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Comment by speedgoose | Hacker News Reader

Foot pound second is such a weird unit. How did that pass the quality controls?

I also don’t understand how one team of educated engineers could decide to use imperial units instead of metric units.

As a former mechanical engineer, any calculations involving pounds in any form I pretty much always converted to metric and then back at the end if need-be. The trinity of pound-mass, pound-force, and slugs is incredibly easy to screw up--at least as a student.

Smh, Americans will literally convert into metric and back to keep using their bald eagles per shotgun squared units.

Admittedly, engineers are pretty much the only people making calculations with those kind of units and they generally use metric as far as I know. I went to school a long time ago and there were probably enough legacy imperial units hanging around that it seemed worthwhile to at least be exposed to examples you wouldn't have encountered in day to day living.

Since the 60s, America has literally been officially a conversion of standard SI units. A yard is defined as most of a meter (exactly 0.9144 meters) and an inch is derived from that.

Since the 60s? The yard measurement dates to sometime before the Norman invasion of England. And it's not even used that widely in the US outside of some specific contexts like sports. A gallon, from which a quart is derived is of a similar vintage. Without digging into history, I assume some SI volume and length measurements roughly mirrored existing Imperial units as opposed to the other way round.

The point is that the American units are now defined relative to metric units.

As recently as the 1800s, the yard was defined as:

"the Straight Line or Distance between the Centres of the Two Points in the Gold Studs of the Straight Brass Rod now in the Custody of the Clerk of the House of Commons whereon the Words and Figures "Standard Yard 1760" are engraved"

... That standard yard was destroyed in a fire in 1834, and a new standard yard was made using several copies of the original standard yard as reference.

But now, it's just defined as an exact fraction of a meter. First as 36/39.370113 m, now as exactly 0.9144 meters.

It's how literally all engineering in this country was done a single generation ago. When I studied mechanical engineering it was all units like slugs and foot-pound-second.

> A slug is defined as the mass that is accelerated by 1 ft/s2 when a net force of one pound (lbf) is exerted on it.[2] One slug is a mass equal to 32.1740 lb

this is like if you tried to explain the difference between a newton and a kilogram to an alien an they didn't quite get it

It's not that complicated. A slug is the Imperial unit of mass, like the kilogram. It's just that while in metric the unit of mass is primary and the unit of force (Newton) is derived, in foot-pound-second it is the reverse. Pounds is a fundamental unit of force and the slug is derived.

Pound force is the unit for force. Pound mass is the unit for mass. Colloquially people say pound.for both of them

This is true colloquially. In engineering (or at least aerospace engineering, which is what I was studying) we more generally used slugs for mass in our calculations. Gravity/weight isn’t always constant after all, even on earth.

Not universally. We still talked about a rocket’s “pounds of thrust.” But for things like angular momentum, center of gravity, etc. those were usually calculated with slugs.

Aerospace engineering is still a mix of both units. Especially on the aeronautics side, imperial is still widely used. Part of the reason for this is that past reference materials and tooling at many manufacturers still using imperial tooling. A team of good engineers is expected to be proficient with both unit systems --- its not really a big deal to use imperial on projects.

> its not really a big deal to use imperial on projects

but it does look pretty embarassing when it destroys a project

I guess an engineer should handle a imperial screw but deciding to use imperial units to exchange data in a software project seems so wrong.

Im American but have heard it’s common for 2-by imperial lumber units be used in Canada and uk for example. But metric would be used for cuts (which i think is easier often too)

Many years ago, probably about 40 years or so, I remember my dad pointing out plasterboard in the building supplies store that was marked as 1200mm x 8'6", because standard UK ceilings are 8'6" but everything else (kitchen units and appliances for example) are in multiples of 600mm...

Given a 2x4 is 1.75 by 3.75 inches, it sure doesn't make using Imperial measure any easier than metric. It's more that certain demographics are oppositional-defiant when it comes to change and they tend to make up the "base" of a powerful political parry.

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