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

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.

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).

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

Real scientists use SI.

Baluncore said:

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.

chrisbroward said:

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.

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.

Baluncore said:

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)

Dale said:

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: ...

chrisbroward said:

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.

difalcojr said:

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.

chrisbroward said:

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

chrisbroward said:

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

sophiecentaur said:

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".

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

sophiecentaur said:

They are not "British Units".

What about the BTU?

Wiki the weights and measures act of 1824.

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

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.

1980:

2024:

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.

difalcojr said:

. 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.

gmax137 said:

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..

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.

Baluncore said:

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.

And to complete this discussion:

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.

chrisbroward said:

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.

jack action said:

And to complete this discussion:

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

difalcojr said:

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

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

DaveE said:

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

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

chrisbroward said:

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.

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

difalcojr said:

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.

Hornbein said:

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?

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!"

diogenesNY said:

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.

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!"

Vanadium 50 said:

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/

Baluncore said:

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.

Baluncore said:

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!