[CharlesS]

I always thought that pounds were a unit of weight (== force), not mass. The Oxford dictionary included with OS X seems to agree with me in that regard. However, I suppose that since the imperial system doesn't really have a separate unit for mass, it would sort of make sense that the pound would get co-opted for this role as well. After all, it's always the best when you have units for completely different things sharing the same name, distinguishable only by context, right? Like the ounce which can be a unit of weight, volume, or, apparently, mass - that's not confusing at all. Hooray for the imperial system!

[OreoCookie]

Originally Posted by CharlesS 

I always thought that pounds were a unit of weight (== force), not mass. The Oxford dictionary included with OS X seems to agree with me in that regard.

No, it's not.
The mass of an object does not depend on the gravitational field: whether you have a kilogram on the moon, on the equator or in outer space, it is always the same. The gravitational force is very different and depends on the location. Now, you could say that this is just a small difference: well, if you're doing semi-precise measurements (we're talking about the precision of percent or a fraction of a percent), then you'll see differences of gravitational force measurements of the same objects at different locations. Your measurement result then depends on the choice of scale!

If you use an old-school balance, you always measure the mass. If you measure with a spring scale, you actually measure force. If you were to get a very precise spring scale, you could see the differences and inconsistencies.

This is not just a theoretical effect, whenever you do something away from the Earth or you do precision measurements, you see these problems.

Originally Posted by CharlesS 

However, I suppose that since the imperial system doesn't really have a separate unit for mass, it would sort of make sense that the pound would get co-opted for this role as well.

I would think so.

Originally Posted by CharlesS 

After all, it's always the best when you have units for completely different things sharing the same name, distinguishable only by context, right? Like the ounce which can be a unit of weight, volume, or, apparently, mass - that's not confusing at all. Hooray for the imperial system!

Don't forget about all the different conversion factors which you better memorize!

[sek929]

I wish I learned it as a kid, but now it's way to late. Especially so given my career in construction.

I can eyeball lengths (in inches) within a 1/4" regularly. Same goes with feet. I can kinda visualize meters since it is similar to a yard, but centimeters? Fuggetaboutit!

2x4s would be 5.08s x 10.16s. 3/4" plywood would be 1.905 centimeter plywood.

I get that Metric is easier overall and actually makes sense, but I don't really see a need for it in adult life. Maybe if I were in the Physics or Engineering fields I'd feel more strongly about this.

[dzp111]

Originally Posted by sek929 

2x4s would be 5.08s x 10.16s. 3/4" plywood would be 1.905 centimeter plywood.

Which is why in Canada while being metric since the 80's, you'll find 2x4, 2x6, etc. in virtually every lumber yard/store. Using metrics in construction, especially any type of framing would be way too laborious and unnecessary.

I wonder if that's the case in other metric countries?

[CharlesS]

Originally Posted by OreoCookie 

No, it's not.
The mass of an object does not depend on the gravitational field: whether you have a kilogram on the moon, on the equator or in outer space, it is always the same. The gravitational force is very different and depends on the location. Now, you could say that this is just a small difference: well, if you're doing semi-precise measurements (we're talking about the precision of percent or a fraction of a percent), then you'll see differences of gravitational force measurements of the same objects at different locations. Your measurement result then depends on the choice of scale!

If you use an old-school balance, you always measure the mass. If you measure with a spring scale, you actually measure force. If you were to get a very precise spring scale, you could see the differences and inconsistencies.

This is not just a theoretical effect, whenever you do something away from the Earth or you do precision measurements, you see these problems.

For crying out loud, I know what mass is. The issue is not whether mass and weight are the same thing (they're obviously not), but what the pound is. It's always been my understanding that the pound was a measure of weight or force, and if you were measuring something in space or on the moon or wherever, there'd be a 99.999% chance that you were using the metric system anyway, so you'd be using grams/kilograms. I've never heard of a balance that measured in pounds (except, I guess, for the ones in doctors' offices), and I've never heard of pounds being used for anything other than terrestrial uses such as measuring personal weight, construction projects, etc., and only in the US at that. I've always thought it was that if you weigh x pounds on the earth, then you weigh y<x pounds on the moon. If that's incorrect and pounds are being used for mass somewhere, then so be it. But there's no need to be so insulting, especially since all my comments in this thread have been fairly tongue-in-cheek.

[OreoCookie]

I think you misunderstood something, I didn't mean to be insulting at all.

To answer your question: the pound is a unit of mass and defined as 0.453… kg (I can only remember the first three digits). Scales always measure mass, forces are measured with force gauges.

[CharlesS]

Originally Posted by OreoCookie 

I think you misunderstood something, I didn't mean to be insulting at all.

I think you misunderstood something, because you were trying to give me a high-school physics lesson as if I'd been saying that mass and weight were the same thing, which I clearly wasn't.

To answer your question: the pound is a unit of mass and defined as 0.453… kg (I can only remember the first three digits). Scales always measure mass, forces are measured with force gauges.

Huh? Balances measure mass. If you have another object of a known mass, you can compare something's mass against it, which is what a balance does - you move the little masses around until you find the combination of masses that add up to the same mass as whatever you're measuring. A regular scale that just has a spring in it measures the downward force that the earth is exerting on the spring due to your standing on it, which is equal to your weight, not your mass, and it's going to give you a different result if you take it to the moon than it will give you on the earth.

[CreepDogg]

Originally Posted by OreoCookie 

I think you misunderstood something, I didn't mean to be insulting at all.

To answer your question: the pound is a unit of mass and defined as 0.453… kg (I can only remember the first three digits). Scales always measure mass, forces are measured with force gauges.

Really? My bathroom scale measures mass? And here I always thought it measured force and converted it to mass using a known gravitational constant. It must have some really small, intricate balancing system inside it...

I've always viewed it as pounds are used interchangably for force and mass, with the force unit defined as the earth gravitational force of a 1 pound (mass) object. Which works well on earth, but requires conversion elsewhere.

...and I think this is also a good demonstration of the value in knowing both systems...

[Eug]

Originally Posted by dzp111 

Which is why in Canada while being metric since the 80's, you'll find 2x4, 2x6, etc. in virtually every lumber yard/store. Using metrics in construction, especially any type of framing would be way too laborious and unnecessary.

Another pet peeve of mine: 2x4s don't actually measure 2" x 4" in-store, even in the US.

P.S. My bicycles for example are metric. Thus, I need to keep to two toolsets around, one for my imperial stuff and one for my metric stuff. Annoying.

[sek929]

Originally Posted by Eug 

Another pet peeve of mine: 2x4s don't actually measure 2" x 4" in-store, even in the US.

Framing in older houses usually have 'true' 2x4s and the like, actually a huge pain in the ass to remodel with modern size framing.

All framing is usually 1/2" smaller than the printed size which is the rough size of the lumber before the final trim. 2x4 = 1.5x3.5, 2x6=1.5x5.5. However, 2x8s tend to be 1.5x7.25 (7 1/4) as well as 2x10s, 1.5x9.25 (or 9.375).

Finally, 2x8 and 2x10 pressure-treated lumber can range anywhere from 9.5 to 9.75. Also, most plywood is sold by the 32nd, instead of 1/2" plywood it will be marked as 15/32" (which is slightly under a true half inch).

[OreoCookie]

Originally Posted by CharlesS 

I think you misunderstood something, because you were trying to give me a high-school physics lesson as if I'd been saying that mass and weight were the same thing, which I clearly wasn't.

Apparently I was wrong in my assumption that you were unclear about the difference and I apologize. Mea culpa. I'm a physicist and I care about these things, because they are important to my everyday life. (And I teach which makes it worse.) Peace?

Originally Posted by CharlesS 

A regular scale that just has a spring in it measures the downward force that the earth is exerting on the spring due to your standing on it, which is equal to your weight, not your mass, and it's going to get you a different result if you take it to the moon than it will give you on the earth.

Most modern scales (the ones you see in the supermarket) measure force, because the approximation that the gravitational acceleration is approximately constant is good enough so as to not change measurement results appreciably. However, it's still correct to say that you're measuring the mass of an object (e. g. you find out that you've gained yet another half kilo after a weekend of good food), because all you do is relate a force to a mass (in a semicontrolled environment). As a matter of fact, most scales these days actually measure a voltage (they use a small strip of metal that changes shape under the strain by whatever you would like to measure; this changes the resistance of this element which can then be measured accurately).

High-precision scales work like balances (e. g. via little electromagnets that acts as counterweight), because they'd be able to detect the tiny differences in the Earths gravitational field.

[CharlesS]

Very well, but how could the pound be a unit of mass though? The unit predates Newton's discovery of gravity by centuries. I doubt the concept of mass was even known at the time the pound was originally thought up.

[OreoCookie]

I would guess because people initially thought forces and masses are the same thing. Although I have no idea about how the pound came to be, historically speaking.
The modern-day definition is derived from the kilogram, though, because scientists need all those digits after the comma for squeezing results out of their measurement data.

[ghporter]

A pound is a unit of weight. Weight is a measure of the force exerted on the mass of a body by a gravitational field.

[OreoCookie]

A pound is defined in terms of the kilogramm, a measure of mass, not force. If it were a force, it'd be defined in terms of Newtons. (And yes, in terms of physics, weight is a force, not a mass.)

[CreepDogg]

Originally Posted by OreoCookie 

A pound is defined in terms of the kilogramm, a measure of mass, not force. If it were a force, it'd be defined in terms of Newtons. (And yes, in terms of physics, weight is a force, not a mass.)

It's also a measure of force, which makes sense if a lb-ft can be a measure of torque.

[OreoCookie]

That's pound-force and not pound:

Originally Posted by OreoCookie 

If you measure forces in `pounds' (put properly: pound-force), you equate the gravitational force of an object of one pound (mass times acceleration due to the Earth's gravitational field) with the mass (1 pound) itself. In terms of numbers the difference is just a factor, but physically, the bare definition is ill-defined and the implicit definition is misleading.

[CreepDogg]

Originally Posted by OreoCookie 

That's pound-force and not pound:

Yes, although either reference is correct in common use.

Originally Posted by The Linked Wiki Article

The pound-force or simply pound (abbreviations: lb, lbf, or lbf) is a unit of force.

[CharlesS]

I think what we can take out of this discussion is that the imperial system is confusing.

[CreepDogg]

How so? Seems very simplified to me in that it re-uses units for different measures!

[OreoCookie]

Convenient, huh? Next thing you know and you use yard to measure the volume of beer.

[CreepDogg]

See how easy that is?

[Face Ache]

Originally Posted by sek929 

I can eyeball lengths (in inches) within a 1/4" regularly.

This makes him popular in the bathhouses.

See now, inches always seem slightly smutty to me. Despite my country using metric, nobody considers their junk in centimetres - the Imperial inch is maintained specifically for penile prowess. So, as far as I'm concerned, using Imperial is like comparing everything to the size of your penis. Just define the average penis as a length of measurement and get it over with. The Phallic measurement system.

[ghporter]

The point we were talking about was foot-pounds. Since pound CAN be a measure of force (whether it was later redefined in terms of the kilogram or not), and since foot-pounds is a measure of torque as in distance times force, can we agree that the unit of measure "foot-pound" is NOT "nonsense?"

When one "weighs" anything, one is measuring the force exerted by gravity on the body being measured, so while "pound" can be shoved into a "mass" pigeonhole, it is primarily a measure of the force exerted between the body and the Earth.

[turtle777]

Originally Posted by sek929 

All framing is usually 1/2" smaller than the printed size which is the rough size of the lumber before the final trim. 2x4 = 1.5x3.5, 2x6=1.5x5.5. However, 2x8s tend to be 1.5x7.25 (7 1/4) as well as 2x10s, 1.5x9.25 (or 9.375).

Oh WTF ?

Let me say this clearly: if this system wasn't backed by the US, it would have been shelved a long time ago.

All hail the power of tradition

-t

[CreepDogg]

Originally Posted by turtle777 

All hail the power of tradition

I'd always chalked that one up to the power of profit & marketing.

[OreoCookie]

Originally Posted by ghporter 

Since pound CAN be a measure of force (whether it was later redefined in terms of the kilogram or not), and since foot-pounds is a measure of torque as in distance times force, can we agree that the unit of measure "foot-pound" is NOT "nonsense?"

It is non-sense, because pound and pound-force is not the same thing: 1 pound-force = 32.17405 pound x ft/s^2. The unit pound-force is usually abbreviated by lbf (as opposed to lb or lbm). Two different units that happen to share the same name (in colloquial terms). The confusion arises, because most of the time, pound-force is abbreviated to pound and some people start to think it's the same thing or the same unit.

The unit pound (≠ pound-force) has always been a measure of mass, so the sister unit has always been kilogram and not Newton (1 N = 1 kg m/s^2). It has been redefined, because the Imperial system was not precise enough for applications. At one point, conversion factors were fixed -- including the one for pounds.

Originally Posted by ghporter 

When one "weighs" anything, one is measuring the force exerted by gravity on the body being measured, so while "pound" can be shoved into a "mass" pigeonhole, it is primarily a measure of the force exerted between the body and the Earth.

No, the two are physically distinct. Unfortunately in English, weighing is ambiguous, because in colloquial terms, it refers to measuring mass, but the weight is, physically speaking, the gravitational force that's pulling on a body due to its mass. It's not shoving one thing into another pigeonhole, and it's not done a posteriori. When you measure the mass of an object on a balance, for instance, you do not (and cannot) measure the gravitational force, but only the mass of an object.

[Laminar]

I just don't know what I'd do without slugs.

[ghporter]

The unit "foot-pound" is well established, whether it makes sense in an SI world or not. It is, by its nature "colloquial" because it comes from a time when there was no reason to separate mass and the effect of gravity on the mass of a body. It isn't useless, idiotic, inane or "nonsense" (in the colloquial sense) because it has real uses and makes sense in its proper context, which is in a terrestrial, static setting.

And all of this highlights the fact that one cannot wave some magic wand and say "today we're all SI!" There are way too many issues that have to be worked through, from physics and engineering applications (not to mention mechanics, where foot-pounds is almost universally used in the U.S.), to day to day units like how big a cup of coffee is, and how fast traffic is allowed to flow. To say that the U.S. is behind in going metric is a vast understatement, but to say that the units with which we as a society operate are stupid or "nonsensical" is rude. They are what our society has developed with, and our society must make the group decision to move beyond them to the simpler and easier to use SI units.

[CreepDogg]

Originally Posted by OreoCookie 

When you measure the mass of an object on a balance, for instance, you do not (and cannot) measure the gravitational force, but only the mass of an object.

I don't think that's entirely true. Aren't you basically comparing the gravitational force of the objects on each side of the balance? How would you use the balance in space?

Using a balance assumes the force of gravity is the same on each of its sides. Which, in general, is a pretty valid assumption, but an assumption nonetheless.

[Spheric Harlot]

Originally Posted by Face Ache 

This makes him popular in the bathhouses.

See now, inches always seem slightly smutty to me. Despite my country using metric, nobody considers their junk in centimetres - the Imperial inch is maintained specifically for penile prowess. So, as far as I'm concerned, using Imperial is like comparing everything to the size of your penis. Just define the average penis as a length of measurement and get it over with. The Phallic measurement system.

followed immediately by:

Originally Posted by ghporter 

The point we were talking about was foot-pounds.

Well, good grief, we'd all be whales if we dealt in penile pounds, wouldn't we?

[OreoCookie]

Originally Posted by ghporter 

The unit "foot-pound" is well established, whether it makes sense in an SI world or not.

This has nothing to do with SI units, it's an inconsistency that exists due to the Imperial System itself: it tricks people into thinking pound and pound-force are the same.

Originally Posted by ghporter 

It is, by its nature "colloquial" because it comes from a time when there was no reason to separate mass and the effect of gravity on the mass of a body. It isn't useless, idiotic, inane or "nonsense" (in the colloquial sense) because it has real uses and makes sense in its proper context, which is in a terrestrial, static setting.

You always have to separate the gravitational force from mass, even in the context you use. That's why there are two units (which `just' have the same name). The problem is that people conceptually do not separate mass and force -- which is probably Newton's biggest insight. (And Newton probably used pounds to measure mass, I reckon.)

The difference between mass and force becomes important even in the setting you quote: momentum is mass times velocity. The change in momentum (which is Newton's law of motion) is given by a force. If you drop something in free fall, then

dp/dt = m dv/dt = m a = F_g = m g.

(m is mass, a is acceleration, g is the Earth's acceleration.) So even on Earth in a very boring, elementary setting, you immediately see that you cannot and must not identify F_g and m.

Originally Posted by ghporter 

And all of this highlights the fact that one cannot wave some magic wand and say "today we're all SI!" There are way too many issues that have to be worked through, from physics and engineering applications (not to mention mechanics, where foot-pounds is almost universally used in the U.S.), to day to day units like how big a cup of coffee is, and how fast traffic is allowed to flow.

Again, this has nothing to do with SI units: engineers, physicists and such know that 1 lb ≠ 1 lbf (1 pound ≠ 1 pound-force). Otherwise they couldn't work. It's not understanding the physics behind it properly -- which is true no matter which system of units you use.

[OreoCookie]

Originally Posted by CreepDogg 

I don't think that's entirely true. Aren't you basically comparing the gravitational force of the objects on each side of the balance?

The force is due to gravity, yes, but the gravitational acceleration cancels out.

If the balance is in, well, balance, then the two forces must be the same:

F_1 = m_1 g = F_2 = m_2 g
==> m_1 = m_2

This works for any g, be it the one on Earth, on the Moon or on Jupiter. If you were to apply a force on the other side of the balance rather than putting weights with known mass on them, you would measure a force, yes. Of course, you assume that g is approximately the same on both sides, but that's an assumption you can verify or falsify.
[QUOTE=CreepDogg;3827809]How would you use the balance in space?
You wouldn't, it doesn't work in space.

[ghporter]

Originally Posted by OreoCookie 

It's not understanding the physics behind it properly -- which is true no matter which system of units you use.

My point was that it doesn't take an understanding of the low-level physics behind torque to properly use a torque wrench.

And I think we've sort of beaten a pretty big derail path between the two of us, so maybe we should agree to disagree on the "sense" of established, Imperial units developed before the British (or Americans) felt the need to think seriously about physics when using applied mechanics... (There's a whole "classist" subtext in this that would take a few volumes of history to even skim over...)

[CreepDogg]

Originally Posted by OreoCookie 

The force is due to gravity, yes, but the gravitational acceleration cancels out.

If the balance is in, well, balance, then the two forces must be the same:

F_1 = m_1 g = F_2 = m_2 g
==> m_1 = m_2

This works for any g, be it the one on Earth, on the Moon or on Jupiter. If you were to apply a force on the other side of the balance rather than putting weights with known mass on them, you would measure a force, yes. Of course, you assume that g is approximately the same on both sides, but that's an assumption you can verify or falsify.

So although mass and weight (force) are separate entities, we cannot observe or measure mass without applying any force (unless, I guess, we had an object with a known density and could measure its volume).

I think that's where a lot of the confusion originates, and probably the reason why the pound developed in use the way that it did. Because although a pound-mass and pound-force are different, a 1-pound (mass) object exerts 1 pound (force) on the surface it sits on when still (assuming base altitude and gravity on earth).

[OreoCookie]

Originally Posted by CreepDogg 

So although mass and weight (force) are separate entities, we cannot observe or measure mass without applying any force (unless, I guess, we had an object with a known density and could measure its volume).

Of course we can observe mass independently of forces: momenta. By measuring momentum and velocity, you can measure the mass. This has nothing to do with measuring force. Alternatively, you could measure energy and velocity.

That's exactly the reason why mass and force aren't just two sides of the same coin, but different concepts. This is the genius of Newton, because up to then, only mass was known.

Originally Posted by CreepDogg 

I think that's where a lot of the confusion originates, and probably the reason why the pound developed in use the way that it did. Because although a pound-mass and pound-force are different, a 1-pound (mass) object exerts 1 pound (force) on the surface it sits on when still (assuming base altitude and gravity on earth).

That were true only if all forces were gravitational in origin, they're not! You can use levers, pulleys or electromagnetism to create forces that have nothing to do with mass. The pound was and still is a measure of mass. The pound-force is defined just as in SI units: mass times acceleration and in that respect, there is no difference between SI units and Imperial Units.

[CreepDogg]

Originally Posted by OreoCookie 

Of course we can observe mass independently of forces: momenta. By measuring momentum and velocity, you can measure the mass. This has nothing to do with measuring force. Alternatively, you could measure energy and velocity.

So how do you measure/observe momentum?

That were true only if all forces were gravitational in origin, they're not! You can use levers, pulleys or electromagnetism to create forces that have nothing to do with mass. The pound was and still is a measure of mass. The pound-force is defined just as in SI units: mass times acceleration and in that respect, there is no difference between SI units and Imperial Units.

Right - which is why I related it to an object sitting still (and, OK, with no other levers, pulleys, or electromagnetic forces applied to it). I'm not trying to prove you wrong (you're not), I'm merely exploring the source/origin of the confusion.

You are absolutely right from a physics perspective, but my point, and I think that of ghporter and others, is that for a lot of practical applications, the difference is moot. Confined to earth and bound by gravity, mass and weight are directly proportional (differing only by the gravitational constant), so you can readily determine one by measuring/observing the other.

[OreoCookie]

Originally Posted by CreepDogg 

So how do you measure/observe momentum?

For microscopic objects, you need quantum mechanics and there are no forces in quantum mechanics anyway. But there, you use that the wavelength of a particle is inversely proportional to the momentum.

For macroscopic objects, you can use elastic collisions with particles of known mass, for instance and then conservation of energy and conservation of momentum to calculate it. (Think of two billiard balls.)

It's probably simpler to measure kinetic energies, e. g. by converting it to another form of energy (say, deformation in a totally inelastic collision or heat).

Originally Posted by CreepDogg 

You are absolutely right from a physics perspective, but my point, and I think that of ghporter and others, is that for a lot of practical applications, the difference is moot.

What this amounts to is that most people don't know that foot-pound should more properly be called foot-pound-force. If you're thinking of people cranking widget, they don't have to know the difference, but that's just ignorance. My sister also doesn't know what `bar' means, but she does know how many bars she needs on her bicycle tires.

[CreepDogg]

Originally Posted by OreoCookie 

For microscopic objects, you need quantum mechanics and there are no forces in quantum mechanics anyway. But there, you use that the wavelength of a particle is inversely proportional to the momentum.

For macroscopic objects, you can use elastic collisions with particles of known mass, for instance and then conservation of energy and conservation of momentum to calculate it. (Think of two billiard balls.)

It's probably simpler to measure kinetic energies, e. g. by converting it to another form of energy (say, deformation in a totally inelastic collision or heat).

Right - so, quantum mechanics aside (we've been talking Newtonian physics to this point), any measurement involves application of a known outside force (as in colliding a billiard ball into another).

What this amounts to is that most people don't know that foot-pound should more properly be called foot-pound-force. If you're thinking of people cranking widget, they don't have to know the difference, but that's just ignorance. My sister also doesn't know what `bar' means, but she does know how many bars she needs on her bicycle tires.

Fair enough - though not everyone is, or needs to be, a physicist.

[OreoCookie]

Originally Posted by CreepDogg 

Right - so, quantum mechanics aside (we've been talking Newtonian physics to this point), any measurement involves application of a known outside force (as in colliding a billiard ball into another).

The type of force during elastic collisions is immaterial, all that matters are initial and final state.

Originally Posted by CreepDogg 

Fair enough - though not everyone is, or needs to be, a physicist.

Or engineers. Or anyone else who wants to build stuff.

[Dakar V]

I want to build an awesome hoagie.

[Laminar]

Originally Posted by Dakar V 

I want to build an awesome hoagie.

Well will it be a footlong or a 30.48 centimeter-long?

[Dakar V]

Who cares? All I know is it's going to require 30 pounds of force to shove into your mouth.

[Laminar]

Originally Posted by Dakar V 

Who cares? All I know is it's going to require 30 pounds of force to shove into your mouth.

Pounds-force or pounds-mass?

[sek929]

I think the correct unit for that is like Spheric said, penile pounds.

[Eug]

This thread is even geekier than the anti-brushed metal threads.

[Laminar]

But not as geeky as the Blu-ray vs. HD thread.

[Eug]

Originally Posted by Laminar 

But not as geeky as the Blu-ray vs. HD thread.

That was pretty geeky, but this one easily beats it, considering some of the posts are basically a bunch of equations.

[ghporter]

...and now I think I see a reason to buy a Blu-Ray player-LOTR is coming out on BR soon... DRAT!

But will the discs play in an American machine? I mean, what with our Imperial units of measure and everything...

[Laminar]

Originally Posted by Eug 

That was pretty geeky, but this one easily beats it, considering some of the posts are basically a bunch of equations.

Uh...a couple posts containing equations is FAR less geeky than over EIGHT THOUSAND posts arguing a freaking video standard.