So I was going to write a comment about how you actually can adjust weight and mass independently based on W = mg if you can control gravity, and that comment relied on the assumption that kg measures mass and lb measures weight…
Except the latter is not true! In US customary and other systems that use the avoirdupois pound, 1 lbm := 0.45359237 kg exactly. However, there is also a pound-force, which is lbf := (1 lb) × (gravitational acceleration in ft/s^2 ). This gives rise to the slug, yet another unit of mass defined as slug := (1 lbf) ÷ (1 ft/s^2 ) = 32.17404 lbm. I actually used this unit in my differential equations class because my professor was really old-school.
The source of the confusion is that the “pound” as a unit of measurement (really one of several units) predates the distinction between mass and weight.
Moral of the story: if handed US customary units, just convert to metric (SI). And I guess I have to write lbf or lbm now whenever I’m forced to use US customary.
I’m so unbelievability tired and sleepy and exhausted can you rewrite this as pretend my english level is that of a 5 year old so i can understand
Short version: lbs and kgs aren’t just different measurements of the same thing. They are the same thing. Pounds are mathematically defined by kilograms.
I’m somehow more confused, pounds and kg are like miles and km right? that’s what I assumed when posting the meme
They were thinking that lbs and kgs are different types of measure:
Lbs measuring weight, which is characterized by the amount of gravitational force applied, and having SI units kg⋅m⋅s⁻² . And kg measuring mass, which is an intrinsic property characterized by the sum of all of an object’s atom’s masses, and having SI units kg.
But they realized in the U.S. a lb is now directly defined by a mathematical proportion to the kg, and therefore is now a measure of mass, not weight. Thus, they are indeed the same type of measure.
The confusion stems from the fact that laymen use force and mass interchangeably as they are always on earth and changes in altitude aren’t significant enough to worry about. Standing on a European scale and seeing a measurement in kg isn’t entirely accurate- it’s actually measuring Newtons and implying your mass in kg from that. Standing on an American scale, however, is literally measuring your weight in lbf. However, there is also a confusing unit called lbm or pounds mass which measures the mass of a 1lbf weight object on earth. The average person will never use lbm realistically, but this is technically the unit that converts directly to kg.
That’s definitely part of it, but the lb dates way back to before there was any conception of difference between weight and mass. Nowadays, the kg, and thus the U.S. lb, is defined in terms of universal constants (the Planck constant and the speed of light), but traditionally the lb would have been defined by some sort of standard physical object, whereas the kg was defined as the mass of a litre of water. There was an implicit reliance on the force of earths gravity in the measure of the lb, which wasn’t part of the measure of the mass of a kg. So, I think historically speaking it’s understandable to think of the lb as a unit of weight, not mass.
If I’m understanding correctly does this imply that a space colony might measure things in kilograms but have an entirely different value due to differing gravity?
Edit: rereading I’m realizing that the opposite is true, lbs and kgs would be the same on and above every planet
Nope! But also kinda! All US customary units are defined by metric. That said the pound is a unit of weight not of mass. That means it’s a unit of force though we usually call that the pound force (lbf). But also since the pound is a unit of weight we generally use it as a unit of mass, but with the added qualifier of assuming it’s at the gravitational constant.
So in summary the pound is actually more closely related to the newton than the kilogram, but also its defined by the kilogram because we basically always mean pound mass.
Also I don’t think American engineers switched to metric before we stopped putting people on the moon, so yeah, in addition to doing math by hand we dealt with these insane pre renaissance units.
Also we were nearly the second country to metricate. Our second president loved the metric system and requested the French to send us metric standards but the boat sank so we didn’t switch.
@[email protected]’s comment is great as a short answer. Below is a longer answer that I’ve tried to write simply.
Kilograms and pounds both measure mass, how much “stuff” is crammed into an object. It is something that every object has, and it doesn’t change depending on where you are.
Weight tells you how heavy it feels to lift an object. This can change on other planets or even on Earth if you climb a mountain.
So if you climb a mountain without losing any mass, technically you will lose weight (but not a lot).
I think most of us were taught in school that pounds measure weight, which is mostly wrong. However, if pounds hypothetically did measure weight, then the person in your post would be technically be able to change their weight in pounds without changing their mass in kilograms (by climbing up a mountain).
Additionally, in some sense, a pound is “a smaller version of a kilogram”. You can get a pound by cutting off a part of a kilogram. In comparison, you cannot get a foot by cutting a kilogram. They don’t measure the same things.
Hope you get some good sleep soon!
No, pounds in the traditional usage refer to lbf, or weight. If you stand on a scale, it measures the force you’re exerting on the scale, which is absolutely distinct from mass because the exact same scale would show a different value on Mt Everest despite you not losing any mass. Every practical use will be measuring lbf. Ie PSI, or pounds per square inch, is clearly referring to force over an area, not mass.
1 lbm weighs 1 lbf on earth, which implies that accelerating a 1lbm object at a rate of 32.2ft/s2 requires 1lbf.
Engineers are the few types of people that actually use lbm and slugs. Sensible ones will prefer to just use metric.
The comment to which you replied is a heavily-simplified version of a previous comment.
No, pounds in the traditional usage refer to lbf, or weight.
I alluded to that in the more-complex first comment. The thing is that the notion of a “pound” was conceived before the distinction between mass and weight was understood.
However, the pound has been legally defined by the United States and a bunch of other countries that use the avoirdupois pound to be a unit of mass since 1959. It’s been 65 years.
If you stand on a scale, it measures the force you’re exerting on the scale, which is absolutely distinct from mass because the exact same scale would show a different value on Mt Everest despite you not losing any mass.
That’s what I said.
Every practical use will be measuring lbf. Ie PSI, or pounds per square inch, is clearly referring to force over an area, not mass.
As I stated in the more complex comment, I acknowledged that both lbm and lbf exist, but that lbm is the unit both US Customary and British Imperial systems use to define “the” pound. This makes lbf a derived unit in those systems.
So technically, PSI really should be called “pounds-force per square inch”.
Engineers are the few types of people that actually use lbm and slugs. Sensible ones will prefer to just use metric.
Actually, the equations of physics in electrical engineering vary more severely with the choice of units than those in other engineering subdisciplines, particularly Maxwell’s equations. For that reason, and because exact conversions exist between SI and US customary, I literally always convert US customary or other “weird” units to SI and convert back the SI result if US customary is required. So I basically don’t use slugs, lbm (except to weigh myself), or US customary in quantitative work unless I am literally forced to do so like I was in my diff EQ course. I’m just pedantic.
Yes it is very pedantic to refer to a legal definition lol. Realistically, eliminating the idea of lbm entirely would make the entire conversation much more sensible.
Grams, kilograms and pounds all measure mass. They’re just “stretched” differently by this or that factor.
There is also a “pound” - lbf - that’s a confusingly named separate unit for force. E.g. pressure can be measured in pounds per square inch - it is a force distributed over an area, not a mass.
and, most cursed of all, the gravitational acceleration on earth is one pound per pound (g = 1 lbf/lb)
Nothing makes you understand the need for metric like doing physics in us customary
Go to sleep sleepy head
TL;DR use football stadiums
Yeah, the greatest moral of the story is if you’re considering engineering school, try europe or Canada. American professors sometimes like to make you use slugs just to keep you on your toes, and they often love to make memorizing the metric-us customary conversion ratios part of the course
You may have just corrected a misconception my High School physics teacher told us that I haven’t thought about since, but had been carrying in my head. Thanks!
Dude needs to stop hitting the gym and start hitting the books.
Time to hit the books
Gotta lower my lilo-byte and increase my kilo-gromms
Eat more and get mugged?
*only works if you’re bri’ish
how long is a piece of string…
twice half its length
