What the 3/32-scientifics in TV say has no relevance. Here, they seem to introduce new units like "size of a football-field", "power of a Jumbo-Jet" or "weight of a VW Rabbit".
Access to SAE co$t$, so I can't definitely say what the standard says. But I guess that this describes the situation quite well:
cite: | In the United States we often use the pound as a unit of force. | This unit is the source of much confusion.
but this is really blowing my head off (cite continued): | The pound is a unit of weight but some people confuse it with a unit | of mass. A pound is equivalent to 4.448 Newtons.
You can also find "degrees Kelvin". Hey, it is an dot EDU-link!
Which shows you the limitations, and occassional depth of ignorance, displayed by Google entires. The unit of mass in the English/American system is the "poundal." That is, the mass when multipied by the accelleration due to gravity (32 feet/second/second) yields a weight (force) of one pound. Look in most elementary physics books or any of hundred of reference books.
The first dozen websites I got from googling "poundal" cite the poundal as a unit of force and the pound as a unit of mass. One reference acknowledges the parallel usage of pound force, slug mass. My old physics books used pound force and slug mass. Machinery's Handbook 23rd edition (1988) converts a pound to .4535924 kilograms (mass), a pound-force to 4.448222 Newtons and a poundal to 0.138255 Newton -- force.
Common usage, whether "correct" or not, does affect clarity of communications. The usage is usually clear in context, should be defined if it isn't clear. Pressure (force per unit area) is often cited in pounds per square inch though the Pascal is 1 Newton/meter^2. If you ask the butcher for a pound of bacon, he knows what you want. Many weighing scales can display either pounds and ounces or grams, though they are measuring force in all cases.
If one says "pound force" or writes "lbf", the meaning is clear. If one says "pound mass", it would usually be understood that this connotes a mass that weighs one pound under some set of standard conditions and would balance with a known mass of 454 grams.
MathCAD treats "lb" as 0.454 kilograms and "lbf" as 4.448 Newtons.
Nonsense. You have a weird notion of what it means "to measure" something.
What you measure is the thing you can assign a numeric value to. Consider, for example, the classic balance scale--which were the only types of scales anybody used for the first 7,500 years after people started weighing things, the only scales used by anybody more than two centuries ago.
Let's use a classic two pan, equal arm balance--the same thing applies to other scales as well.
When you put your known weights in one pan of the balance, what is it you know about them? It is their mass.
======================================================== Ed sez:
And how do you know their mass? Have you accelerated them with a known force and measured their velocity? Do you have some way to measure their inertia?
Wake up, Gyngaard. You know that GRAVITY IS APPLYING THE SAME FORCE TO BOTH OF THEM. That's all you know. From your definition of mass, you CONCLUDE that, because gravity is applying the same force to each, they have the same mass. BUT YOU HAVE NOT MEASURED THEIR MASS! You only have drawn a conclusion, based upon an experimentally developed derivation, that the FORCE of gravity will be the same if their mass is the same.
Look, this is getting ridiculous. "Pound," as I said, can be either force or mass, but there was no way to measure mass when the pound was introduced, and engineers use it, to this day, as a unit of force. The use of pound as mass is primarily in the fields of commerce and common household usage.
Let's see what some authorities say about it. First, weight:
Encarta: Weight, measure of the gravitational force exerted on an object (see Gravitation). The weight of an object may be determined by using either a comparative method, as with a chemical-laboratory balance, or by measuring the gravitational force directly by means of a spring scale, such as the familiar bathroom scale
Now, pound:
Columbia University Encyclopedia:
pound, abbr. lb, unit of either mass or force in the customary system of English units of measurement. Two different pounds of mass are defined, one in the avoirdupois system of units and one in the Troy system. The avoirdupois pound (lb avdp) is now defined in terms of the kilogram, the metric unit of mass; 1 lb avdp is equal to 0.45359237 kg. The Troy pound is used only for the measurement of precious metals and is defined as 5760/7000 of the avoirdupois pound. The apothecaries' pound is identical to the Troy pound. As a unit of force, or weight, the pound is the weight that a mass of
1 lb avdp has when the acceleration of gravity has its standard value (9.80665 meters per second per second). In ordinary usage, the term pound is often used without specifying whether force or mass is meant, but for scientific purposes it is important to make this distinction.
Wolfram Research Science World:
Pound: A unit of weight in the British engineering system (O'Hanian 1985, pp. 14 and 96) equal to 4.448 newtons that is commonly used in the United States. Unfortunately, there is a great deal of confusion about the definition of the pound, with many authors using the unit to denote the mass-equivalent of 4.448 newtons, namely 453.592 grams.
Here are some examples of usage:
"Jet Engine." Encyclopedia Americana. New York: Anderson, 1996: 51: ""The engine, which was designated the whittle W1, produced 860 lb of thrust and powered the Gloster airplane to a maximum speed of 338 miles per hour (544 km/hr)."
"Jet Propulsion and Aircraft Propellers." Marks Standard Handbook for Mechanical Engineers, 8th Edition. New York: Zucrow & Reese, 1978: 11-84: ""Because of the limited air induction capacity of the centrifugal compressor, also called the radial compressor, engines for developing thrust above 7,000 lb (31 kN) at static sea level, employ axial-flow compressors."
The Russians almost always used kilograms until the late 1980s. The European Space Agency, the Chinese, and the Indians and some others often still use kilograms today.
Sometimes, of course, they use "tonnes" instead. Same thing, 1 tonne-force =3D 1000 kgf.
You often see that also in the strange use of "seconds" for specific impulse. People often try to justify using them in part on the basis that they are the "same" in English units and metric units; yet the proper SI units for specific impulse are N=B7s/kg.
Then, of course, we have the similar still-common use of metric horsepower (often PS in English, from the abbreviation of the German name, or sometimes CV from hte abbreviation of the French name). This unit is slightly smaller than the English horsepower. It is defined as 75 kgf=B7m/s, or about 735.5 watts vs. 745.7 watts for the English horsepower.
No. A poundal is a unit of force, not a unit of mass. It is the force which will accelerate a mass of one pound at a rate of one foot per second squared. Standard acceleration of gravity is irrelevant to its definition. This is the absolute foot-pound-second system of units.
The "the mass when multipied by the accelleration due to gravity (32 feet/second/second) yields a weight (force) of one pound" is the pound, not the poundal. This is the engineering foot-pound-second system of units. It is not quite a "coherent" system of units, because of that factor related to the acceleration of gravity on Earth.
Those are in two different, specialized foot-pound-second subsystems of mechanical units, systems used only to simplify calculations. You can use whichever you choose for that purpose. To use either of those systems, you might have to convert your units into the system first, and often you have to convert out of the system again to get the units you want for the final result.
There are also several other such systems, such as inch-pound-second systems, and there is also a different, third foot pound second system, the gravitational fps system. In that system, the mass which a force of one pound-force will accelerate at a rate of one foot per second squared is called a "slug" It is about 32.17 lb.
In the gravitational inch-pound-second system, the unit of mass is 1 lbf=B7s=B2/in, often used without any special name for it, though there are a couple of names that have been used on rare occasions. This 1 lbf=B7s=B2/in is equal to 12 slugs, or about 386.1 lb, but it is never used together with slugs since they are in different systems of units.
1=2E There is nowhere on Earth where it is legal to sell meat by the newton.
2=2E Everyone outside the United States sells meat by the kilogram.
3=2E In the United States, it is legal to sell meat by the kilogram, but it is most often sold by the pound.
4=2E Why in the world do you suppose the law bothers defining a pound in the first place? The pound which can be used for the sale of meat in the United States is the one legally defined as 0.45359237 kilogram, exactly.
5=2E Those scales, no matter what their internal mechanism, are calibrated, tested, and certified on the basis of their in measuring MASS, in the VERY LOCATION IN WHICH THEY ARE USED, not on the basis of their accuracy in measuring force. Haven't you ever thought about what a government inspector does to test those scales?
6=2E "Whether correct or not"? What in the world are you thinking about? Not only do we measure mass, but that's logically exactly what we want to measure. We do not measure force, and ideally we would not want to measure force. We would not want a "pound" or a "kilogram" to be more bacon at one place on Earth than it is at some other place on Earth. Not a quarter to half a percent less bacon in Barrow, Alaska than in Honolulu, Hawaii, for example, just because gravity will pull it so much harder towards the table or the frying pan, or the weighing scale, in Barrow..
No. You have it ass-backwards.
Using "pound mass" means we are using the pound which itself is officially defined (internationally since 1959) as 0.45359237 kg, and values very close to that before 1959 (about one part in 8 million more than that in the United States for the 66 years that it had already been defined in terms of the kilogram, before 1959). Gravity never comes into play here.
It is using the pound-force which would "usually be understood" that this "connotes" a force that would "under some set of standard conditions" would correspond to the force exerted due to gravity by a pound of mass. This is where gravity comes into play, in our understanding of what "pound-force" means.
Of course, those "standard conditions" are not officially, uniformly defined. So a pound-force has some ambiguity in its definition. Back in 1901, the CGPM did define a "standard acceleration of gravity" of
980.665 cm/s=B2 for purposes of defining a gram force or a kilogram force, as well as manometric pressure units such as millimeters of mercury.
But nobody has officially defined a universal standard acceleration of gravity for purposes of defining a pound force. We often borrow the value official for defining grams force to define pounds force as well, but we do not have to do so, and in fact other values have been used for that purpose. One such value, used but different, is 32.16 ft/s=B2. Witness this Google search:
ballistics 450240 208 hits
and consider the kinetic energy formula E =3D =BDmv=B2, the fact that there are 7000 troy grains in an avoirdupois pound, and that 450240 =3D 2 x
7000 x 32.16.
That should be newtons, of course, lowercase. But yes, those lb and lbf are the standard symbols used by NIST (the U.S. national standards laboratory), NPL (the UK national standards laboratory), and most professional standards organizations as well, such as SAE.
There are many different, little subsystems of units which people can use to simplify calculations. None of them are in general use.
But your statement is TOTALLY FALSE. It is simply not true. You have confused parts of two separate and distinct system of units.
In one foot-pound-second system of units (gravitational), we have
t ONE ft/s=B2. (not at 32.1740 ft/s=B2)
You messed up 3), if that's what you intended
In a second foot-pound-second system of units, we have
at 9.80665 m/s=B2 or some other value such as 32.174 ft/s=B2 or 32.16 ft/s= =B2
You messed up 1), if that's what you intended.
In a third foot-pound-second system of units, the oldest coherent system of English units, we have
In one inch-pound-second system of units (gravitational ips), we have
t any special name, though on rare occasions some (esp. at NASA) have calle= d it a "slinch", and some others have called it a "snail"
s=B2/in at ONE ft/s=B2..
Other, often partial and less comprehensive, systems also see limited use (e.g., ballistics with mass in grains troy, speed in ft/s, and energy in ft=B7lbf).
Wrong. The number 10 is about the third most important number in SI, the modern metric system.
The most important number is 1, due to the fact that this is a "coherent" system of units, as that term is used in the jargon of metrology. There is one unit for each quantity, and that one unit (whether or not it has a special name) is a unitary combination of the base units.
Another important number is 1000; the preferred prefixes are those which are multiples of 1000.
The other huge advantage of metric is that it is less ambiguous. Little chance of confusing units of the same name, especially in SI which doesn't have a handful of different small calories as well as a handful of different large calories roughly 1000 times as big.
Might be a semantic nit in what you mean by "measuring". Mass is indeed usually the quantity of interest, and scales are calibrated in pounds or kilograms. But they are responding to the force resulting from the mass as being accelerated by gravity, either by balancing internal moments against a reference mass or by noting the deflection of a spring. A moment is force times distance. Deflection of a spring is proportional to force and spring constant. Many scales used in trade are now spring scales, using straingages which depend on the elasticity of the material upon which they are mounted.
Get out your postage scale and leaf blower, aim the leafblower at the scale. Does moving air have more mass than still air? Push it with your finger. Does your finger gain mass when you push harder?
Exactly. They are calibrated to "measure" mass by responding to the force it exerts while being accelerated by gravity. F=MA
The gov't inspector uses a reference mass, under the supposition that gravity doesn't change much from day to day in a given location. It's as reliable as ...uh...gravity! The force it takes to support it is the same from day to day, so noting the force a mass exerts when accelerated by gravity is a reliable inference of how much mass is present.
That's why the government inspector comes round. If you ship the scale *and* the reference mass from Pike's Peak to Death Valley, the scale will read slightly differently -- but if they are returned to Pike's Peak the scale will read what it did before it left. Therefore, the calibration is a local correction for inferring mass from the force it exerts while being accelerated by local gravity.
I said "would usually be understood". I've asserted that the pound is a unit of mass. What you understand when someone says "pound mass" is entirely up to you.
Again, I have asserted that the pound is a unit of mass. Gravity always comes into play because it is always present. Comparisons of masses can be done in presence of gravity with a balance in which the moments cancel if the dimensions of the balance are perfectly known. Moments have to do with force and distance.
Which merely recites that a grain is 1/7000 of a pound.
OK. There are plenty of examples both ways, but lowercase is probably correct usage when spelled out. When abbreviated it is always N.
Sir Isaac Newton's name was always written uppercase. -> N Others to mention: Blaise Pascal, James Watt, Mes. Ampere, Signore Volta, Mr. Gray, Herr Hertz, Oersted, Mr. Joule, Mr. Henry, Herr Röntgen, ...
Whoops! You're right, of course. I meant the "slug" kind of poundal. My senior moment aside, the Van Nostrand Scientific Encyclopedia states that "pound" is used both as a unit of mass (equivalent to 453.59 grams) and also the weight (force) that such a mass produces at an accelleration of one standard g. The poundal is the force that provides an accellaration of 1 ft/sec/sec to a one pound (mass). My old physics professor (Mark Zemanski) said: "There are two kinds of mensuration systems: the metric and the barbaric!"
What everybody seems to be forgetting is that a pound is a "dependent" measure... How much it weighs or what force it applies depends on how big the hammer you're using happens to be :)
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