[tkgally]

I thought this looked like good advice until I got to:

> Mass/weight distinction

> Incorrect: He weighs 70 kg; Correct: His mass is 70 kg

> Incorrect: She weighs 50 kg; Correct: She weighs 490 N on Earth

Really?! We’re not supposed to use kilograms to describe a person’s weight? And to describe someone’s weight, we have to use newtons and specify what planet they’re on? Good luck enforcing those rules.

[jeroenhd]

There's a 5% range when converting mass into weight (depends on where on Earth you are). Santa loses a few kilos when he comes down to the equator. It's not enough for people to care, but it can be important if you're building something that needs to carry a lot of weight.

This is a very important distinction to make in scientific papers. Completely meaningless for most writing. If anything, physicists got the definition for the word "weight" wrong and patched up their mistake by adding the concept of "mass". Most mass is measured standing still on earth, so "weight" is fine unless you need to be precise.

[kelnos]

> It's not enough for people to care, but it can be important if you're building something that needs to carry a lot of weight.

The problem is that the article presents these "corrections" without being clear what context they're important in. The examples given make it sound like the author is railing against people who use these forms in regular conversational speech. And even the longer explanations after the table still make it seem like the author isn't only concerned with formal/scientific writing.

So yes, in scientific papers, I do think this list is a reasonably good guide (though good luck getting particle physicists to give up electron-volts, or people writing about electrical power systems to give up kWh). But for everyday use it feels unnecessarily nitpicky.

For the specific mass vs. weight example, especially, regular conversation (spoken or written) is pretty much always going to use the term "weight", and using "mass" would feel stiff and unnatural.

[ndsipa_pomu]

The issue is that it's very common for people to use weight when they're referring to mass. We get away with it as our weight doesn't change very much when on Earth, but if we start populating other planets, then we're going to have to become more precise.

[immibis]

It's all pedantry. Physics makes a distinction between mass, a (mostly) invariant property of an object, and weight, the force proportional to your mass and the planet you're standing on divided by the square of the radius. Only physicists care about this distinction.

[USiBqidmOOkAqRb]

I would like to believe engineers care too. A simple action like standing up or being on a bus that's taking a curve will make items on your person weigh more.

[creata]

Yeah, this is the only one that seems flat out wrong. When I'm on a bridge, I absolutely want to see "The bridge can hold up 3000 kg", NOT "The bridge can hold up 29 kN". Partly because "holding up" (as opposed to, say, withstanding) force sounds nonsensical, but primarily because knowing the maximum load in kg is much more useful.

[nayuki]

Structures like bridges are a complicated case because you can't describe its load capacity as a mass or a weight (force).

I'll assume that a bridge fails when the downward force on its deck exceeds a certain constant number. But that force is inclusive of the dead weight of the bridge.

Let's say a bridge has a mass of 1000 kg and it can hold up an additional 1000 kg of vehicles on Earth. That means it can hold up about 20 000 N of weight (assuming that g = 10 m/s^2).

The Moon's surface gravity is 1/6th of the Earth's. 20 000 N of weight would be 12 000 kg of mass on the moon. Subtract the bridge's mass of 1000 kg, and you can put 11 000 of vehicles on the bridge - which is substantially more than 6× the mass of vehicles allowed on Earth.

[isoprophlex]

You never know! Tomorrow that bridge might be on the moon, and how on earth (or how on the moon, indeed) will you know if it will hold up when you walk your herd of elephants over it!

[kelnos]

I mean, that's still fine? The max load of that bridge on the moon will just be a different, higher value in kilograms. Since we don't move bridges between planets, it's fine to not advertise a universal value that applies everywhere.

(I get that you're joking, but still...)