Weight vs. Mass: I know their NOT the same, but cannot comprehend why

[Jackissimus]

Ah, conversions from one unit of measure to another . . . such a 'small' thing relatively speaking but with such capacity to cause a bad, bad day; even to the tune of $655,200,000

Yep, I use that bit to tease my American colleagues when I drive them around in Czechia and they start making fun of kilometres

[CrazyFizik]

Coming a bit late into the discussion, but anyway:

In my work I often perform installations of scientific instruments which are either American-made, Japan-made or German-made. I have to say that I like the German instruments the most. The Japanese have their horrible "Japanglish", which makes the manuals nearly unreadable, but the imperial units sometimes really boggle my mind. Like the first time I installed an antivibration table with the air pressure measured in psi. Pounds per square inch? Really? How do I convert that into bars? My toolbox is basically twice the size because I need to carry the metric tools and imperial tools. And what's with the weird ratios on the allen keys? How is it easier for anyone to have an allen key of the size 5/32 inch or 7/64 inch?

That said I have to say that the little inconsistencies in SI units also tick me off sometimes, being the perfectionist that I am. Why is the kilogram the base unit of mass? Shouldn't it be simply the gram without the prefix? Why don't we use megameters or gigameters (well, except KSP)? Why don't we use gigagrams instead of tons? Shouldn't this be the more logical system?

I also think we shouldn't be using degrees in angle measurement anymore, it's just completely arbitrary to have 360 degrees, 60 minutes, 60 seconds. We should be using radians instead, it's just a matter of getting used to it and it makes the math simpler.

Regarding Dichebach's point: if you grow up in metric, you don't think of imperial. I have no concept of the ceiling being 12feet high, I have to convert to meters. I never measured in feet and when I try to use my feet to measure something, I quickly realize that my foot is smaller than 30cm. Usually when I try to guess the height of a ceiling, I use me as a quick mental measuring stick, because I am about 180cm high.

At my work I have the eternal problem when using of U.S. and Russian microwave connectors. In Russia inch metric is used only for parts of large diameters. American Microwave connectors all inch. Even diameter of cut-in bayonet connectors differ 1.016 times.

Very frustrating when due to the fact that mixed connectors are breaking equipment that costs tens or hundreds of thousands of dollars

More funny that the microwave electronics with SI also use Gaussian unit system (CGS unit).

[Mr Shifty]

Yeah, pounds per square inch are still in common usage in the U.S. I hear them all the time; though it's often shortened colloquially to just "pounds", e.g. "the boiler is kept at 600 pounds of pressure." For vacuum, I usually see torr used, which are approximately equal to 1 mmHg, so those units are also still in wide use here. Torr are often combined with SI prefixes: mTorr or uTorr.

[Javster]

Basically, an apple has a MASS of 100g, and on Earth it weighs 1N. On the Moon, it would still have a mass of 100g, but would weigh less.

[Kerbin Dallas Multipass]

I appreciate the merits of metric and I try to use it whenever possible. Certainly in the papers I have published where units of measurement are involved I've used metric.

But in defense of the old Imperial system . . . I think for length at least, feet and inches are more 'intuitive' for the 'hands on' contexts where non-scientists (but including craftsman, workmen, and just plain homeowners) tend to use them. Compare:

My ceilings are 8ft ceilings vs. 2.438 meters ceilings

Objection your honor!

Let's look at the hands on approach. OK, first of all I would always measure my actual room height in all corners and not just believe that they are precisely 8ft (they never are).

Now lets say you put in wooden floors first. That will reduce your ceiling height by lets say half an inch. What have we got now?

8ft - 1/2 inch = 7ft 11 1/2 inches or 95.5 inches.

I'm sorry but thats just not intuitive, and its the most simple real world example I could think of.

Feet and inches are nice because theyre easy to guess with the naked eye, but the imperial system is by no means intuitive.

[Sochin]

Depends on what you are used to handling measurments in.

[PakledHostage]

There's not much to say about this topic that hasn't already been said, but I do think the lowly Slug has been overlooked. According to Wikipedia, the Slug is:

a unit of mass associated with Imperial units. It is a mass that accelerates by 1 ft/s2 when a force of one pound-force (lbF) is exerted on it.

I saw that Bunsen mentioned it, but nobody seems to have noticed. I always thought that the Slug was the "official" imperial unit of mass? (I was raised with the metric system so I don't know for sure...)

[lajoswinkler]

Oh maaaan ... Random outbursts of laughter in my office ... I am trying to hold it in now, I hope my boss doesn't notice...

Some gems (sorry for OT):

http://4.bp.blogspot.com/_wCTbikLCeBk/TDUlbuWiZwI/AAAAAAAAA_o/StjKZyVVrlM/s1600/engrish.jpg

http://delviewmedia.weebly.com/uploads/3/0/9/9/3099487/8740348_orig.jpg

Pascals are derived, so they are recommended, and all derived units are actually really easy to derive, there are never any conversion constants to remember (with the sole exception of Celsius, which is Kelvin + 273). Bars are actually non-SI units, you are right, but they are exactly 100 000Pa, so they are sort of pegged to Pascals. We don't usually use pascals to talk about gas pressure, only the weathermen use them for this (they usually say "we will have 1013 hectopascals today", how is it your country?). Everyone uses bars here, but they could just as easily be using Pascals, the real pressure unit.

I haven't seen that one, with the river.

1 bar = 100,000 Pa, though it depends on who do you ask for a definition.

There are bars, atmospheres, technical atmospheres... Too complicated. I say let's use atm for everyday talk. It's 101325 Pa. It's intuitive. For labeling gas tanks, pascals should be used.

Yes, weathermen use hPa here.

Yeah, pounds per square inch are still in common usage in the U.S. I hear them all the time; though it's often shortened colloquially to just "pounds", e.g. "the boiler is kept at 600 pounds of pressure." For vacuum, I usually see torr used, which are approximately equal to 1 mmHg, so those units are also still in wide use here. Torr are often combined with SI prefixes: mTorr or uTorr.

mmHg is still used in the medical community in most of countries in Europe, but for any laboratory work or calculations, pascals (newtons per square metre).

[tavert]

There's not much to say about this topic that hasn't already been said, but I do think the lowly Slug has been overlooked. According to Wikipedia, the Slug is:

I saw that Bunsen mentioned it, but nobody seems to have noticed. I always thought that the Slug was the "official" imperial unit of mass? (I was raised with the metric system so I don't know for sure...)

Slugs allow your physical equations to be consistent, so that F = m a without needing any unit conversions of standard gravity thrown in (BTW, this unit conversion factor is why g0 is in the specific impulse equation). But no one really uses them in serious communication. You could alternately use poundals as a unit of force, but same problem.

I worked for a US aerospace company for a few years, and our highly technical work was done in imperial units with legacy code, tools, and ingrained practices. It was terrible, nothing was consistent, the code and physical modeling tools had magic-number unit conversions thrown around everywhere.

We'd measure temperature in Rankine. Mass flow rate in pounds-mass per minute. Distances in inches, unless it was altitude which was always in feet. Area and volume in inches squared and cubed (resp). Energy in BTU. Shaft power in hp, but thermal power in BTU per minute (unless it was per hour). Pressure in pounds-force per square inch (psi). Humidity in grains of water per pound-mass of dry air. Converting between all of these and doing rudimentary thermo- and fluid dynamics, isentropic flow equations and the like, was made much more painful by the stupid choice of units.

Edited September 3, 2013 by tavert

[dronkit]

If I may add to the discussion, at least here in South America, the Kilogram is very often used for force everyday, meaning what has already been said, the weight of a 1 Kg body at standard earth gravity.

In colleges and schools there's a clear distinction between "Kilogram mass" (or just Kilogram) and "Kilogram force", abbreviated Kgf or "Kg" with a little arrow over it. I see the Kilogram force is not a non-existant thing, see here: http://en.wikipedia.org/wiki/Kilopond

Of course, in order for the equations to work, you have to usea a *consistent* set of units. In our case, we want to use F = m A, and for this to work, mass must be in Kg, force in N and acceleration in m/s^2. This system we call it International System.

If you mix Kg mass with Kg force, the equation does not work.

BUT, there can be another consistant set of units which is called "technical system" at schools and colleges: you can use Kg force for force, m/s^2 for acceleration, and a make-up unit for mass which we call UTM ("technical unit of mass") which is such that fixes up the equation, so F [Kg f] = m [uTM] A [m/s^2]

You'll see inmediately that 9,8 UTM = 1 Kg mass.

I never use that system, though.

[Diche Bach]

Objection your honor!

Let's look at the hands on approach. OK, first of all I would always measure my actual room height in all corners and not just believe that they are precisely 8ft (they never are).

Now lets say you put in wooden floors first. That will reduce your ceiling height by lets say half an inch. What have we got now?

8ft - 1/2 inch = 7ft 11 1/2 inches or 95.5 inches.

I'm sorry but thats just not intuitive, and its the most simple real world example I could think of.

Feet and inches are nice because theyre easy to guess with the naked eye, but the imperial system is by no means intuitive.

I'm sure you're right. If I had grown up using metric I'd not see Imperial as being more intuitive.

Some great points in here. Sheeze, who could've guessed that measuring was so complicated!

[Kerbin Dallas Multipass]

BTW: Do you "imperialists" know by heart that 8ft == 96 inches? Or that 3 yards == 108 inches == 9 feet?

(or that 2 gallons = 462 cubic inches or 256 fluid ounces)

If you ask 100 people in the street in a "metric country" how many litres fit in a cubic meter you will probably get 3³ correct answers, so its not trivial either I guess.

[xZise]

base 12 is actually much more useful than base 10. You know how the table of 2 and the table of 5 are really easy because they keep repeating the same pattern? For base 12 you have that for the table of 2, 3, 4 and 6. And the universe really doesn't care what numerical system we use, the laws of mathematics apply whether you use base 10 or binary. The problem about the imperial system is that it isn't self consistent like the metric system is. 1 kilometer is 1000 meter and 1000 millimeter is 1 meter, but a mile is 1760 yards and 36 inches is 1 yard.

Also, base 12 can be very intuitive. Ancient Egyptians used base 12. You have 12 finger bones in your 4 fingers, and you can keep track with your thumb.

Now as far as I know, only feet/inch use a 1 to 12 ratio. Almost any other unit uses another factor like yard/feet = 1/3. Now statue mile/feet = 1/5280 does use a multiple of 12 (440) but how often to you count in 440 ft steps. Both tons (1 short ton = 2000 lbs, 1 long ton = 2240 lbs) don't have multiple of 12. Now I don't know all imperial units, maybe there are values in between them to make them multiple of 12 but these are the most common.

The biggest problem with this 12 factor is, that our numbering system is usually decimal. The SI units would work in a duodecimal system (1728 (base 10) m = 1000 (base 12) m = 1 (base 12) km) but how many really use the duodecimal system. The same applies to binary prefixes (1024 B = 1 KiB, 1000 B = 1 KB): You can faster say that 12 265 843 B ≃ 12 MB by simply cutting 6 digits away, but to convert it into a binary prefix it not that straight forward. But as soon as you have the number binary, you simply can cut 20 bit away and get your MiB value. Also 1000 or 100 has a lot more divisors than 12, okay granted not 1/3 or 1/6.

When you step on the scales, you are using weight kilograms. When you read how heavy a plane is? Weight kilograms.

Depend on your scale: A scale with a spring usually measures only Newtons and convert it into kilograms with a fixed factor. A modern scale COULD measure the default weight (for example the tray) and then can deduct the local gravity, although I don't know if any scale does this as all are used on Earth. But when you have a scale were you put a weight on another end to counteract the probed mass it measures a mass.

[edit]Okay the type of scale where you place a mass on another end is then called a “balanceâ€Â. So it appears that a balance measures the mass and the scale the weight and simply label the measured value incorrectly.[/edit]

Fabian

Fyi: hPa is totally legitimate as hecto is the prefix for 100. hm would be 100 m.

Edited September 4, 2013 by xZise