General > General Technical Chat
UK back to "imperial" measurements ?
IanB:
--- Quote from: TimFox on June 05, 2022, 11:10:58 pm ---I explained the legal meaning of a 5 lb bag of rice in a post above (reply 141).
--- End quote ---
I'm not sure that you did. You described a prescribed process for checking the accuracy of a point of sale weighing device to within a certain accepted tolerance by government inspectors.
If we look to a regulatory source, namely "NIST Handbook 130: Uniform Laws and Regulations in the areas of legal metrology and fuel quality (2022)" we find something more precise. Firstly, we find throughout the document the statement:
--- Quote ---When used in this law (or regulation), the term “weight” means “mass.”
--- End quote ---
This note appears following every use of the term weight in the regulations. It is supported by some explanatory text:
--- Quote ---The mass of an object is a measure of the object’s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity, or the force needed to support it. The pull of gravity on the earth gives an object a downward acceleration of about 9.8 m/s2. In trade and commerce and everyday use, the term “weight” is often used as a synonym for “mass.” The “net mass” or “net weight” declared on a label indicates that the package contains a specific amount of commodity exclusive of wrapping materials.
--- End quote ---
Secondly, we find a definition of the pound mass avoirdupois in Appendix A:
--- Quote ---1 pound = 453.592 37 g exactly
--- End quote ---
If the amount of a pound depended on local gravity, it could not be given exactly to 8 s.f. like this.
Therefore the legal meaning of a 5 lb bag of rice is that it contains 2267.96185 g of rice, or sufficiently close to comply with the applicable regulations and measurement standards.
--- Quote from: TimFox on June 05, 2022, 11:10:58 pm ---In careful scientific and engineering usage, the kg is an SI unit of mass, the N is an SI unit of force or weight, and the lb av is a customary unit of force or weight.
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As a careful scientist and engineer, I disagree with the latter part of this. The lb (mass) is a unit of mass, and the lb (force) is a unit of force. Where the context is not clear, these are sometimes abbreviated as lbm and lbf .
coppice:
--- Quote from: bd139 on June 05, 2022, 09:48:45 pm ---Kg was never used as a force. Force is N (newtons)
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You weren't there man. :) Before the Newton was added to the SI units, we used Kg (often written as Kgf, for Kilo equivalent of force) as a unit of force. Our physics text books changed during my time at school.
TimFox:
Re: IanB
Your discussion about the difference between a "pound mass" and "pound force" is consistent with my discussion of using lb av at the grocery store to weigh food.
However, here is an example of my reasoning not to use lbm:
Imagine a spacecraft whose mass is 100 lbm. We then apply a force of, say, 10 lbf to accelerate it in a desired direction.
Applying Newton's second law straight out of the bottle, F = m A will give the wrong result, unless we throw another factor gN, the defined standard acceleration of gravity.
In careful US usage, one should use pounds of force and slugs of mass, just as in careful metric usage one uses Newtons of force and kg of mass.
I remember in high-school physics class (11th grade), an important initial hurdle for the students was to understand the difference between mass and weight. The instruction was in both customary and metric units, and I learned some important factors for mental calculation, such as 88 ft/sec = 60 mph (exactly).
themadhippy:
--- Quote ---If we look to a regulatory source, namely "NIST Handbook 130: Uniform Laws and Regulations in the areas of legal metrology and fuel quality (2022)"
--- End quote ---
But only applicable to america,if i was served 0.473177 liters in my liquid pint glass id be questioning the parentage of the bartender
vk6zgo:
--- Quote from: Terry Bites on June 05, 2022, 03:14:14 pm ---Skeggy OMG. Too many wasted childhood hours. I know a bloke who lives in the middle another town of Lincoln county - Scunthorpe.
--- End quote ---
Dorset will see your Scunthorpe, & raise you "Shitterton!"
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