Pounds is a unit of mass or weight

[holtvg]

I've been looking around and some places tell me pounds is a english unit of weight and other tell me it's a unit of mass i.e lbf and lbm. I thought it was a unit of weight but I'm not so sure because you're able to convert kg to pounds right and kg is a unit of mass not weight.

 

[madmike159]

Its a unit of mass. 1 pound = 0.45359237Kg. People some times get weight and mass confused. e.g. " how much do you weigh" should be "what is your mass"

 

[PhanthomJay]

I've been looking around and some places tell me pounds is a english unit of weight and other tell me pounds is a unit of mass i.e lbf and lbm. I thought it was a unit of weight but I'm not so sure because you're able to convert kg to pounds right and kg is a unit of mass not weight.

The pound unit gets confusing, as there are several definitions related to separate terms like lbf, lbm, poundals, etc. For most physics problems using Newton's laws, pound is defined as unit of force, used primarily in the USA. In the SI system, the force unit is the Newton. 1 kg weighs 9.8 Newtons, or 2.2 pounds, more or less. That's a conversion factor used to equate mass with weight on planet Earth.

 

[holtvg]

I think it's both it's a unit of weight if it's pound-force and a unit of mass if it's pound-mass (lbf and lbm) lb generally refers to lbm

 

[PhanthomJay]

I think it's both it's a unit of weight if it's pound-force and a unit of mass if it's pound-mass (lbf and lbm)

CORRECT

lb generally refers to lbm

INCORRECT. lb or lbs (pound or pounds) refers to a unit of force or weight, not mass. I told you it would be confusing! If a man who weighs 200 pounds hangs from a rope, the rope tension is 200 lbs, a force unit.

 

[LURCH]

A person who weighs 200 bls on Earth would weigh 33 lbs on the Moon, so "lbs" is a unit of weight, not mass.

Holtvg; do you live in the US? (just curious; because if I lived anywhere else, I don't think I would bother learning the Empirial system.)

 

[D H]

I think it's both it's a unit of weight if it's pound-force and a unit of mass if it's pound-mass (lbf and lbm)

Correct - and a unit of money as well. For a long time, one pound (mass) of silver was worth one pound (sterling) in money, by fiat. Now that we have switched to fiat money the old fiat no longer applies.

lb generally refers to lbm

INCORRECT. lb or lbs (pound or pounds) refers to a unit of force or weight, not mass. I told you it would be confusing! If a man who weighs 200 pounds hangs from a rope, the rope tension is 200 lbs, a force unit.

holtvg is CORRECT, PhantomJay. lb, without an additional qualifier, indicates the pound (mass). The National Institutes of Standards and Technology is the standard bearer in this regard. See, for example, http://physics.nist.gov/Pubs/SP811/appenB9.html".

Sorry about the bad puns. I couldn't resist.

 

[PhanthomJay]

holtvg is CORRECT, PhantomJay. lb, without an additional qualifier, indicates the pound (mass). The National Institutes of Standards and Technology is the standard bearer in this regard.

When did that standard come out? As far as I'm concerned, I weigh 200 lbs, a force unit. Now I might weigh 200lbf, but I've never, or seldom, seen it that way. And either in layman's terms or in physics, I've never seen seen lb used to designate mass. They're going to have to change a lot of textbooks in engineering statics if this is the case, but I doubt it will ever happen. The unit of mass in the USA system is the seldom used 'slug', which weighs 32 pounds (lbs) on Earth. A net force of 1 pound (lb) will give an object with a mass of 1 slug, an acceleration of 1ft/sec^2, per Newton 2. I have no idea how to apply Newton 2 if the mass is given in lbm or 'lb'. I'd flunk Physics 101 for sure that way!

 

[Janus]

When did that standard come out? As far as I'm concerned, I weigh 200 lbs, a force unit. Now I might weigh 200lbf, but I've never, or seldom, seen it that way. And either in layman's terms or in physics, I've never seen seen lb used to designate mass. They're going to have to change a lot of textbooks in engineering statics if this is the case, but I doubt it will ever happen. The unit of mass in the USA system is the seldom used 'slug', which weighs 32 pounds (lbs) on Earth. A net force of 1 pound (lb) will give an object with a mass of 1 slug, an acceleration of 1ft/sec^2, per Newton 2. I have no idea how to apply Newton 2 if the mass is given in lbm or 'lb'. I'd flunk Physics 101 for sure that way!

Here's the thing. There are two ways to express both the MKS and FPS systems, absolute and gravitational. In absolute terms, the unit of mass in the FPS system is the lb_m and the unit of force is the poundal. Under the same terms, the Kilogram is the unit of mass and the Newton the unit of force for the MKS system. In gravitational terms, the slug is the FPS unit of mass, the lb<sub>f[/sub} is the FPS unit of force, the kgf is the MKS unit of force, and the kg-sec²/m is the MKS unit of mass.

Commonly in the US, you will compare lb to kg (1 kg = 2.2lb for instance) and the kg is considered a unit of mass. You wouldn't directly compare a force to a mass, you compare a a mass to a mass.</sub>

 

[holtvg]

Yes i live in the us don't won't to learn the imperial system just curious as i was confused about that part. So what's the difference between a poundal and lbf

 

[PhanthomJay]

Yes i live in the us don't won't to learn the imperial system just curious as i was confused about that part.

If you live in the US and want to be a civil or structural engineer, or want to buy a pound of apples, you're not going to make it on either count. 50 years ago they told me I had to convert to metric. I'm still waiting, and I'm in no hurry!

So what's the difference between a poundal an lbf

I don't know and I don't care! (If I had to buy a poundals worth of apples, I'd be the one in BIG trouble!) (I'd like to make a smiley face, but I don't know how, let me try: :smile)

 

[D H]

When did that standard come out?

Since 1832, officially. Unofficially, far before that.

Now I might weigh 200lbf, but I've never, or seldom, seen it that way.

That is because in lay and legal terminology, weight is synonymous with mass. Since the NIST sets the legal standards of weights and measures in the US, it is governed by legal terminology.

The unit of mass in the USA system is the seldom used 'slug', which weighs 32 pounds (lbs) on Earth.

No, it is not. The legal unit of mass in the US is the pound. The slug is a seldom used derived unit.

I have no idea how to apply Newton 2 if the mass is given in lbm or 'lb'. I'd flunk Physics 101 for sure that way!

Newton did not say that force is mass times acceleration (F=ma). What he said instead is that force is proportional to the product of mass and acceleration, or F=kma. (Actually he didn't say that either; he said "Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur," or "The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.") In Physics 101, we ignore variations in mass, so change in momentum is simply mass times acceleration. Thus Newton's second law is, ignoring changes in mass, F=kma.

The metric system was designed so that the proportionality constant has a numerical value of 1, exactly. If you measure mass in pounds and force in pounds-force, the proportionality constant is 1/32.17405 lbf-sec²/lb/ft.

Or you could use pounds force and slugs, which are defined as 32.17405 pounds, and then you have F=ma.

Or you could use pounds mass and poundals, which are defined as 1/32.17405 pounds-force, and then you again have F=ma.

Or you could just view us Americans as incredibly stupid and use metric units, and then you again have F=ma.

 

[PhanthomJay]

Since 1832, officially. Unofficially, far before that.

That is because in lay and legal terminology, weight is synonymous with mass. Since the NIST sets the legal standards of weights and measures in the US, it is governed by legal terminology.

No, it is not. The legal unit of mass in the US is the pound. The slug is a seldom used derived unit.

Newton did not say that force is mass times acceleration (F=ma). What he said instead is that force is proportional to the product of mass and acceleration, or F=kma. (Actually he didn't say that either; he said "Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur," or "The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.") In Physics 101, we ignore variations in mass, so change in momentum is simply mass times acceleration. Thus Newton's second law is, ignoring changes in mass, F=kma.

The metric system was designed so that the proportionality constant has a numerical value of 1, exactly. If you measure mass in pounds and force in pounds-force, the proportionality constant is 1/32.17405 lbf-sec²/lb/ft.

Or you could use pounds force and slugs, which are defined as 32.17405 pounds, and then you have F=ma.

Or you could use pounds mass and poundals, which are defined as 1/32.17405 pounds-force, and then you again have F=ma.

Or you could just view us Americans as incredibly stupid and use metric units, and then you again have F=ma.

I'm glad I'm a science guy and not a lawyer!

 

[Janus]

Yes i live in the us don't won't to learn the imperial system just curious as i was confused about that part. So what's the difference between a poundal and lbf

A poundal will accelerate a 1 lb mass at 1 ft/sec²
A lb_f will accelerate a 1 slug mass at 1 ft/sec²

IOW, a poundal is about 1/32 the force of a lb_f

 

[Dale]

Since 1832, officially. Unofficially, far before that.

Are you sure about that? Because as of late 90's engineering professors were still teaching that a pound is a unit of force. Also, US auto manufacturers still quote torque as lb.-ft., which indicates that the auto industry thinks that lb. = lbf. != lbm, and since the specification sheets are legal documents I would think they would need to use the right units.

 

[rcgldr]

< General usage of pound

The english unit for torque is foot pounds, where pound is a force.
How much someone weighs (a force) is also pounds, but weight force is relative to mass.

As mentioned, the English unit of mass is a "slug".
Gravity is usually defined as 32.174 ft / sec^2
1 pound = 1 slug foot / second^2.
So a slug = 31.274 pound(mass).

In SI units, describing a mass by it's weight could be express as Newtons, but this is rarely done. I'm not sure of the history of why using pound-mass became common.

 

[LURCH]

So, if I weigh 200 lbs on Earth, I will weigh 200lbs on the Moon? That is not what I learned since ellementary school.

 

[LURCH]

So, if I weigh 200lbs on Earth, I weigh weigh 200lbs on the Moon?

 

[Janus]

So, if I weigh 200 lbs on Earth, I will weigh 200lbs on the Moon? That is not what I learned since ellementary school.

Well, it depends on what type of scale you use. If you use a spring scale, it actually measures force, which it displays as lbs assuming an Earth surface force of gravity, and it will read about 33 lbs. However, if you use a beam balance scale which measures mass, it will read 200 lbs.

 

[D H]

As of late 90's engineering professors were still teaching that a pound is a unit of force.

A pound is both a unit of mass and a unit of force. If engineering professors don't teach that they are wrong. The National Institute of Standards and Technology, formerly the National Bureau of Standards, formerly the Coast and Geodetic Survey Office of Weights and Measures, defines standards in the U.S., not engineering professors.

NIST defines the avoirdupois pound to be exactly 0.45359237 kilograms and the pound force to be exactly 0.45359237 kg × 9.80665 m/s² = 4.4482216152605 Newtons. When not qualified, the term "pound" means the avoirdupois pound. For more info see http://physics.nist.gov/Pubs/SP811/".

Also, US auto manufacturers still quote torque as lb.-ft., which indicates that the auto industry thinks that lb. = lbf. != lbm, and since the specification sheets are legal documents I would think they would need to use the right units.

Officially, that unit is lbf-ft, not lb-ft. See NIST Special Publication 811.

As mentioned, the English unit of mass is a "slug".

No, it is not. Before the official adoption of the metric system in 1893, the official unit of mass was the grain. One (avoirdupois) pound was (and still is) exactly 7000 grains, while one troy pound was (and still is) exactly 5760 grains. Since 1893 (http://en.wikipedia.org/wiki/Mendenhall_Order" ), the official unit of mass is the kilogram. The Mendenhall Order was supplanted in 1959, slightly changing the definition of the pound mass.

I'm not sure of the history of why using pound-mass became common.

Commerce. Dumping the contents of a one pound can of peas on a spring scale calibrated for Earth standard gravity will yield a weight of 1.0016 pounds (force) in Nome, Alaska but only 0.9966 pounds (force) at the Mauna Kea observatory. That isn't good for commerce. The contents of a one pound can of peas has the same legal weight in Nome as it does at the observatory -- one avoirdupois pound, or one pound for short. Legally, weight is synonymous with mass, not force.

 

[Topher925]

As far as engineers are concerned, a pounds is a unit of weight, NOT MASS. For example you don't measure the density of something in lbm/ft^3 you measure it in slug/ft^3. Thats great if the NIST says that a slug isn't a unit for mass but practically speaking, it is.

 

[stewartcs]

This is exactly why it is important to correctly write lbf or lbm...it avoids ambiguities (although the context will quite often make it more clear which is meant).

The largest part of confusion arises from the use of the USCS. The unit of mass is lbm, the unit of force is lbf. These are NOT the same thing. They coincidentally have the same value at 9.81 m/s^2. Hence, in general conversation, a "lb" is considered to be the weight of an object in lbf. However, "lb" is truly meaningless without a qualifier (m or f).

CS

 

[Q_Goest]

The English system uses both pound force (lbf) and pound mass (lbm). A lbm is equal to the mass that produces a force of 1 lbf on the Earth. A slug is also a unit of mass equal to 32.174 lbm. Attached are excerpts from 2 of my textbooks from college, so you don't have to take my word for it. I use lbm and lbf every day, so although I don’t really like the idea of lbm because it’s a bit confusing, it’s very common in engineering in the US.

 

[D H]

This is exactly why it is important to correctly write lbf or lbm...it avoids ambiguities.

I agree completely.

The unit of mass is lbm, the unit of force is lbf. These are NOT the same thing. They coincidentally have the same value at 9.81 m/s^2.

To be picky, it is 9.80665 m/s², not 9.81.

Hence, in general conversation, a "lb" is considered to be the weight of an object in lbf. However, "lb" is truly meaningless without a qualifier (m or f).

In general conversation, a pound is considered to be the weight of an object in pounds mass. As far as most people are concerned and as far as the law is concerned, weight is mass, not force. Both NIST and Congress are quite clear on this. http://ts.nist.gov/WeightsAndMeasures/h130-06.cfm" says "Statements of weight shall be in terms of avoirdupois pound and ounce."

 

[stewartcs]

To be picky, it is 9.80665 m/s², not 9.81.

Indeed it is...however, I assumed those reading this knew that (I was too lazy to look up the exact value).

In general conversation, a pound is considered to be the weight of an object in pounds mass. As far as most people are concerned and as far as the law is concerned, weight is mass, not force. Both NIST and Congress are quite clear on this. http://ts.nist.gov/WeightsAndMeasures/h130-06.cfm" says "Statements of weight shall be in terms of avoirdupois pound and ounce."

I suppose I should clarify "general conversation". By that, I mean general conversation in physics. Weight is force (W = mg), therefore, when one hears the terms weight in a physics context, one immediately thinks of force (i.e. lbf in the USCS, not lbm).

I do agree that in the legal sense that the term weight (in the US and some other places) means mass, but not in the standard physics definition.

CS

 

[LURCH]

LOL! just an example of how convoluted our language has become:

...In general conversation, a pound is considered to be the weight of an object in pounds mass...

Makes one ask, "In general conversation, how does one express the mass of something in pounds force?"

 

[Dale]

To whom it may concern:

SI please!

-Many Thanks
Dale

PS I won't hold my breath while waiting.

 

[D H]

LOL! just an example of how convoluted our language has become:

In general conversation, a pound is considered to be the weight of an object in pounds mass.

In this case, it is scientists, not the general public, who are guilty of convoluting the language. Weigh and weight are very, very old words, predating Newton by many hundreds of years. The first definition of weight in my unabridged dictionary (Random House Unabridged Dictionary, 2006) is "1. the amount or quantity of heaviness or mass; the amount a thing weighs". The second definition is the scientific one: "2. Physics. the force that gravitation exerts upon a body, equal to the mass of the body times the local acceleration due to gravity". That Physics is italicized means that Random House at least considers the physical definition of weight as force due to gravity to be technical jargon. Collins Essential English Dictionary similarly qualifies weight as gravitational force as scientific jargon.

The American Heritage Science Dictionary, on the other hand, presents the jargon scientific definition as the primary definition of weight. It also has a very erroneous usage note regarding the translation of kilograms into pounds: "Although most hand-held calculators can translate pounds into kilograms, an absolute conversion factor between these two units is not technically sound. A pound is a unit of force, and a kilogram is a unit of mass." A pound is a unit of mass and a unit of weight, and when used as a unit of mass, a hand-held calculator can indeed convert kilograms into pounds and vice-versa with complete technical soundness.

In summary, one needs to be very careful in talking about weight.

• Legal weight is synonymous with mass. A balance scale measures legal weight.
• Actual weight is a force, tautologically defined as mass times the acceleration due to gravity. The actual weight of an object cannot be measured directly. It can only be computed via the tautological definition.
• Apparent weight is a force, defined as the total force acting on an object less the actual weight of the object. A spring scale measures apparent weight.

For an object at rest on the surface of the Earth, the apparent weight of an object differs from object's actual weight in direction and in magnitude. Actual weight is directed roughly downward (and is unmeasurable); apparent weight is directed exactly upward (and is measurable). The magnitude of the actual and apparent weight of an object at rest on the surface of the Earth differ in that apparent weight includes a centrifugal force term.

 

[madmike159]

So, if I weigh 200lbs on Earth, I weigh weigh 200lbs on the Moon?

No weight depends on gravity (w = mg). If your mass was 200lb on Earth it would be the same on the moon. Most people get confused because in every day conversation you would say "how much do you weigh" not "what is your mass". Spring scales measure the force applied and change it to a mass (depending on the force of gravity), so if you went to live on Mars you would need some new scales.

I think a pound is both mass and force because (in the U.K) people would still put 1lb of butter etc into a cake mixture, but people also say a rocket has so many 1000's of pounds of thrust.

 

[Nim]

To be picky, it is 9.80665 m/s², not 9.81.

Sorry. I know I am dragging out an old thread, but I had to comment on this. Awhile back I was messing around with the equation to find out Earth's gravitational acceleration and couldn't figure out why I wasn't getting 9.81. I kept getting 9.80065745906891. But that was because I was using "Earth's Equatorial Radius". If you use "Earth's Quadratic Mean Radius" then you get 9.81708900527711. If you use "Earth's Polar Radius" then you get 9.86670995442209.

I actually came to this thread doing on a search on Google about pounds. I was wondering, if using the imperial system, how exactly would you do the equation weight = mass * 9.81? If pound = mass and you have 185 mass, then is your weight 1814.85 pound-force?

 

[stewartcs]

I actually came to this thread doing on a search on Google about pounds. I was wondering, if using the imperial system, how exactly would you do the equation weight = mass * 9.81? If pound = mass and you have 185 mass, then is your weight 1814.85 pound-force?

If using lbm, then it is:

$$lbf = \frac{lbm \cdot \frac{ft}{s^2}}{g_c}$$

where gc is the gravitational constant and has the value/units of 32.17 $$\frac{lbm \cdot ft}{lbf \cdot s^2}$$

Essentially, 1 lbm = 1 lbf if g (local acceleration) equals gc (i.e. you are at sea level and 45 degrees latitude on earth). Hence the confusion when using pounds to mean force and mass without a qualifier.

Otherwise, just use:

$$lbf = slugs \cdot \frac{ft}{s^2}$$

If you have the choice just use SI as it is less confusing.

Hope this helps.

CS

 

[mgb_phys]

Sorry. I know I am dragging out an old thread, but I had to comment on this. Awhile back I was messing around with the equation to find out Earth's gravitational acceleration and couldn't figure out why I wasn't getting 9.81. I kept getting 9.80065745906891. But that was because I was using "Earth's Equatorial Radius". If you use "Earth's Quadratic Mean Radius" then you get 9.81708900527711. If you use "Earth's Polar Radius" then you get 9.86670995442209.

Then if you take into account the local geology and the rotation you get more variation -
The average is 9.80665 m/s²

 

[Dr.D]

"Because as of late 90's engineering professors were still teaching that a pound is a unit of force." You bet they were, and they continue to do so because it works!

As an engineering dynamicist who works with this stuff on a daily basis, I see it occasionally in SI units but usually not. I can report to you that the pound-force is alive and well.

The Imperial system of units is just about as dead as the British Empire, but what is commonly used in the US today is called the US Customary system (USC). It comes in two versions: the Foot-Pound-Second (FPS) system and the Inch-Pound-Second system (IPS) where the only difference is in the choice of the length unit. Notice that the force unit is the pound which is a fundamental unit to both systems. Big systems like buildings and dams are usually described in FPS while machines are invariably described in IPS.

The mass unit that is used in dynamic calculations in the FPS system is the slug, which is not so rare as has been suggested, even if it is not often encountered in trade. For dynamic calculations in IPS, the mass unit is the lb-s^2/in. Now I am sure that some of you will be laughing all over, saying that this is ridiculous, but I assure you that I am dead serious and that I, and many others use this unit with great regularity to good results.

It is very important that we be able to write F = m*a without the need for any additional proportionality constants as has been suggested above (and many other places as well). The mass units described here enable that to be done without any difficulty whatsoever, so we may say that they work.

One final word: The pound-mass is a terrible idea that leads to massive confusion whenever attempts are made to apply it in advanced mechanics. I would not touch this concept with a 20 ft. pole!

 

[mgb_phys]

I would not touch this concept with a 20 ft. pole!

That's 6.1m for our international readers!

 

[Nim]

Then if you take into account the local geology and the rotation you get more variation -
The average is 9.80665 m/s²

Ya I know, and distance from the surface adds variation too. But I thought it was the "Quadratic Mean Radius" that gives you the average. But it's the "Equatorial Radius"?