What is the difference between the metric system and the us long measure?

Answer 1

Many. The American system is very similar to the Imperial (British) system on which it was based, though some of the units such as the Ton and gallon differ in size.

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The main point about those is that the various units are not mutually consistent, and their subdivisions / multiples are not all consistent with themselves.

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1 Statute Mile = 1760 yards, 1 yard = 3 feet, 1 foot = 12 inches. And so on. In engineering the Inch is divided in binary fractions (half, quarter, etc) or decimals to 1, 2 or 3 decimal places as required. So 0.001 inch is a thousandth of an inch, generally called a "thou" in machine-shop conversation.

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The Metric System, and especially its rather peculiar ISO descendant, is entirely decimal in its scales. It was the first system to be based on artificial rather than old trade, units, although nods towards them in basic form. It was invented in France to overcome a bewildering plethora of regional units. The metre was based on a fraction of the Earth's polar circumference as closely as that could be determined at the time, and is a little over a yard in length. (I don't know why they didn't base it on sub-dividing the Nautical Mile, which is calculated from angles of longitude.)

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All metric units are expressed in powers of ten described by symbols: M[ega] for Million, m[illi] for one-thousandth, etc.

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The Systeme International (SI) scale used for international trade and science, took the original metric system, kept the kilometre, second of time, kilogramme and metre, added the volt, ampere and I think the Joule (energy) and radian (angle), and composed a collection of compound units based on them. (I use the proper spellings throughout - they are French words.)

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The point is that you can combine selections from these 6 fundamental units to make any other compound unit to suit the technical field in which you are working. This works because physically, 'work' is a transfer or conversion of energy; and energy itself, which is immutable whatever its form, can be measured in a single unit, the Joule (J).

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One oddity is that SI is unlike any other system, in which the compound units are self-explanatory multiples or divisions of the fundamental units with each other, facilitating dimensional analysis (algebraic analysis of formulae to determine and verify correct dimensions). SI named the compound dimensions after scientists instead, so for example the standard Pascal (Pa) for pressure is 1Newton/square metre.

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Another problem is that many of the Si units are not very practical for everyday use, and the bizarre situation in the UK is that schools teach the Metre and centimetre as the main length units whereas the millimetre (again these are the correct, French spellings - a "meter" is a measuring device) is actually the officially-approved division and with a few exceptions, only the m and mm are used in science and engineering. Used to both systems, I find I have to convert cm to mm to gain an idea of a quoted length - then often translate to approximate inch equivalents! (10cm = 100mm = approx. 4")

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The ISO has recognised that the SI system is fine for physicists and engineers but always very useful for ordinary practical use, so reluctantly accepts use of the Bar (= 100 000Pa = 1 atmosphere, of pressure, though the pedants say not very accurately!) and the Degree for angles.

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The Pascal is a ridiculously tiny pressure, useful in a practical sense really only for very fine physics experiments, but consistent with the fundamental kg and metre. Yet it's too big for acoustics: the minimum sound pressure our ears can detect is a mere 20 micro-Pa (20 one-hundred-thousand- millionths of atmospheric pressure!). That is used as the reference-level for the dimensionless logarithmic ratio of the decibel, so the faintest whisper to us is 0deciBels re 20µPa on the scale used for sound measurements in air.

The Radian = the angle subtending an arc of a circle of arc length = radius. it's about 57degrees, and 2*pi Radians = 360 degrees. Got that? There will be a test later. The radian (rad) is useless for dividing circles equally, but 360 has so many very useful factors, many equating to the geometry of so many common, valuable, regular polyhedra, that we keep the degree for everyday work. However, the radian comes into its own for technical calculations because it is based on pi so simplifies all manner of circle- and sine- related calculations in Mechanics, Harmonic Analysis, etc. (You may know the sine and cosine rations in right-angled triangles only, but they work together with circles in a very beautiful way!)

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I hope that lot helps.

Answer 2

The two systems use different measurements for almost everything.