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Comment by braythwayt | Hacker News Reader

Disclosure: A rare self-post. I'd be very grateful for any and all corrections to the story, as I am far from an expert is astrophysics, geology, or pretty-much anything else.

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> At some point, a satellite the size of Mars came crashing into the earth, so hard and so fast that about a third of the earth was blasted into space. It didn’t go far–the debris formed a ring around the earth, and gravity eventually compacted it into a single satellite, our moon, but a satellite much larger than our planet’s gravity would normally be able to capture.

Something seems off about "a third": according to https://en.wikipedia.org/wiki/Moon, the Moon's radius is a bit less than a third of Earth's, but its volume is 2% and its mass is just 1%. I'm no expert, but "about a hundredth" seems more accurate.

For what it's worth, I only looked it up because I've come across this discrepancy before, so perhaps I'm missing something!

I could be wrong about this, but note also that not all of the material ejected from the impact wound up in Earth's orbit.

Small typo: "Neutron stars to not hang, fixed in space"

I'm guessing that it should be "do not"

https://github.com/raganwald/braythwayt.com/commit/dc07b496d...

Another: "May tiny such creatures, when they died"

Many?

Probably another typo: "planet our side" -> "planet our size"?

Also, this is a beautiful piece of writing! I love it.

https://github.com/raganwald/braythwayt.com/commit/981362fad...

Thank you or sharing your appreciation. I will treasure that.

> I would have found that while my scale tells me that I am about 86 kilograms on Earth, I would have weighed 8.6 trillion kilograms on this neutron star.

Kilograms are a measure of mass, not weight. Pounds would be a more appropriate unit there.

Most of the people world weigh themselves in kilograms.

True, but WaxProlix is correct. Ten kilograms is 22 pounds on Earth, but 2.2 trillion pounds on the given Neutron Star.

I was going to jump in and say that the pound is also a unit of mass. But it turns out its both.

https://en.wikipedia.org/wiki/Pound_(force) https://en.wikipedia.org/wiki/Pound_(mass)

And then there's the foot-pound, a unit of torque much prized by the owners of muscle cars until eclectric vehicles went from obscure to early-adopter status.

Also, assuming we're talking about rest mass, since without that distinction the word mass is ambiguous in accelerated contexts.

Or just general torque in the United States. Electric vehicles still have tires that are joined to wheels which are fastened to an axle by the tightening of a nut on a threaded stud. For safety, we generally state that the nut should have X foot-pounds of torque applied to ensure it doesn't fall off. Unless Tesla et al. have suddenly decided to use a more universal standard in the states, I'd assume they still publish that value in ft/lbs, since 98% of tools designed to measure these things owned in the U.S.A. have this unit on them.

Well, there's also https://en.wikipedia.org/wiki/Kilogram-force

They roughly infer their mass (in kilograms) by assuming 9.8m/s/s gravitational acceleration and using a scale to judge how much mass the resulting 'push' would require given that pull. A scale would tell you that you weight slightly less at the top of a mountain than at sea level, though in reality you'd have the same mass. Same is true on a neutron star.

It's an easy shorthand for the most part, since we're not 'weighing' ourselves in space, on the moon, etc, but in an article about space and physics and whatnot, it can come off as a bit sloppy.

Every single thing about life as we know it, humanity, and our brains is exceedingly sloppy. What makes it work? Error correction!

So, thank you.

> A scale would tell you that you weigh[] slightly less at the top of a mountain than at sea level, though in reality you'd have the same mass.

For an illustration of a pedantic distinction, this seems awfully unlikely to be correct. You gain mass by eating and drinking and lose it by urinating, defecating, and breathing; odds are you're not going to have the same mass on the mountaintop.

This is physics, we're clearly assuming a perfectly spherical human with lossless and instantaneous modes of travel.

> pedantic

Think you're maybe projecting? It was meant to be a silly example and taking a super uncharitable interpretation (object x at different times or states with different masses) of my statement just to be able to pick a nit strikes me as more pedantic than responding to a request for corrections with my high school level understanding of basic units.

> A scale would tell you that you weight slightly less at the top of a mountain than at sea level, though in reality you'd have the same mass.

A pound of force uses 'standard gravity' as a constant, even though it varies across the earth. Sounds just as bad?

A pound of force is defined [1, 2] as 4.448222 N . No gravity necessary (but the IPK is, until the upcoming redefinition of the SI).

Yes, that definition was reached using a notion of "standard gravity", but once fixed, it is nothing but a number.

[1] https://physics.nist.gov/cuu/pdf/sp811.pdf [2] https://en.wikipedia.org/wiki/Pound_(force)

Still not any more precise than kg to measure human weight (unless you position the human at the exact right spot on earth).

A properly-calibrated scale/balance will correctly determine the mass of any object in any gravitational field (gravity gradients excepted).

Not just as bad, because you'd be measuring the correct thing, which is force. Not mass. That's all I was getting at - they're different things and depending on context, one can change (weight) where the other does not (mass). That's all.

9.8 m/s^2 need not be assumed. As long as you have a calibration standard, any scale can be calibrated correctly in any gravitational field.

If you care about absolute masses at the ~percent scale or better, calibration is requisite.

Because a hike like that will really burn off the pounds ... er, I mean kilograms. (Joke)

Because they're interested in knowing their mass, not their weight. Weighing is done using scales, which have a hardcoded division by gravitational acceleration of Earth on its... scales.

https://github.com/raganwald/braythwayt.com/commit/b79d17066...

My pleasure. Great piece, hope I didn't come off as being a turd or anything.

Honestly, I don't think it is any more correct to use pounds instead of kilograms as a substitute for force, as they are both units of mass. I didn't mind the use of kilogram in your specific context, for what is worth.

A pound is also a measure of mass [1]. Unless you're thinking of the 'pound-force' which is just the force of gravity applied to one pound-mass. The same definition exists for kilograms (kilogram-force) but is not part of the international unit system. Gravity is slightly different close to the poles etc so a *-force unit will never be precise in this context.

The majority of humanity has absolutely no idea what 'one pound' is when not referring to the currency, and will merrily say they weigh X kilograms.

[1] https://en.m.wikipedia.org/wiki/Pound_(mass)

Well, at least you recalled the story toothfully.

Writing is an exercise in literary mastication.

Is that you racing cross on the book cover?

Yes it was, when the stars were younger than today :-)

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