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The Saint
2006-Feb-28, 10:42 PM
British Commonwealth and US technology, industry and science progressed very well on the foot, pound, inch, miles, ounces etc.

Apart from the issue of world standardisation, did the Metric system offer any really practical advantage?

Could an SI system have been developed and made to work based on Imperial measures?

Fr. Wayne
2006-Feb-28, 10:47 PM
very many advantages once you get past the size of a acre and the length of a mile. People could use just about any system even septigesimal if all could agree.

hhEb09'1
2006-Feb-28, 10:47 PM
O sure. Base 10 systems are naturally unwieldy. Just look at the mile--it used to be one thousand paces (mille passus) and now look at it. :)

The Saint
2006-Feb-28, 11:21 PM
The entire Apollo Project and Moon landings were made using the Imperial system.

Would it have been any easier practically for the scientists, engineers and builders to use the Metric one, or nothing in it?

Sock puppet
2006-Feb-28, 11:21 PM
British Commonwealth and US technology, industry and science progressed very well on the foot, pound, inch, miles, ounces etc.

Apart from the issue of world standardisation, did the Metric system offer any really practical advantage?

Could an SI system have been developed and made to work based on Imperial measures?

I wouldn't underestimate the practical advantages of almost never having to carry out unit conversion in a situation where you would be doing so many times per day otherwise. It's surprisingly easy to botch and, even with care, when you have to do it constantly mistakes will be made. In my opinion, it also makes it vastly easier for laymen to carry out calculations.

The Saint
2006-Feb-28, 11:26 PM
On the other hand, they say that the brain is like a muscle: if it isn't regularly force-exercised, it whithers.

Might having to do those constant daily conversions have kept the previous generations more mentally accute than ourselves?!

Chuck
2006-Mar-01, 01:53 AM
Some day we'll all have computer chips implanted in our heads. Then it won't matter which system we use. We'll automatically know all of the conversions.

ASEI
2006-Mar-01, 03:06 AM
There are coherent, nicely ordered systems of english units. One of them is slugs, ft, sec (with lbf as a force measurement, not a mass measurement). 1 slug*ft/sec^2 = 1 lbf.

The thing that's nice about the english systems is that they use units that correspond to easily estimable quantities. You can get a mental picture of 3 inches, or a quarter of a foot. A quarter of a meter is a bit more difficult. Feet or inches come out in nice quantities. Foot-lbs and psi also. Pascals, on the other hand, end up standing for a very small amount of pressure.

This is partially due to the fact that the english system was constructed from common measures in a series of powers of two and three, not ten. You can visualize a half or a third of something. Dividing a well known unit into ten smaller units, or assembling ten of them into larger units mentally is much more of a challenge. You can do the multiplication with SI, but not the visualization.

The Supreme Canuck
2006-Mar-01, 03:28 AM
The thing that's nice about the english systems is that they use units that correspond to easily estimable quantities. You can get a mental picture of 3 inches, or a quarter of a foot. A quarter of a meter is a bit more difficult.

Not for someone brought up using only metric. I can easily picture 25 cm.

Tog
2006-Mar-01, 08:48 AM
Almost all the (limited) math and physics that I know I lerned from the 1972 Wold Book Encyclopedia. All the formulas there were Imperial so that's what I'm comfortable with. I did learn the conversion factors to make things metric without having to look most of them up, but it takes time to do that when often I really don't want to.

That said, the metric system has some advantages over Imperial that I didn't see mentioned yet.

1 cc = 1 ml This is a lot easier than trying to figure out how many cubic inches in a pint.

1 Km = 1000m = 1,000,000 mm Takes a minimum amount of thought to convert in your head.
Quick, in your head, how many inches in 1.65 miles?

Sure, there are some that could eventually figure it out, and few that could do it very quickly, but for the average person, it needs a paper and pencil, and for many (tragically) a calculator.

The argument that having to do less arithmetic will cause people to think less is something I've thought of a lot since I got cell phone. I used to be able to remember numbers at a glance. After seeing it for about 1 second, I could retain a credit card number for a few days, longer if I actively tried to remember it. After 5 seconds I could recall any bit of information from either side of a military ID card. Thanks to cell phones, I have no idea of my best friend's or my Girlfriend's home phone numbers. There was no need to learn them, the phone knows them. I think being able to do math in your head is different though. It's like writing.

It's probably obvious that I rely on my spell checker a lot. These forums don't have one, so I mistype and misspell a lot of things. Being a touch dyslexic doesn't help. (The letters 'P' and 'B' ar often interchangable to me, that can't be a typo). It doesn't mean I don't have a decent vocabulary, or that I use use words without really knowing what they mean, it's that I have to rely on an outside device to have them come across correctly.

Given a formula I use often, M*V^2/450451, where mass is in grains (1/7000 pound) and V is in feet per second. I can compare the foot pounds of any two moving objects... eventually. This came up one time at work talking about the bird that Nolan Ryan hit with a fastball. I converted the weight of the ball and the estimated speed to units I could use. Metric would have made the conversion faster, but working the formula would be the same.

In this case, the calculator is my spell checker (to make sure I spell 2870 grains correctly), whereas the formula is my vocabulary.

Two areas I think it's interesting where Metric has NOT entered the mainstream are in measuring angles, and time. Grads have 100 degrees to a right angle, but no one seems to care. I know there are people who have done Metric time, but I still go to work at 11 pm or 2300 hours, not 0.96 of a day.

Overall, I think Metiric is a better system, and if I'd grown up with it, I would be just as comfortable with it as I am now with Imperial units. I really don't think I would be a less effective thinker any more than I would be if I spoke a language that was more phonetic and had fewer contradictory rules than English.

gwiz
2006-Mar-01, 09:07 AM
British Commonwealth and US technology, industry and science progressed very well on the foot, pound, inch, miles, ounces etc.

Apart from legacy products, UK industry has been using SI units since 1975. As someone who has used both systems for engineering design work, I know which I prefer, and it's not the one with slugs in it.

The Saint
2006-Mar-01, 09:35 AM
Just like the US and UK have resisted dropping the mile, WI they had held on to the Imperial system for science and engineering longer? Would the UK have even been allowed to join the EEC?

Indeed, are pounds, ounces, hundredweight, pints, quarts, acres, inches, feet, yards and miles still taught in UK schools? And in the US?

Are there plans to drop the mile for the kilometre? Does anyone else still use the mile apart from the US and UK?

Why hasn't the foot as an international airline measure of altitude been dropped?

UK fruit stall vendors have been taken to court and even jailed for continuing to sell bananas in pounds!

THE US chose the continental system of driving on the right to symbolise the independence from Britain. Why did they not adopt Napoleon's Metric system at the same time?

astromark
2006-Mar-01, 09:59 AM
I am 54 years young, and in my working life have used both systems. New Zealand converted to metric in the early 70's. It was a struggle for many including myself. Many of my friends and piers did not want or feel the need to change. They were wrong. Following is two cases which I think covers the debate well.
As you all know the polar explorer space craft plunged into Mars because the calculations of foot pounds and of thrust kilo watts were not done correctly. If the metric system had been applied to the building and test programs the error could not have happened. The detail of what actually happened will be of some debate but, my point is made. The conversion of one to the other is fraught with error and is prone to stuff things rather drastically. One day on the building site we needed to find the center of a garage opening for a center pole fixture. The older fallow used the trusted imperial system of feet and inches. I used my own metric tape measure. divided by two gave me the center point quicker than it took me to type this. While the boss was still converting feet to inches. I put the jug on for another coffee.
Yes the boys at NASA did get to the Moon in 1969 and continue to do many wonderful things using the imperial system. It would have and would be so simple if the metric system were to be used universally

captain swoop
2006-Mar-01, 11:08 AM
In the UK a few traders who refused to display Metric measures as a protest against membership of the EEC were taken to court. You can display imperial measures alongside your metric. Beer is sold in pints but wine and spirits are sold in metric measures as are things like milk, soft drinks etc. Fuel is priced and sold in Litres but car economy is still expressed as Miles Per Gallon. Distance in the UK is in miles mainly I think because of the cost of changing all the roadsigns to metric, the Military use Ks for distance. Imperial measures haven't been in British schools since the mid 1970s, I was educated with both as the changeover happened. I don't think you will find anyone in the UK under the age of about 35 who would know what a foot or an inch was.

Any further extension of Metrication is seen as 'surrendering to Europe' by the Tabloid press and the right wing so we are forced to chug along in a semi metric mess with kids leaving school knowing kilometers but learning to drive in vehicles with MPH as the main speed indication and miles as the distance on road signs. Even Maps are metric wit han added scale of miles.

gwiz
2006-Mar-01, 11:57 AM
Even Maps are metric wit han added scale of miles.
The Ordnace Survey changed their map scales to metric in the mid 1970s. Their most popular product was scaled up slightly from 1 inch to 1 mile (1:63360) to 1:50000. Even the old scale maps had an overlaid kilometre grid, not a mile one.

gwiz
2006-Mar-01, 12:00 PM
The entire Apollo Project and Moon landings were made using the Imperial system.

Would it have been any easier practically for the scientists, engineers and builders to use the Metric one, or nothing in it?
The scientists were using metric units even then, so the moon maps used for the EVAs had metric scales.

ASEI
2006-Mar-01, 01:30 PM
Well, no one likes a unit system to be imposed on them from on high, anymore than a language or a government. Especially by the French!

Perhaps someday we'll use metric, but only when we're comfortable with it and can visualize it, not when it is decreed that we will do so.

Bob B.
2006-Mar-01, 01:31 PM
Do countries using the metric system still use nautical miles and knots for sea and air navigation? Unlike statute miles, nautical miles have a real world derivation that actually makes good sense. A nautical mile is the length of one minute of arc of a great circle, and knots are just nautical miles per hour. Nautical miles seem to me to be a better unit for navigating oceans than kilometers.

captain swoop
2006-Mar-01, 01:39 PM
Well, the RN does

Eta C
2006-Mar-01, 01:51 PM
Do countries using the metric system still use nautical miles and knots for sea and air navigation? Unlike statute miles, nautical miles have a real world derivation that actually makes good sense. A nautical mile is the length of one minute of arc of a great circle, and knots are just nautical miles per hour. Nautical miles seem to me to be a better unit for navigating oceans than kilometers.

There's also a nice coincidence that 1 n.mi. is almost exactly 2000 yards. (within a percent, I believe 2026 is the exact number). This leads to people expressing distances in one of the oddest units I've ever enountered: the kiloyard (1 n.mi. ~ 2 kyd). It's actually practical in the nautical context once you get used to it. On the other hand, I once worked with a sonar prediction program that measured ranges in kyd, depths in feet, but sound velocities in m/sec. Talk about mixed units!

Argos
2006-Mar-01, 01:57 PM
Do countries using the metric system still use nautical miles and knots for sea and air navigation?

Yes, they do.

Down here airplane pilots use feet as a matter of custom (well, I´ve never seen a metric altimeter anyway). Mountains are measured in meters, though. In the deep countryside I have seen some folks using the league and the fathom.

Swift
2006-Mar-01, 02:33 PM
The thing that's nice about the english systems is that they use units that correspond to easily estimable quantities. You can get a mental picture of 3 inches, or a quarter of a foot. A quarter of a meter is a bit more difficult.
As The Supreme Canuck said, I have no problem estimating 3 mm or 30 C. I actually feel more comfortable with millimeters than 1/8 inches.

I think the comfort level of "feeling" is somewhat of an illusion. People act like they can feel the difference between an air temperature of 23F and 27F (wow, it's only 23F today, that's 4 degrees colder than yesterday), but they don't know how to dress for 0C versus 10C. I think it is just what you get used to.

As someone involved in industry, and who has worked for companies that have customers all over the world, almost all of our work is metric. Whether metric or imperial is "better" (beta was better too ;) ) the standard globally is metric and there is a huge advantage to having one system. I've dealt with situations where one customers was buying in kilograms and another in pounds, and it gets messy (heck, when most people were buying 100 kg or 1000 kg orders, we had one customer that insisted in buying in ounces! :wall: ). It is hard to change - my previous company switched over, parts sizes went from 1100xyz (for 1.1 inches) to 28xyz (28 mm). It probably took almost 10 years to switch completely. But once done, it was not a big deal.

hhEb09'1
2006-Mar-01, 05:37 PM
Perhaps someday we'll use metric, but only when we're comfortable with it and can visualize it, not when it is decreed that we will do so.When FIFA starts playing soccer with other than 8 foot by 8 yard goals :)

Bobunf
2006-Mar-01, 06:06 PM
The thing that's nice about the english systems is that they use units that correspond to easily estimable quantities...A quarter of a meter is a bit more difficult...

A quarter meter is easy--it's about 10 inches, a little less than a foot.

The thing I like about feet is that, if I have to pace something off, the measuring tool is very handy.

Bob

kzb
2006-Mar-01, 07:05 PM
Think the metre is new-fangled and French? Not according to a book I saw a few months back. It's related to the Sumerian "kush".

It's also very close to the pendulum length with a period of 1 second. Newton and other notables worked on this, the idea being to standardise a measure in terms of period of swing. That would enable anyone to standardise a metre by finding the length of rod with a period of exactly 1 second. Somehow though, the idea of the metre being 1/10 millionth of the circumference of the Earth got the upper hand, in the story of how it came about.

The book also contends that the Imperial and metric systems are more connected than we think at first sight.

Parrothead
2006-Mar-01, 07:50 PM
In Canada, conversion happened in the early-mid 70's. Some problems arose from the fact that not all industries could readily convert between systems of measurement. The fun part of course is the need to keep, an imperial and metric set of tools on hand. ;)

Kaptain K
2006-Mar-01, 08:10 PM
The fun part of course is the need to keep, an imperial and metric set of tools on hand.
Not to mention Whitworth, if you work on old Brittish cars!

hhEb09'1
2006-Mar-01, 09:21 PM
Think the metre is new-fangled and French? Not according to a book I saw a few months back. It's related to the Sumerian "kush".

It's also very close to the pendulum length with a period of 1 second. Actually, it's a pendulum that has a period of two seconds (http://www.roma1.infn.it/~dagos/history/sm/node3.html).
Newton and other notables worked on this, the idea being to standardise a measure in terms of period of swing. That would enable anyone to standardise a metre by finding the length of rod with a period of exactly 1 second.The problem was that gravity varied from place to place, so that wasn't a real standard--and you needed a standard second as well.
Somehow though, the idea of the metre being 1/10 millionth of the circumference of the Earth got the upper hand, in the story of how it came about.It's 1/10 millionth of a quarter of the circumference. When they noticed that, they were able to swing government funding for a nice little exploring expedition.

If a meter were defined so that it had a period of two seconds, then the acceleration of gravity, g, would be pi squared exactly. It almost is.

The Saint
2006-Mar-01, 10:37 PM
You mean 32 fpsps?

astromark
2006-Mar-02, 10:29 AM
There are exceptions to this metrication. . . aviation uses feet, or thousands of feet when talking of altitude. A global practice. To avoid the confusion of different scales. The nautical terms of fathoms and knots also seem to still be in favor. For myself, .I am a fan of the metric system. In every application the standardization would solve many a balls up. If you consider for a moment the practicality of the metric idea. 10. when you look into the imperial system you find not ten but, 1/32nd and 1/8th, 12, 36, and when talking of fluid measurement. 1 litre of water is 100mm X 100mm and its weight is 1 kilo. You just can't do these things with the imperial system. ,And speaking of this is it true the USA gallon is not the Imperial one at all. Do the US have there own? . . . Back to Astronomy, Why don't we metricate time?
History should teach us that changing from one to the other has caused the loss of millions of dollars worth of space equipment and the loss of many ( more than one ) space craft. When the media told us of the error coursing the loss of that mars polar lander. I first thought of those poor people who had spent hundreds of hours planning the mars landers mission, and built it wrong. . . . .

Tog
2006-Mar-02, 10:55 AM
In the US, a gallon (128 oz) is 4 quarts (32 oz), 8 pints (16 oz) or 16 Cups (8 oz), making it about 3.8 litres. Not sure what an Imperial gallon is.

A freind and I treid to metricify time, but ran into some snags. We ended up with two incompatible base units and couldn't get around them.

Midnight to midnight was equal to 1 D. Noon was .5 D.
A month was either 35 or 36 days long alternating each time for 10 months.
Leap year would happen on the last day of the last month, which would be the day before the spring Equinox (new years).

Years didn't fit well into this. We ended up with a year being a totally seperate thing and unrelated to days, since there was no way we cold find to make 366 rotations=1 revolution and still have a nice even 10 involved.


We also couldn't fit seconds in anyplace but decided that thy were used in too many formulas to abandon. It wasn't an overly successful trip to Denny's.:D

grant hutchison
2006-Mar-02, 12:12 PM
In the US, a gallon (128 oz) is 4 quarts (32 oz), 8 pints (16 oz) or 16 Cups (8 oz), making it about 3.8 litres. Not sure what an Imperial gallon is.An Imperial pint is 20 fluid ounces, and an Imperial gallon is 8 Imperial pints. However, there is a small difference between the definitions of a fluid ounce in the US and the Imperial measure, with the result the US gallon and pint are ~83% of their Imperial counterparts.

I remember, as a teenager, being baffled by a mnemonic given in one of Heinlein's juveniles (I think it was Have Spacesuit, Will Travel): "A pint's a pound the world around."
It was a pound and a quarter in my part of the world!

Grant Hutchison

Bob B.
2006-Mar-02, 01:30 PM
And speaking of this is it true the USA gallon is not the Imperial one at all. Do the US have there own?
What we use here in the United States is technically called the U.S. Customary System. This system is inherited from, but now different from, the British Imperial System. I don't know what the differences are, but it is entirely possible the U.S. and British gallons are not the same.

I like using the U.S. system in normal every day practice because it is what I was brought up on and I can visualize the units. For example, I can easily create a mental image of someone 5'-9" and 165 pounds, but I cannot 175 cm and 75 kg. To be able to get a feel for the measurements I have to convert to U.S. units.

However, when solving problems, particularly those involving masses and forces, I prefer using the SI system. This is mainly because of the common practice of expressing mass in units of pounds in the U.S. system. We therefore always end up with an equation where "mass" is divided by g to convert to the proper units of slugs. This often causes confusing for laymen and people unfamiliar with the U.S./Imperial system. I like using the SI system to avoid to whole bloody mess.

EDIT:

Of course I imagine you run into the same type of problems in the SI system. For instance, I’m guessing a bathroom scale is graduated in kilograms instead of the proper units of Newtons. This is just as grievous an error as using pounds to measure mass.

Eta C
2006-Mar-02, 02:02 PM
Of course I imagine you run into the same type of problems in the SI system. For instance, I’m guessing a bathroom scale is graduated in kilograms instead of the proper units of Newtons. This is just as grievous an error as using pounds to measure mass.

On the other hand, on a trip to Singapore I was amused at the labels on a locally produced can of diet soda. In the US such a product is usually described as "low calorie." There is was a "Low Joule Cola." (Joule being the SI unit of energy, as if most here didn't already know that. I won't even go into the fact that the "calories" one counts when dieting are actually kilocalories).

grant hutchison
2006-Mar-02, 02:17 PM
For instance, I’m guessing a bathroom scale is graduated in kilograms instead of the proper units of Newtons.But unless the force of gravity is variable in your bathroom, you can estimate your mass quite accurately using properly calibrated bathroom scales.
Maybe the error is that people talk about their weight when they're really interested in their mass. (But we're as likely to fix that terminological inexactitude as we are to get people to use the strict scientific meaning of "theory".)

Grant Hutchison

Bob B.
2006-Mar-02, 02:57 PM
But unless the force of gravity is variable in your bathroom, you can estimate your mass quite accurately using properly calibrated bathroom scales.
Maybe the error is that people talk about their weight when they're really interested in their mass.
I agree that for practical purposes a bathroom scale can be calibrated to accurately read one's mass, thus I don't have a problem with it reading in kilograms. However, speaking as a purist, the scale is actually measuring weight, i.e. force, i.e. Newtons. On the other hand, a balance type scale (like the one at your doctor's office) is actually measuring mass, i.e. kilograms. Calibrating a balance scale to read pounds is technically incorrect.

One of the big differences between the two systems of units is that in the U.S./Imperial system force (pounds) is a base unit and mass (slugs) is a derived unit. In the SI system mass (kilograms) is a base unit and force (Newtons) is a derived unit. As such, preference is given to the base units of pounds and kilograms in the respective systems. We therefore end up using phony units like pounds-mass and kilograms-force in common practice even though it is technically wrong.

grant hutchison
2006-Mar-02, 04:17 PM
However, speaking as a purist, the scale is actually measuring weight, i.e. force, i.e. Newtons.Wow, and I thought I was a purist. :)
I think my bathroom scales actually measure the distance the footplate moves against a compression spring. But Hooke's law tells me the distance is proportional to force, and Newton's tells me the force is proportional to mass.
IIRC, the tachometer in my car measures a force in a slipping magnetic clutch, and the airspeed indicator in a plane measures a pressure difference.
It seems like things would get very complicated if we were only allowed to calibrate measuring devices in the quantity being directly measured, rather than in the quantity of interest.

Grant Hutchison

Bob B.
2006-Mar-02, 05:05 PM
In my business to we typically use a method similar to an airspeed indicator to measure liquid flow rates. Pressure differences are measured as the fluid passes through a Venturi; this gives velocity that is then converted to flow rate.

As long as you know what you're measuring and the conversion to the unit you want, you can calibrate the instrument to read out the desired unit.

Whether a scale is calibrated to read pounds or kilograms doesn't make any difference to me. I was just pointing out that what we are actually measuring is not necessarily the unit we read off the dial. And it is different depending on the system of units we are using:

Spring-type bathroom scale -- Imperial units:
.... measure pounds (weight) ---> read pounds (weight)

Spring-type bathroom scale -- SI units:
.... measure Newtons (weight) ---> read kilograms (mass)

Balance scale -- Imperial units:
.... measure slugs (mass) ---> read pounds (weight)

Balance scale -- SI units:
.... measure kilograms (mass) ---> read kilograms (mass)

10.9
2006-Mar-04, 12:50 PM
In Canada, conversion happened in the early-mid 70's. Some problems arose from the fact that not all industries could readily convert between systems of measurement. The fun part of course is the need to keep, an imperial and metric set of tools on hand. ;)

For example, the Gimli Glider (http://en.wikipedia.org/wiki/Gimli_Glider) :eek:

ASEI
2006-Mar-04, 04:21 PM
I have this huge unit conversion spreadsheet. I'll convert and do some of my problems in metric, then convert back to see what it looks like. Stress/strain problems I can do in english pounds and inches. Certain lift/drag/flow problems I do in ft, slugs, sec. I usually use metric for thermal problems, but have to see the Farenheit units to compare the temperatures with the material property charts that we have from ASM.

2300K, what's that? 3680F?! Too hot! 293499Pa? huh? 6137psf.

In any case, we like to nondimensionalize everything (too many things, if you don't know which reference lengths you're using), so if you're dealing a nondimensional number, the system of measurement you use to compare parameters is arbitrary.

Fr. Wayne
2006-Mar-08, 12:10 PM
Thanks everyone from around the world for your updates on measuring units. Seems that Great Britain is where the politics of measurement are most daunting. Good subject Saint. I'd love to hear from the Far and Middle East on this subject too.

Irishman
2006-Mar-09, 09:41 PM
Apart from the issue of world standardisation, did the Metric system offer any really practical advantage?

Could an SI system have been developed and made to work based on Imperial measures?

To answer the OP, yes, an international system could have been devised using Imperial, or US Customary, or new magical numbers nobody yet knows. Just distribute the list and say "This is what we are now using. Violators will be shot."

As for practical advantages, converting dimensions larger and smaller are much easier in metric. As mentioned, liquid measure in particular is pretty nasty in US customary.



The thing that's nice about the english systems is that they use units that correspond to easily estimable quantities. You can get a mental picture of 3 inches, or a quarter of a foot. A quarter of a meter is a bit more difficult.

This is mostly an artifact of the system you first learned. It is somewhat convenient to know 1 inch is approximately the length of the first knuckle of my thumb, and 1 foot is just about the length of my shoe. But those are only approximates, and the approximation varies depending upon the individual.

Actual example, for teaching karate I pace off the floor and put down peices of tape to indicate where the children should stand. The actual distance changes depending upon which shoes I wear, or if I am barefoot. My current shoes are slightly longer than my previous pair. I know this because the tape placement differs against the lines painted on the gym floor.

Once you work with the system you can develop that sense of ability to approximate some reference dimension with some sense of amount.



As you all know the polar explorer space craft plunged into Mars because the calculations of foot pounds and of thrust kilo watts were not done correctly. If the metric system had been applied to the building and test programs the error could not have happened. The detail of what actually happened will be of some debate but, my point is made.

That is not really a problem inherent to the US customary system, but rather to communication across the team. Yes, agreement ahead of time to use consistent units would have prevented that loss, but so would have agreeing to use the U.S. Customary system, or actually checking the specifics in the contract when putting together the database that performed the transfer of data from one team to the other. Putting in the wrong formula is a problem regardless of which units you use, and failing to check that the formula is properly accounting for all unit conversions is a freshman error. Painful to experience, but no less valuable and painful if you fail to check within a consistent set of units.



Perhaps someday we'll use metric, but only when we're comfortable with it and can visualize it, not when it is decreed that we will do so.

But we'll only become comfortable through consistent use, and without a decree we're very likely not to use the new to learn the familiarity.



One of the big differences between the two systems of units is that in the U.S./Imperial system force (pounds) is a base unit and mass (slugs) is a derived unit. In the SI system mass (kilograms) is a base unit and force (Newtons) is a derived unit. As such, preference is given to the base units of pounds and kilograms in the respective systems. We therefore end up using phony units like pounds-mass and kilograms-force in common practice even though it is technically wrong.

In college engineering classes, we dealt with both systems. I got really perturbed when I encountered a problem with Kilograms Force units. There's no excuse for that. The pounds-mass problem is a historical artifact of equating amount of material to weight. But metric was established after the distinction between the two was recognized. So the correct units should be easy enough to use! (Okay, I do see how lazy thought can apply the "base units" in the wrong context. But there's no excuse in an Engineering text book.)

Bob B.
2006-Mar-09, 10:13 PM
In college engineering classes, we dealt with both systems. I got really perturbed when I encountered a problem with Kilograms Force units. There's no excuse for that. The pounds-mass problem is a historical artifact of equating amount of material to weight. But metric was established after the distinction between the two was recognized. So the correct units should be easy enough to use! (Okay, I do see how lazy thought can apply the "base units" in the wrong context. But there's no excuse in an Engineering text book.)
Kilograms-force really perturbs me as well. The only time I ever use it is as an intermediate step in a problem if doing so saves me from having to perform a conversion later on. I'll never express a final answer in kg-force.

I attended engineering school in the late 1970s and we used both system of units, though perhaps US units slightly more than SI. I don't recall us ever using the terms kilograms-force or pounds-mass. In SI units we always expressed mass in kilograms and force in Newtons. When working in US units we always expressed mass in slugs and force in pounds. We would, however, often refer to the "weight" of something, which of course is really the same thing as pounds-mass though we never called it that. At typical problem might give us the "weight" of something in pounds and we would then immediately convert it to slugs.

Bob B.
2006-Mar-09, 10:30 PM
History should teach us that changing from one to the other has caused the loss of millions of dollars worth of space equipment and the loss of many ( more than one ) space craft. When the media told us of the error coursing the loss of that mars polar lander. I first thought of those poor people who had spent hundreds of hours planning the mars landers mission, and built it wrong. . . . .

According to the following article by Jim Oberg,

Mars orbiter faces NASA’s ‘Ghoulish’ past (http://www.msnbc.msn.com/id/11718925/from/RS.1/)

the story popularized by the media regarding the loss of Mars Polar Lander is false, though confusion over units was partly to blame.

Irishman
2006-Mar-10, 06:37 PM
Regarding metric time, besides the inherent difficulty of transitioning from the current to any new system, there is an inherent problem that the normal blocks of time that interest us are not related by integer values. Ignore the partial elements of a day, let's look at the "natural" time values - day and year. (A week is arbitrary, we just happen to like 7 days, there's no real reason for it to be so. A month originally tracked the cycle of the moon, but now is just an arbitrary block of time.)

There are ~365.25 days in a year. That breakdown does not change (significantly). We want days because of the tracking of the light/dark cycle, and we want years because of the cycling of the seasons. There's no even breakdown of days into the year. Furthermore, there's no even breakdown of sets of days to make "weeks" or "months". There are different ways to accommodate this, and they have been tried. Some systems make an even number of months and then have the extra days as a holiday at the end of the year. But our system works as well as any other.

Okay, what about metrifying the Day? This could be done, redefining "hours" into 10 or 20 or 15 or however many preferred number of time blocks per day. Then you adjust the pieces evenly for "minutes" and "seconds". Even give them new names to help with the confusion. In the end, what does metric time gain? Improves the ease to calculating fractions of hours from minutes? There might be some value gained there, but since the metrifying of days would stop there and not extend, it is difficult to build up the impetus to implement such a system. France tried it - didn't last.



the story popularized by the media regarding the loss of Mars Polar Lander is false, though confusion over units was partly to blame.


Depends on what you mean. I read the report when it finally came out, and the description matches what Oberg said. The proximate cause of the loss of the orbiter was the inaccurate units conversion caused by one team developing in one system and the other team computing thrust in the other. That is a units conversion problem. One can dig deeper to find what caused that problem, and trace it back to the root cause, which was poor management. But that doesn't negate that the physical and immediate cause of the loss was too much thrust because the computer was given values in one system and treated them as values in another.

As my physics instructor stressed - CHECK YOUR UNITS! The answer to the problem is not a number, it is a number and units. If you only wrote the value and not the units, he took off points.