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Ansuz07 t1_ja91gf8 wrote

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.

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DankChunkyButtAgain t1_ja94q1r wrote

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

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jerwong t1_ja9eeun wrote

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

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zmz2 t1_ja9teo6 wrote

In the unholy metric imperial system flb is just a femtopound

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Any-Growth8158 t1_jaa1xck wrote

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

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ZerexTheCool t1_ja998ew wrote

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.

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PralineCrunch t1_ja9a4if wrote

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

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Fred2718 t1_ja9dl7c wrote

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.

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Brave_Promise_6980 t1_ja9pkgh wrote

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 ?

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Ansuz07 t1_ja9q4y3 wrote

> 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.

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KennstduIngo t1_ja9twbm wrote

>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

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