Comments by "Laurence Fraser" (@laurencefraser) on "How much thrust does a ceiling fan produce?" video.
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@busimagen The difference is mostly units of measure of volume, if I recall correctly. That and possibly units of length smaller than an inch, which no one actually uses in the normal course of events.
The Imperial system is basically just 'every specialised unit of measure of things relevant to a specific field, standardized such that unit X is always unit X, with people then prefering to reuse existing units rather than create new ones if the existing units were sufficient to their new task'. Over time some fell out of use as the specialty they came from was less significant, and then sometimes things were rationalized a bit when that left gaps that became an issue later. Imperial units are Very Good at the things they're intended for, and Terrible at everything else, and conversions are a pain.
US customary units Started Out as British Imperial units, but then things happened.
SI units (kind of sort of metric but not exactly) are intended primarily for scientific applications, and for stuations where great precision is needed, as well as to minimise, simplify, and/or eliminate conversions wherever possible. Officially only the base units and multiplying or dividing it by 1000 are actually Things in SI, but you'd be hard pressedd to find a country that uses the metric system that doesn't add additional units for practical reasons: Centimetres (1/100th of a metre) are pretty much universal, for example. Some places use decilitres (1/10th of a litres), most places will use teaspoons (5ml), tablespoons (10 or 15ml depending on country), and cups (250ml, or 1/4th of a litre, or 200ml, or 1/5th of a litre, depending on where you're talking about... good odds on one of those having it's origin in American units and the other in British, though it's quite possibly unrelated, but it's very confusing when you end up with the wrong one!) for cooking because it's just substantially more practical for things of that scale for that particular application. ... and don't be surprised if you find the occasional stray imperial unit still floating around for certain niche applications.
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