Stoopid basic physics question / just checking (rusty brain).

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I have a vague memory of "poundals"
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http://en.wikipedia.org/wiki/Poundal
So *that's* what "slugs" are. Even though I got the question right on University Challenge, I didn't know what they are.
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Today is Setting Orange, the 66th day of Bureaucracy in the YOLD 3178
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On Thu, 11 Oct 2012 16:30:46 -0700, pastedavid wrote:

Its mass is 1600kg. Its weight varies according to where you are (e.g. slightly less at altitude, and a lot less on the Moon).
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On 12/10/2012 00:52, Bob Eager wrote:

If you are suspected of overloading it, you will be taken to a weighbridge where *weight* will be measured, not mass.
While it is not uncommon to see weights (incorrectly) quoted in kg, engineers would quote them in kgf, (i.e. kilograms force, or the force exerted by gravity on a mass of one kilogram).
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On 12/10/2012 14:50, newshound wrote:

Don't think I ever recall seeing anything quoted in kgf...
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John Rumm wrote:

Is a unit of force, not of weight.
As I said the common meaning of weight is not the force exerted by a mass due to gravity. A lb of flour is still a lb of flour even in a freely falling lift shaft.
It exerts a force of a pound force when its at rest at the earth's surface. That why we have masses in kilograms and weigh things in kilograms, not kilogram force. That's why atoms have weights that are independent of gravity. We talk of atomic weight, not atomic mass, although we probably shouldn't.
But as you can see its a mess. with people getting confused between how much stuff is in the bag of flour, and what force it exerts when its at rest in 1g. And what to call the separate things.
I side with common usage here.
Not pedantic half baked physics masters who probably don't understand the difference either.
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On 13/10/2012 00:44, The Natural Philosopher wrote:

Yeah I know what it is, just don't recall ever seeing it used. pounds force, yes, and newtons, certainly.
[snip]

I side with whichever is appropriate for the context...
If I am working out the work done lifting a mass then I will use weight in its "proper" sense. If on the other hand I am "seeing how much I weigh", I am content with an answer in stone.
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On 13/10/2012 01:18, John Rumm wrote:

I can assure you that "kgf" and "tonf" are very widely used in mechanical engineering design calculations. Although the purists like to see the proper SI units it's often easier to "sanity check" kgf because we all recognise that ballast comes in 25 kg bags, etc. Modern design tools like MathCAD are brilliant because they understand units so you don't have to do any conversions (although you have to watch out because a MathCAD "ton" is is a "short" ton, i.e. 2000 pounds).
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On 13/10/2012 00:44, The Natural Philosopher wrote:

It was drummed into me that we should talk of atomic masses! (And centripetal rather than centrifugal.)
And, colloquially, people with a bit of understanding say things like "A falling bag of flour is weightless". But I am sure they do not think that it then has no mass.
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On 13/10/2012 00:44, The Natural Philosopher wrote:

Weight is a *force* = mass x acceleration.
I agree that almost no one in practice uses kg.f any more than experienced chemists use IUPAC names for common chemicals.
It is even worse in the US where aerospace uses foot-poundals and short measure antique British gallons but NASA uses SI and Mars probes smash into the planet and planes fall out of the sky as a result.

But common usage does not clearly distinguish between mass and weight. We are so used to constant Earth gravity that for most practical purposes setting g=1 makes no difference and is very convenient.
Physics does and you would actually find that if you tried to use a pair of bathroom scales or Tescos checkout in freefall the products would all be weightless and a mass balance wouldn't work either.
The products would all be weightless so you would have to weight them by applying a known specific impulse and measuring their change in velocity.

It is good enough to within 0.5% over the entire surface of the Earth. But it is generally weight that is measured these days even in fancy laboratories - in the old days kitchen scales had a set of weights and labs had beautiful chemical balances to measure mass but not any more.
Even mass spectrometers are actually measuring the mass to charge ratio. It has proved very difficult to tie the standard reference mass into the fundamental constants with anything like acceptable precision.

Common usage *and* what is actually being measured.

It is actually rather important to understand that things in freefall are weightless though - that is the foundation of general relativity.
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On 13/10/2012 09:57, Martin Brown wrote:

Things have changed then. Even the most fancy balances I used at Uni were balances - illuminated readouts and all.
<fx googles>
Well B***r me, things HAVE changed.
"Sartorius analytical balances are lab weighing instruments that feature the highest accuracy for analytical weighing processes" ... "Their world's first monolithic weigh cell meets unique prerequisites for ensuring high measurement accurracy. " [sic]
And yet it's still called a BALANCE! Andy
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On Sat, 13 Oct 2012 00:44:37 +0100, The Natural Philosopher

1,600 kg.

kg x g > >>>> 1,600 x 9.81 = 15,696 N.

are (e.g.

kg,
force
Same thing!

a

are

Weight is not exerted by a mass, it is exerted on the mass by the gravitational field. Why do you think that because an object is moving, it has no weight?

how

its at

There we go again
in 1g. And what to call the separate things.

understand

to

are

of a

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On 12/10/2012 00:30, snipped-for-privacy@gmail.com wrote:

Assuming you live on Earth, its weight is, for most practical purposes, 1,600 kg force and its mass is 1,600 kg mass. It will weigh slightly more at sea level at the poles than it will at sea level at the equator, and it will weigh slightly more at the foot of Everest than it would if you could get it to the summit, but its mass will be the same wherever it is.
Colin Bignell
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I'm curious why this in fact matters?
Brian
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On Friday, October 12, 2012 8:23:10 AM UTC+1, Brian Gaff wrote:

I have an over-active mind, and not enough sleep!
I was just watching television, so naturally mind drifted to matter less depressing, and I was idly thinking about the force required to get my car to whatever speed, and then it occurred to me that I couldn't remember the formula, so I looked it up and found F = ma, and W = mg and due to my tired state, I confused myself. It happens.
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snipped-for-privacy@gmail.com wrote:

It's becaus'e you're suffering from grocer's' apos'trophe s'yndrome.
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Tim

"That excessive bail ought not to be required, nor excessive fines imposed,
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On Friday, October 12, 2012 8:52:03 AM UTC+1, Tim Streater wrote:

One misplaced "it's", and they breed. I think we can all learn from this error.
Anyway, after sleeping, I think Ive cracked it.
;-)
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On 12/10/2012 00:30, snipped-for-privacy@gmail.com wrote:

No, you are perfectly correct if taking their terms literally.
However it is very common for mass to be used to interchangeably with weight, that its hard to argue they are wrong as such. If someone ask how much do you weigh, you probably don't answer in Newtons.
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Cheers,

John.

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On 12/10/2012 12:08, John Rumm wrote:

That would make BMI calculations more difficult.
Colin Bignell
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On Friday, October 12, 2012 2:02:43 PM UTC+1, Nightjar wrote:

Yes, your weight is less on the moon, but your BMI is the same.
Robert
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