Because the measurement of force on an object of a particular mass due to gravity is effectively identical everywhere on planet Earth. For the general purposes of needing to know weight, fluctuations in the force applied by gravity aren't even rounding errors - they are too minuscule to matter.

Given that that force is effectively a constant in Earth-gravity, we can convert between mass and force on Earth.

While pounds are indeed a unit of force, most people treat them as a unit of mass assuming Earth's gravity.

When someone says that 1 pound is 454 grams, they mean a mass that weighs 1 lb in Earth's gravity has a mass of 454 grams.

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There are two types of pound. Pound (force) and pound (mass). So just have to use the appropriate one.

When you see someone doing this, it's because they're taking a shortcut, usually without even realizing that they're doing it. On the surface of the earth, where acceleration due to gravity is roughly 9.81m/s^2 no matter where you are (it DOES change a bit but not by a huge amount), then we can use the force an object applies to the ground to determine what its mass is. In other words, we can treat a unit of force like a unit of mass. Doing so will get you yelled at by physicists, so don't do this in any scientific or engineering environment.

Pounds are a unit of force, while kilograms are a measure of mass, true. However, on Earth they are interchangeable because the difference between a measure of force and a measure of mass in this case is gravity. The pound takes into account gravity, and mass does not.

However, so long as they are in the same gravitational field, they are effectively the same unit. You can think of it as adding gravity to kilograms to make it a force or removing gravity from the pound to make it a unit of mass. Either way, you end up with comparable results if the gravity is the same.

It also does not matter what gravity it is, on the Moon, Earth, Jupiter, or a neutron star. So long as the gravity for both is the same, kilograms are effectively a unit of force and pounds a unit of mass.

Nope, a kg is indeed a unit of mass. A unit of weight in metric is Newton.

...Grams are absolutely a measure of mass. You body has the same amount of kg on earth as you do on the moon and beyond. Pounds is a measurement of force and will change depending on elevation from the earth.

This is why in engineering you learn that pounds can be either a force or a mass, designated lbm or lbf. Pounds mass is based around the gravitational constant only, so it is a weight but because the gravitation constant is standardized it can also be a form of mass. Pounds force accounts for the gravitational constant where the object physically is or needs to be studied. For earth, pounds mass and pounds force are the same. Anywhere else, they will differ.

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Edit: For those who don't know, a slug is the actual imperial unit of mass

Pounds of mass and pounds of force are both named pounds. 1 pound of mass has a weight of 1 pound of force while on Earth. We usually don't distinguish between them, but as long as you're on Earth it doesn't really matter.

Gravity applies a practically constant force on you at earths surface. If you don’t go deeeep underground or really high above earth, 9.8 meters per second is a pretty good number.

So, on earth’s surface, one kg of mass is approximately 2.2 lbs of weight and 9.8 newtons.

Go somewhere where gravity is half as much as ours and the same kg weighs 1.1 lbs and 4.9 newtons.

Force is only worthwhile as a PROXY for mass when measuring on equal gravity. So, for most experiments on a single planet with a strong gravity, it’s fine to use force for calculations.

Someone here says they’re 400 lbs and you may be able to picture their fitness level. Someone on the sun’s gravity weighs 400 lbs and they’re the size of a baby. A 100 kg person weighs differently on the sun too, 30 times more, but they would still be 100kg. And dead, but this is a thought experiment.

When you’ve got multiple gravitational fields to think about, knowing the mass of an object makes your life way easier. Even in the US, physicists use metric for this very reason.

How did I do for simplicity?

Even more to the point.

Every scale ONLY measures the force an object puts on the scale. No scale measures the mass independent to a force being exerted on it.

Quick way of doing it in your head: add 10% and then double it. It's not precise but pretty close.

Edit: sorry, it's the other way round. Lbs->kg would be dividing by 2 and substracting 10%

Check: 1lbs is 0.453592...kg

1/2 = 0.5

0.5 - 0.05 = 0.45 --> off by about 3.592g

The other way round: 1kg is 2.2046...lbs

1*2 = 2

2 + 0.2 = 2.2 --> off by about 0.0046lbs

Pound or lb is a unit of mass defined as 0.45359237kg,

Pound-force or lbf is a different unit defined as weight of one pound. That is of course not a fixed quantity, but depends on location, Earth's gravity is not exactly the same everywhere.

Would be interesting to have a space-scale that measures mass by pushing on and then stopping it.

Pound is not a unit of force.

>1 pound (avoirdupois)= 0.453 592 37 kilogram

https://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf

Sorry, only read the title. Still leaving this here, might be useful for some.

As others have explained you specify lbf or lbm to distinguish force vs mass. You use a reference assumption about the gravitational “force” to convert weight to mass.

If you want your mind really blown, know that gravity isn’t really a force. Weight is a lie!

That is exactly how "space scales" work. Astronauts get wobbled back and forth by the scale, which compares the force to the movement, and computes the mass.

Ah yes, lbf, not to be confused with flb or ft-lb, which is an unit of torque.

A kilogram is a unit of mass. Just read the definition of it

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>"The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs."[1]

The SI unit of force a 1Newton that in SI base unit is 1 kilogram meter / second ^2

Yeah, pound is a mass unit, but we treat it as a force unit via the [pound-force](https://en.wikipedia.org/wiki/Pound_(force)).

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Is earth gravity different in Death Valley vs top of Everest ? I mean so altitude play a part adjusting or do we use a constant like light speed is a constant ?

I had thought the closer we got to the Center of the mass the more intense the gravity and the further away the weaker ?

> Is earth gravity different in Death Valley vs top of Everest ?

Technically yes - gravitational force does decrease the further you get from the Earth.

However, the distance in question reduces the force by such a minuscule amount that it doesn't matter outside of science or engineering. You have to get very far away to notice - even the ISS, about 250 miles above the surface of the Earth, still experiences about 90% of Earth's gravitational pull.

From the calculations I was able to find, the difference in weight from sea level to the summit of Everest is about 0.4% - and that is about as extreme as you can get and still technically be "on Earth". For most practical purposes, you can treat the force as constant.

Edit: Just to expand on this, the highest city in the world is El Alto, Bolivia, at about 13k feet - half the height of Everest. A 50k lb shipping container would only weigh about 100lbs less if taken from sea level to El Alto - and that isn't a meaningful difference.

Since the pull of gravity is pretty much constant on Earth’s surface, a measure of mass implies a weight and vice versa.

You are right, mass is measured in kilograms and slugs, and weight in Newtons and pounds. However, people don’t bother with the difference because we something with 454g of mass has 1 pound of weight (while on Earth).

In the unholy metric imperial system flb is just a femtopound

>I had thought the closer we got to the Center of the mass the more intense the gravity and the further away the weaker ?

As the other answer said, it is a very small difference. The earth has a mean radius of ~4000 miles while going from Death Valley to the top of Everest is only about 6 miles, so a pretty small change proportionally.

Edit: said diameter instead of radius

That is a completely different unit, with a completely different abbreviation of lbf. Pound-force is defined as: "weight of one pound"

Pound has abbreviation of lb, it's a unit of mass and is equal to 0.45359237kg

You cannot treat pound as unit of force, because it isn't one. Same as you can't treat pound as unit of torque or unit of energy. Pound-force, pound-foot and foot-pound are all completely different units of completely different quantities and they are definitely not the same unit as pound.

It is, formally speaking, a different unit. But for our purposes here on the Earth, where the vast majority of practical use of the units is conducted, the two are proportional and that proportionality is constant enough for the difference not to matter.

If you can't tell a difference between force and mass you might as well not bother measuring anything and just eyeball it.

And the difference of how much a pound weighs varies significantly, at poles its half a percent more than on equator. That's a lot, half a percent makes a difference between a buy and a sell.

Well done! I love mixing metric system prefixes with real units as well.