What is the purpose of two units of mass in the Imperial system?

[chrisbroward]

TL;DR Summary

What's even the point of putting lb-m in most loading or force problems? Why can't we just stick to one unit.

I'll see pound-mass (lb-m) & slug.
I'm an undergrad student and always confuse between the two at times. It seems to me slug is a lot easier to use at times (given its easy conversion into force for most problems) (i.e 1 slug * 1 ft/s^2 = 1 lbf)

What's even the point of putting lb-m in most loading or force problems? Why can't we just stick to one unit.

 

[sophiecentaur]

The SI system manages to avoid most of those basic problems. I realise that comment doesn't help you but having a word that stands for both mass and force is confusing.

The problem goes right back to Galileo when people hadn't found the distinction between Mass, Weight, Momentum and Energy. Your two quoted terms / units in the question are used because of the general ignorance about this many hundred years old problem.

Basically, I'd say you have to avoid getting cross about this and just be bi-lingual about it. Avoid taking up a career where Imperial Units rule (if you can).

 

[Lnewqban]

Welcome, Chris!

The confusion does not stop at force and mass.
Please, see:
https://en.wikipedia.org/wiki/Pound_(force)

https://en.wikipedia.org/wiki/Short_ton

 

[Baluncore]

Real scientists use SI.

 

[chrisbroward]

Real scientists use SI.

Can't say I use much SI besides in my Physics I & II

In most of the classes related to engineering & design. It's almost exclusively in British Units. kip, psi, psf, lbf*in, mile, yard, etc.

 

[Baluncore]

Can't say I use much SI besides in my Physics I & II

In most of the classes related to engineering & design. It's almost exclusively in British Units. kip, psi, psf, lbf*in, mile, yard, etc.

Then you are probably in the USA, not Britain or the EU.
Those are US customary units.

 

[gmax137]

When I finished my physics bachelors, I went to do my masters in engineering. I thought the units were funny - things like gallons per minute. But you know what? They all work just fine. I admit: pounds mass, pounds force, and where to put the 32.2 are confusing. Well, they are confusing until you figure out what's going on.

Real scientists use SI.

Well, I think that's overstating things. I still see "MeV" used a lot, for example. And how about the cosmology / GR guys? from thread https://www.physicsforums.com/threads/what-about-the-big-ben-paradox.1057911/post-6973643: (emphasis added)

I wanted to add the actual GR math. The outcome of this is exactly as everyone who has any experience in GR said. Indeed, from first principles it could be no other way. But I had time yesterday to play around with this. All equations are using geometrized units where c=G=1.

We start with the weak field metric in cylindrical coordinates: ...

 

[difalcojr]

TL;DR Summary: What's even the point of putting lb-m in most loading or force problems? Why can't we just stick to one unit.

I suggest you start amassing many conversion sheets for all the different units used in all the many different sciences. Start a 'units' folder, you'll need conversions for future problem solving too. It will grow. Measurement units can be very confusing.

I like an old 1974 book, "The World of Measurements" by H. Arthur Klein. He examines and explains your question well. Out of print, but there are probably others, newer, that also explain the unit measurements, some history, and conversions needed to get to other measurement systems.

 

[sophiecentaur]

I suggest you start amassing many conversion sheets for all the different units used in all the many different sciences.

That would require a lot of dedication. There is a wide choice of websites with conversions between almost all units known to man. As you say, there are a number of books that discuss the history of unit and measurements but don't think in terms of reading them from cover to cover.

The OP should realise that the point in using multiple units is to reconcile the results of different paths through history. People can be very resistant to change and old habits die hard.

It's almost exclusively in British Units.

They are not "British Units". The name is "Imperial" because, with the exception of the weights of babies and quantity of beer (oh yes - and mph), you can live your life in the UK without touching feet and inches. It's amazing that the US left the Empire hundreds of year ago but they are still in love with 17/64", thou's and Farenheit. Such hard work. Just look at what's used by NASA, Bell Labs and JPL; that's the way to go.

In school, I went through fps then cgi then mks but came to rest in SI. Bliss

 

[pbuk]

In most of the classes related to engineering & design. It's almost exclusively in British Units. kip, psi, psf, lbf*in, mile, yard, etc.

They are not "British Units". The name is "Imperial" because, with the exception of the weights of babies and quantity of beer (oh yes - and mph), you can live your life in the UK without touching feet and inches.

It's even worse than that, the US does not use Imperial units for volumes - US liquid pints, quarts and gallons are all smaller than the Imperial units with the same names, but US fl oz are larger! And that doesn't even cover the US dry pints, quarts and gallons!

The correct term is "US customary units".

 

[Baluncore]

The bushel, a US customary volume, that had a different US customary weight in each US State.
https://en.wikipedia.org/wiki/Bushel#Weight

 

[Vanadium 50]

They are not "British Units".

What about the BTU?

 

[gmax137]

Wiki the weights and measures act of 1824.

Just which empire does anyone think the "imperial" moniker refers to?

 

[Vanadium 50]

My high school teacher had a 19th century algebra book and would now and again give us problems out of it, One involved converting Danish ells to French ells to Swedish ells etc. Back in the day, it was importtamt to convert between these differing national standards.

Now with SI, things are less...um...ellish.

 

[gmax137]

1980:

2024:

 

[difalcojr]

If I had to start again an undergraduate physics, engineering or other science course of studies, I would want, first thing, an introduction course in metrology. First thing. An overall view of all the different units would be very helpful in further studies. An overview would be very useful to avoid questions like that valid question posed in this thread.
The book I mentioned above even could be a course textbook, it is just that good. Encyclopedic, informative, fun. I went to school when only hand calculators were in use, no computers, so I photocopied many conversion charts. Haliday/Resnick physics texts had good conversion charts in the appendixes.

 

[sophiecentaur]

. I went to school when only hand calculators were in use, no computers, so I photocopied many conversion charts.

You need to 'get some time in' my boy. A book of four figure log tables was all we had and copying by hand was the only thing available. Godfrey and Siddons Twelfth Edition (a cherished Christmas present) was my source of that sort of info. The units in there were not suitable for today, of course.

Eee it were tough in them days.

 

[sophiecentaur]

Wiki the weights and measures act of 1824.

Just which empire does anyone think the "imperial" moniker refers to?

The British Empire really did rule the waves and most of the units, although the French had the kilogram first. But they were busy cutting people's heads off at the time. (I happen to be in the middle of 'A tale of two cities' at present. Dickens doesn't mention anything about Physical Units apart from quarts of madeira etc..

 

[Baluncore]

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, but now we have SI.

 

[sophiecentaur]

It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, but now we have SI.

Please Sir, I want some more.

 

[jack action]

And to complete this discussion:

Subway refuses to answer my questions about whether it's an International Footlong or a US Survey Footlong. A milligram of sandwich is at stake!​

 

[difalcojr]

Yes, absolutely, and why not also blame the ones who were at the start of the units problems?
The cubit wasn't good enough. Nooooo, they had to have a "royal" cubit.

 

[pines-demon]

TL;DR Summary: What's even the point of putting lb-m in most loading or force problems? Why can't we just stick to one unit.

I'm an undergrad student and always confuse between the two at times. It seems to me slug is a lot easier to use at times (given its easy conversion into force for most problems) (i.e 1 slug * 1 ft/s^2 = 1 lbf)

Oh boy are we really complaining of imperial units of mass? Wait until you see how many units of magnetic field there are in cgs units. I can only wonder what kind of abomination would have come from the imperial units if it had electromagnetic units.

 

[pbuk]

And to complete this discussion:

Very timely, but Randall would probably appreciate a link: https://xkcd.com/2888/

 

[DaveE]

I suggest you start amassing many conversion sheets for all the different units

google. That's the one thing is does nearly perfectly.

 

[Baluncore]

google. That's the one thing is does nearly perfectly.

What is the current exchange rate between the USD and the brass razoo?

 

[difalcojr]

TL;DR Summary: What's even the point of putting lb-m in most loading or force problems? Why can't we just stick to one unit.

Physics/engineering undergraduate student's starter kit for the physical sciences. 1976 style.
Except for the slide rule and U.S. math tables (also a 12th edition; 5 figure tables!) 17/64th scale ruler.
SI units were in full use then, too, but complete globalization to SI can never happen, as you all point out.
Texas Instruments and HP calculators were great tools, especially the programmable ones.
The change from slide rule to calculator (handheld computer) was just revolutionary.

 

[boneh3ad]

Another timely webcomic...

https://www.smbc-comics.com/comics/1707076216-20240204.png

 

[Hornbein]

View attachment 339767
Physics/engineering undergraduate student's starter kit for the physical sciences. 1976 style.
Except for the slide rule and U.S. math tables (also a 12th edition; 5 figure tables!) 17/64th scale ruler.
SI units were in full use then, too, but complete globalization to SI can never happen, as you all point out.
Texas Instruments and HP calculators were great tools, especially the programmable ones.
The change from slide rule to calculator (handheld computer) was just revolutionary.

I've still use my photon powered calculator from 1975 or so. It looks exactly the same as it did then.

 

[sophiecentaur]

I've still use my photon powered calculator from 1975 or so.

Do you have room for it in your pocket, plus the Garmin GPS, the cell phone and the 1MB digital camera?

 

[diogenesNY]

I remember way back in second grade (circa 1973) posters on the classroom wall, with various illustrations and conversions, proudly proclaiming: "America's Going Metric!"

 

[Hornbein]

I remember way back in second grade (circa 1973) posters on the classroom wall, with various illustrations and conversions, proudly proclaiming: "America's Going Metric!"

I remember it was scheduled for 1980.

 

[Vanadium 50]

The metric system is, and has been for some time, the preferred (and in some cases, only) legal standard in the US. It still hasn't completely caught on, with exceptions like 2 liter bottles, and 10 mg pills.

But what do you do to fix it? Criminalize the use of feet and pounds? "What are you in for?" "Murder - and you?" "I bough a pint of milk." "A pint! You fiend!"

 

[Baluncore]

But what do you do to fix it? Criminalize the use of feet and pounds?

It was once an offence to carry a tape measure onto a building site in Australia, if that tape was graduated in centimetres.

In Australia, it only took two years from 1974 to 1976, to convert the building industry from feet, inches, and fractions of an inch, to metric.

Australian linear measures must be graduated in millimetres and metres, because then, decimal points and fractions are not needed anywhere in the building industry. Builders only need to do integer arithmetic. Wastage of materials and time has been significantly reduced, resulting in savings that have been growing and compounding now for 50 years.

The thing keeping the US back, is their comedic addiction to customary units.
Long may we benefit from US intransigence.

https://themetricmaven.com/building-a-metric-shed/

 

[sophiecentaur]

It was once an offence to carry a tape measure onto a building site in Australia, if that tape was graduated in centimetres.

In Australia, it only took two years from 1974 to 1976, to convert the building industry from feet, inches, and fractions of an inch, to metric.

Australian linear measures must be graduated in millimetres and metres, because then, decimal points and fractions are not needed anywhere in the building industry. Builders only need to do integer arithmetic. Wastage of materials and time has been significantly reduced, resulting in savings that have been growing and compounding now for 50 years.

The thing keeping the US back, is their comedic addiction to customary units.
Long may we benefit from US intransigence.

https://themetricmaven.com/building-a-metric-shed/

The Australians can cope very well with change. Not long ago they went for a gun law which produced instant reduction in deaths and injuries; no fuss, they just did it.
Their good attitude to regularising the measurement system doesn't surprise me at all.

their comedic addiction to customary units.

There are many "comedic addictions" in the US. Guns and aged politicians are just two more examples. (I DO NOT have a problem with my memory!) What staggers me is the huge number of lovely, pleasant and balanced individuals over there. Pretty clever too!

 

[JT Smith]

Cost-benefit analysis for adopting the metric system cold turkey probably does not favor the adoption -- in the U.S. A small country like Australia (13M in the 1970s) probably had a lot to gain. It more or less had to change. The U.S. doesn't have to change.

For the average citizen it would be a PITA. For industry it would result in significant cost. The long term benefit? Maybe not as much as people imagine. And the old system wouldn't simply go away in a couple of years.

I had always been a little embarrassed that the U.S. never made the full switch. But now I'm not so sure how important it really is.

 

[sophiecentaur]

I had always been a little embarrassed that the U.S. never made the full switch. But now I'm not so sure how important it really is.

It just costs a steady drip drip of money and risk by not choosing to get in line.

 

[Baluncore]

For the average citizen it would be a PITA. For industry it would result in significant cost. The long term benefit? Maybe not as much as people imagine. And the old system wouldn't simply go away in a couple of years.

Said like a true addict. It will cost votes.

The aim is not to eliminate the old system, but to formalise the new.
The size of the country is irrelevant. Big countries make bigger savings.

The size of the industry is irrelevant. Big industries make bigger savings.
The automotive industry, in the US, left the US behind in the late 1970s.

The significant cost is not in changing, it is in not changing.

 

[sophiecentaur]

For the average citizen it would be a PITA.

It would be no worse than learning to use a smart phone or drive a Zoom connection. There's a very sad rump of reaction against decimal in UK. There are even some politicians who promise to allow imperial units for food sales.
The 'average citizen' is well capable of this stuff when they have no option. It makes me cross (you may have noticed).

 

[JT Smith]

Said like a true addict. It will cost votes.

You mean me? I use the metric system every day. But I also think that the old system works just fine for some things. The metric system as a panacea is a kind of groupthink.

I believe that the main reason for the U.S. not changing is about money. If there were money to be made by changing there would be change.

 

[gmax137]

. It makes me cross

To me, it's like being upset with those French people. You know, they have a different word for everything.

 

[sophiecentaur]

To me, it's like being upset with those French people. You know, they have a different word for everything.

Except the French have an interesting language and culture which is worth getting into. Is there anything at all interesting about 17/32"??

 

[Baluncore]

Is there anything at all interesting about 17/32"?

13.5mm is about 17/32"

 

[gmax137]

Is there anything at all interesting about 17/32"??

Ha ha. I have these in my box, I'm sure I never used them.

This one I have used.

 

[jack action]

Ha ha. I have these in my box, I'm sure I never used them.
View attachment 340101

This one I have used.
View attachment 340102

BSF? Best Spanner Forever?

 

[gmax137]

https://en.wikipedia.org/wiki/British_Standard_Whitworth

More than most people ever wanted to know. Although Whitworth threads are long out of fashion, Mr. Whitworth is the guy who really led the charge on standardizing thread forms.

 

[Baluncore]

BSF? Best Spanner Forever?

British Standard Whitworth = BSW, standardised threads at 55°. Later, when metallurgy and machining tolerances improved, there was British Standard Fine = BSF, also 55°. The Whitworth spanners were marked for the size of the thread = the diameter of the bolt. That worked OK when Whitworth was the one and only standard.

60° UNC and UNF threads appeared in the US, then migrated to the UK during WW2. Obviously, the 55° BSW and BSF threads mismatched the 60° UNC and UNF threads. There were also a 60° BSC = British Standard Cycle thread, and (metric) 47.5° BA = British Association (of Engineers) standard threads.

With so many thread standards, a spanner in the UK, (wrench in the US), (key = clé in France), (key = schlüssel in Germany), (key = chiave in Italy), are now all specified Across the Flats = AF.
Some British spanners during the transition were marked with AF, as say 1/2"AF, to distinguish them from 1/2"BSW, or 1/2"BSF, but are now simply marked 1/2".

Standardisation has reduced costs and employment.
The number of inventory items that must be kept in stock as spares will fall gradually as old stock is used or scrapped.

I need a LH 1-1/4" BSF nut to repair an old saw bench. They used to be a commodity, but I could not find one available anywhere. It is not easy to cut left-handed internal threads in an old engine lathe, because the start of the cut cannot be seen.

 

[Ranger Mike]

And to complete this discussion:

View attachment 339508

Subway refuses to answer my questions about whether it's an International Footlong or a US Survey Footlong. A milligram of sandwich is at stake!​

good show

 

[Nik_2213]

If you hunt around, there are real-neat 'Metrinch' spanners that have a sorta-crinkly jaw that grips both eg 3/8" and 10mm 'Across Flats'. Also, they'll usually grip fixing that was one of those before 'wear, tear & rust'...

Tangential: Any-one ever seen an Octal slide-rule ? Or how to craft such ??

 

[Baluncore]

Octal-Decimal slide rule calculator.
https://www.computerhistory.org/collections/catalog/X585.85

https://treasures.scss.tcd.ie/sliderule/Slide_Rules_for_Computer_Programmers_updated.pdf