Hell, with Celine Dion's "My Heart Will Go On" playing in an infinite loop while being forced to read and write reports detailing the socioeconomic, geopolitical and philosophical implications of every *chan post in existence. Your reward for actually producing such a report (already impossible): having the Celine Dion exchanged for Kenny G.

Again, why? For quick order-of-magnitude calculations in the head it is very convenient. Nothing wrong with approximations as long as you know their limitations.

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### Re: Kilogram as a unit of weight

StNowhere wrote:
Ulc wrote:Along with the people that round pi to three.

Came a little late on this thread, but had to comment on this. I firmly believe that this warrants the ultra-special Hell, with Celine Dion's "My Heart Will Go On" playing in an infinite loop while being forced to read and write reports detailing the socioeconomic, geopolitical and philosophical implications of every *chan post in existence. Your reward for actually producing such a report (already impossible): having the Celine Dion exchanged for Kenny G.

I had a lecturer once who rounded pi to 1. Seriously. He started the lecture by writing on the board [imath]c = \hbar = k_B = \pi = 1[/imath]. It was awesome.

Of course this comes from a field where theoretical predictions and experimental results differ by 100 orders of magnitude. So a paltry factor 3 really does not even register.
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### Re: Kilogram as a unit of weight

StNowhere wrote:
Ulc wrote:Along with the people that round pi to three.

Came a little late on this thread, but had to comment on this. I firmly believe that this warrants the ultra-special Hell, with Celine Dion's "My Heart Will Go On" playing in an infinite loop while being forced to read and write reports detailing the socioeconomic, geopolitical and philosophical implications of every *chan post in existence. Your reward for actually producing such a report (already impossible): having the Celine Dion exchanged for Kenny G.

I had a lecturer once who rounded pi to 1. Seriously. He started the lecture by writing on the board [imath]c = \hbar = k_B = \pi = 1[/imath]. It was awesome.

Of course this comes from a field where theoretical predictions and experimental results differ by 100 orders of magnitude. So a paltry factor 3 really does not even register.

Other than the last one, that just seems as though he's stating that he's using Planck units which is perfectly valid. That last step is also valid if you're only interested in knowing what scale an answer is (to within an order of magnitude or two).
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### Re: Kilogram as a unit of weight

Diadem wrote:So a paltry factor 3 really does not even register.

I like how you actually rounded [imath]\pi[/imath] to 3 in this post.

StNowhere
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### Re: Kilogram as a unit of weight

Tass wrote:
StNowhere wrote:
Ulc wrote:Along with the people that round pi to three.

Came a little late on this thread, but had to comment on this. I firmly believe that this warrants the ultra-special Hell, with Celine Dion's "My Heart Will Go On" playing in an infinite loop while being forced to read and write reports detailing the socioeconomic, geopolitical and philosophical implications of every *chan post in existence. Your reward for actually producing such a report (already impossible): having the Celine Dion exchanged for Kenny G.

Again, why? For quick order-of-magnitude calculations in the head it is very convenient. Nothing wrong with approximations as long as you know their limitations.

It just ... makes my spleen twitch, or something. If I'm doing quick calculations, it's really not appreciably slower to at least use 3.14. I'm not going to use e=3, either, though it might round up to the same number. I mean, if you're just doing order-of-magnitude calculations, why not just round everything to the nearest power of 10 and call it a day? Propagate so much error through the calculation that you end up 5 or 6 orders off from what should have been the theoretical solution, never mind the experimental.

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### Re: Kilogram as a unit of weight

eSOANEM wrote:
Diadem wrote:I had a lecturer once who rounded pi to 1. Seriously. He started the lecture by writing on the board [imath]c = \hbar = k_B = \pi = 1[/imath]. It was awesome.
Of course this comes from a field where theoretical predictions and experimental results differ by 100 orders of magnitude. So a paltry factor 3 really does not even register.

Other than the last one, that just seems as though he's stating that he's using Planck units which is perfectly valid. That last step is also valid if you're only interested in knowing what scale an answer is (to within an order of magnitude or two).

Yeah, the first three are just Planck units. Pretty standard. But I found the juxtaposition of those with pi to be rather cool.

StNowhere wrote:It just ... makes my spleen twitch, or something. If I'm doing quick calculations, it's really not appreciably slower to at least use 3.14. I'm not going to use e=3, either, though it might round up to the same number. I mean, if you're just doing order-of-magnitude calculations, why not just round everything to the nearest power of 10 and call it a day? Propagate so much error through the calculation that you end up 5 or 6 orders off from what should have been the theoretical solution, never mind the experimental.

Seriously, 3 is less than 5% off from pi. That's not going to add up to 5 or 6 orders of magnitude in your final answer, unless you have like 40 powers of pi. And of course pi^2 = 10 so that's even less error there.

And I don't know how fast you are in doing calculations in your head, but multiplying something by 3.14 usually takes me *a lot* longer than multiplying with 3. Say I want to know the radius of the earth. I know the circumfence is 40,000 kilometers because that's the definition of the kilometer. So that's 40000 / 2 pi ~= 40k / 6 = 6.67k. You can do that in less than a second. But dividing by 3.14? Really? 40 / 3.14 that's 10 leaves 8.86 that's 2 leaves 8.86 - 6.28 = 2.38 which gives .7 leaves 238 - 210 - 7 - .28 is ehm ... Seriously, I have the aid of typing this out right now and I'm already getting frustrated.

For the record the actual answer is 6,371.0 km mean radius (that's not exactly 40k / 2 Pi. The kilometer was defined by meridional circumfence, and they made a slight error). I'm rather happy with the answer if got in less than a second.
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StNowhere
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### Re: Kilogram as a unit of weight

eSOANEM wrote:
Diadem wrote:I had a lecturer once who rounded pi to 1. Seriously. He started the lecture by writing on the board [imath]c = \hbar = k_B = \pi = 1[/imath]. It was awesome.
Of course this comes from a field where theoretical predictions and experimental results differ by 100 orders of magnitude. So a paltry factor 3 really does not even register.

Other than the last one, that just seems as though he's stating that he's using Planck units which is perfectly valid. That last step is also valid if you're only interested in knowing what scale an answer is (to within an order of magnitude or two).

Yeah, the first three are just Planck units. Pretty standard. But I found the juxtaposition of those with pi to be rather cool.

StNowhere wrote:It just ... makes my spleen twitch, or something. If I'm doing quick calculations, it's really not appreciably slower to at least use 3.14. I'm not going to use e=3, either, though it might round up to the same number. I mean, if you're just doing order-of-magnitude calculations, why not just round everything to the nearest power of 10 and call it a day? Propagate so much error through the calculation that you end up 5 or 6 orders off from what should have been the theoretical solution, never mind the experimental.

Seriously, 3 is less than 5% off from pi. That's not going to add up to 5 or 6 orders of magnitude in your final answer, unless you have like 40 powers of pi. And of course pi^2 = 10 so that's even less error there.

And I don't know how fast you are in doing calculations in your head, but multiplying something by 3.14 usually takes me *a lot* longer than multiplying with 3. Say I want to know the radius of the earth. I know the circumfence is 40,000 kilometers because that's the definition of the kilometer. So that's 40000 / 2 pi ~= 40k / 6 = 6.67k. You can do that in less than a second. But dividing by 3.14? Really? 40 / 3.14 that's 10 leaves 8.86 that's 2 leaves 8.86 - 6.28 = 2.38 which gives .7 leaves 238 - 210 - 7 - .28 is ehm ... Seriously, I have the aid of typing this out right now and I'm already getting frustrated.

For the record the actual answer is 6,371.0 km mean radius (that's not exactly 40k / 2 Pi. The kilometer was defined by meridional circumfence, and they made a slight error). I'm rather happy with the answer if got in less than a second.

Point taken. Admittedly, I tend to fall back on the old 22/7 approximation, which has the benefit of being easy to deal with (ends up with fractions involving sevenths or elevenths, which can be very quickly converted to decimals) and being accurate to two decimal places, instead of just the nearest integer. An approximation, still, but demonstrably more efficient, IMO. In your example, 40k/ 2*(22/7) = 40000 * 7/44 = 280k/44 = 70k/11 = 6,363.63...km. Maybe an extra step or two? Yep (took me less than a second, anyway), and the answer is now off by less than 10 km, instead of 300.

And obviously, my comment about being off by several additional orders of magnitude - admittedly hyperbole - applies mainly to more complicated calculations. You might use orders of magnitude to try to arrive at a ballpark estimate for the Stefan-Boltzmann constant, but if you do it as I jokingly suggested, you'll be off by a couple of orders of magnitude. Some will say that this isn't a big deal, but I've honestly never been impressed by order-of-magnitude calculations. Great; you're in the right ball park, astronomically. Come back to me when you've got an actual answer.

But the most important thing to remember is that this is just a pet peeve of mine. And if you ever use pi = 3 in front of me, I will crucify you, set the crucifix on fire, put anything that isn't charred to ash into an industrial incinerator to finish the job, then piss on your ashes before placing the mixture in an urn marked "BLASPHEMER" and burying it upside-down in the middle of an ancient Indian burial ground where Satanic rituals are held. But we can agree to disagree, right?

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### Re: Kilogram as a unit of weight

StNowhere wrote:it's really not appreciably slower to at least use 3.14

You multiply with 3.14 in your head as fast as you multiply with 3? Wow I am impressed.

Edit: Okay, maybe I should have read the posts above a bit more thoroughly.

gmalivuk
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### Re: Kilogram as a unit of weight

StNowhere wrote:I've honestly never been impressed by order-of-magnitude calculations. Great; you're in the right ball park, astronomically. Come back to me when you've got an actual answer.
Give me a computer or paper and a writing utensil and/or a few extra minutes, and I'll get you your answer. In the meantime, trying to figure out in my head if something is at least slightly reasonable, I'll use orders of magnitude. (Pi is about half an order of magnitude between 1 and 10, so if I'm willing to remember that 0.5 I get quite close, without having to divide anything by 11 or 7.)
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