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Pound a unit of force

Scientific discovery and discussion
servodude
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Pound a unit of force

#646162

Postby servodude » February 11th, 2024, 9:43 am

Moderator Message:
Sub-discussion split from here to keep that discussion on-topic - Chris

9873210 wrote:
So many bad physics teachers. Next you'll be saying the pound is a unit of force.


We can't really blame the Americans for the nonsense they're taught though.... ??

stewamax
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Re: Pound a unit of force

#646171

Postby stewamax » February 11th, 2024, 10:38 am

9873210 wrote:Next you'll be saying the pound is a unit of force.

Don't US physics teachers teach poundals?
We prefer Newtons.

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Re: Pound a unit of force

#646177

Postby DrFfybes » February 11th, 2024, 10:54 am

servodude wrote:
9873210 wrote:
So many bad physics teachers. Next you'll be saying the pound is a unit of force.


We can't really blame the Americans for the nonsense they're taught though.... ??


Surely even they know it is a unit of currency?

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Re: Pound a unit of force

#646355

Postby XFool » February 12th, 2024, 9:32 am

9873210 wrote:So many bad physics teachers. Next you'll be saying the pound is a unit of force.

It is! (1 lb force, 2 lbs force, 3 lbs force... usually called 'weight')

Unless joking, I think you mean it is not a Defined (base) Unit of force - unless a standardised value for 'g' is assumed.

odysseus2000
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Re: Pound a unit of force

#646428

Postby odysseus2000 » February 12th, 2024, 2:31 pm

XFool wrote:
9873210 wrote:So many bad physics teachers. Next you'll be saying the pound is a unit of force.

It is! (1 lb force, 2 lbs force, 3 lbs force... usually called 'weight')

Unless joking, I think you mean it is not a Defined (base) Unit of force - unless a standardised value for 'g' is assumed.


Pound is mass, not force.

When you say lb force you are talking mass x acceleration, or poundals or what ever unit you want to measure force in, but pound, kg, etc are all units of mass, not force.

Regards,

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Re: Pound a unit of force

#646431

Postby XFool » February 12th, 2024, 2:50 pm

odysseus2000 wrote:
XFool wrote:It is! (1 lb force, 2 lbs force, 3 lbs force... usually called 'weight')

Unless joking, I think you mean it is not a Defined (base) Unit of force - unless a standardised value for 'g' is assumed.

Pound is mass, not force.

Yes! But a pound of force is still a pound force...

odysseus2000 wrote:When you say lb force you are talking mass x acceleration,

Quite so - which I alluded to in my comment about 'g'.

odysseus2000 wrote:or poundals or what ever unit you want to measure force in, but pound, kg, etc are all units of mass, not force.

Yes, a "poundal" - force needed to accelerate a mass of one pound at a rate of one foot per second, per second (where a pound mass, a foot and a second are all defined primary units) is the standard definition of the unit of force, in the FPS system of units.

But 32 poundals is still one lb of force (give or take). ;)

https://en.wikipedia.org/wiki/Poundal#Background

odysseus2000
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Re: Pound a unit of force

#646452

Postby odysseus2000 » February 12th, 2024, 3:54 pm

XFool wrote:
odysseus2000 wrote:Pound is mass, not force.

Yes! But a pound of force is still a pound force...

odysseus2000 wrote:When you say lb force you are talking mass x acceleration,

Quite so - which I alluded to in my comment about 'g'.

odysseus2000 wrote:or poundals or what ever unit you want to measure force in, but pound, kg, etc are all units of mass, not force.

Yes, a "poundal" - force needed to accelerate a mass of one pound at a rate of one foot per second, per second (where a pound mass, a foot and a second are all defined primary units) is the standard definition of the unit of force, in the FPS system of units.

But 32 poundals is still one lb of force (give or take). ;)

https://en.wikipedia.org/wiki/Poundal#Background


We think of mass as weight which is a force, but mass is not a force. For example a mass of 1 lb is considered a weight of 1 lbforce on earth, but 1 lb on the moon would weigh less.

Acceleration due to gravity on earth is 9.81 m per second squared, on the moon it is 1.62 m/s**2. The reason for the square is that the Gravitational acceleration on earth 9.81 m/s per second.

Regards,

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Re: Pound a unit of force

#646461

Postby XFool » February 12th, 2024, 4:39 pm

odysseus2000 wrote:
XFool wrote:Yes! But a pound of force is still a pound force...

Quite so - which I alluded to in my comment about 'g'.

Yes, a "poundal" - force needed to accelerate a mass of one pound at a rate of one foot per second, per second (where a pound mass, a foot and a second are all defined primary units) is the standard definition of the unit of force, in the FPS system of units.

But 32 poundals is still one lb of force (give or take). ;)

https://en.wikipedia.org/wiki/Poundal#Background

We think of mass as weight which is a force, but mass is not a force. For example a mass of 1 lb is considered a weight of 1 lbforce on earth, but 1 lb on the moon would weigh less.

Acceleration due to gravity on earth is 9.81 m per second squared, on the moon it is 1.62 m/s**2. The reason for the square is that the Gravitational acceleration on earth 9.81 m/s per second.

I know. But, on the surface of the moon or the surface of the earth a mass of 1 lb still exerts a force, it's weight. On the earth it is 1 lb of weight, which is a force - it can accelerate a mass of 1 lb at a rate of 32 ft (9.81 m) per sec per sec. Or thereabouts. On the moon, significantly less.

My original point is: You can have a pound mass and you can have a pound force.

But, if you can clearly explain to me how the current SI definition of the kg mass works, in terms of the Kibble current balance, I would be grateful!

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Re: Pound a unit of force

#646469

Postby 9873210 » February 12th, 2024, 5:22 pm

Weights and Measures act wrote: the pound or the kilogram shall be the unit of measurement of mass
...
(b)the pound shall be 0·453 592 37 kilogram exactly.


The pound has been a unit of mass pretty much everywhere (e.g. UK, Canada, US, ...) for as long as there has been a concept of mass. Anybody who says pound is a unit of force is clearly and unequivocally wrong, and a scofflaw.

Pound-force is a different unit, you can tell this because it is spelt differently. You will also occasionally come across the use of kilogram-force, which is spelt differently from kilogram because it is also a different unit.

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Re: Pound a unit of force

#646474

Postby XFool » February 12th, 2024, 5:47 pm

9873210 wrote:
Weights and Measures act wrote: the pound or the kilogram shall be the unit of measurement of mass
...
(b)the pound shall be 0·453 592 37 kilogram exactly.

The pound has been a unit of mass pretty much everywhere (e.g. UK, Canada, US, ...) for as long as there has been a concept of mass. Anybody who says pound is a unit of force is clearly and unequivocally wrong, and a scofflaw.

Pound-force is a different unit, you can tell this because it is spelt differently. You will also occasionally come across the use of kilogram-force, which is spelt differently from kilogram because it is also a different unit.

I have never said, nor am I saying, the pound is not a unit of mass... Good grief!

What I said, and I'll say it again because it is true, is that a "pound" can be a defined unit of mass - under the FPS system of measures. But.... it can also be a measure of force - most commonly referred to as 'weight'! This is not a contradiction. You have even said so above in your own comment. It is not "spelt differently" it is still spelled 'pound'.

https://en.wikipedia.org/wiki/Poundal#Background

"...and that is the poundal, which is approximately 1⁄32 pound force."

I regret now ever starting this... :roll:

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Re: Pound a unit of force

#646484

Postby 9873210 » February 12th, 2024, 6:52 pm

XFool wrote:
What I said, and I'll say it again because it is true, is that a "pound" can be a defined unit of mass - under the FPS system of measures. But.... it can also be a measure of force - most commonly referred to as 'weight'! This is not a contradiction. You have even said so above in your own comment. It is not "spelt differently" it is still spelled 'pound'.

Pound is defined as a unit of mass. It is not defined as a unit of force. "Can be" is so weak as to amount to deliberate deception.

Anybody who spells "pound-force" as "pound" is wrong. Anyone who uses "lb" as an abbreviation for pound-force is wrong.

This is a hazing thing for entry into academic engineering. Entrants must want acceptance so much that they are willing to be wrong. Meanwhile for over 5000 years, law, commerce and even most practical engineering today weight means mass and there is no ambiguity on what a pound is.

XFool wrote:I regret now ever starting this... :roll:

And so you should. It's long past time for the academic engineers and wannabes to give up on their stupidity.

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Re: Pound a unit of force

#646584

Postby XFool » February 13th, 2024, 10:09 am

Wanton silliness abounds...

9873210 wrote:weight means mass

Try telling that to a physicist.

We watch with bated breath (well, possibly!) on a daily basis, to see when Wikipedia's egregious 'errors' are corrected.

Finis!

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Re: Pound a unit of force

#646612

Postby servodude » February 13th, 2024, 11:13 am

XFool wrote:Wanton silliness abounds...

9873210 wrote:weight means mass

Try telling that to a physicist.

We watch with bated breath (well, possibly!) on a daily basis, to see when Wikipedia's egregious 'errors' are corrected.

Finis!


I'd have quoted "means" myself..but given the prevalence of using "weight" for relative mass expressed in simple mass units, I think they're synonymous enough that one risks problems if you assume how the reader will take it :(

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Re: Pound a unit of force

#646625

Postby XFool » February 13th, 2024, 11:46 am

servodude wrote:I'd have quoted "means" myself..but given the prevalence of using "weight" for relative mass expressed in simple mass units, I think they're synonymous enough that one risks problems if you assume how the reader will take it :(

Not quite sure what you mean here, servodude. However...

I'd have thought, to a phycisist, 'mass' "means" mass, "means" inertia and a fundamentally defined physical unit in the ISO system of units. (Could be a pound mass under the old FPS system)

Whereas, "a pound of potatoes" means something else! Although related, obviously. A pound of force? Of course! Torque, in the US, is still measured in pound-force-feet, lbf-inch.

https://en.wikipedia.org/wiki/Torque

While all for accuracy myself, I should be fully aware of the degree of absolute and literalistic thinking exhibited by some on TLF (and other BBs). After all, by now, I have surely had enough experience! :(

My fault for originally being a bit flippant?

So, I didn't get the memo that posting on TLF was actually the same thing as taking a Viva for an Oxford Natural Sciences Tripos! Or is that Cambridge? I'm sure I wouldn't know.... :)


PS. Anyone mention the spring 'balance' ? I really should shut up now. :lol:

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Re: Pound a unit of force

#646785

Postby servodude » February 13th, 2024, 8:25 pm

XFool wrote:Not quite sure what you mean here, servodude. However...


Just that if someone gets a form to fill in with a field for their weight it is not normally entered in Newtons ;)

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Re: Pound a unit of force

#646786

Postby XFool » February 13th, 2024, 8:30 pm

servodude wrote:
XFool wrote:Not quite sure what you mean here, servodude. However...

Just that if someone gets a form to fill in with a field for their weight it is not normally entered in Newtons ;)

Has it ever been tried? :|

Yes. It's the weight that counts, not the "mass". Though I suppose, if you are slimming... :)

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Re: Pound a unit of force

#647074

Postby 9873210 » February 15th, 2024, 10:06 am

XFool wrote:
Whereas, "a pound of potatoes" means something else! Although related, obviously. A pound of force? Of course! Torque, in the US, is still measured in pound-force-feet, lbf-inch.

"a pound of potatoes" in commerce* means a quantity of potatoes. Always much closer to mass than to force. As soon as there was a clear definition of mass the legal definition became mass.

XFool wrote:PS. Anyone mention the spring 'balance' ? I really should shut up now. :lol:

New fangled devices. For almost 5000 years weight was determined using a balance and reference weights. That's what "weight" meant in various languages. Then a few years ago somebody invented the spring balance and claimed that measured "weight". The commercial and civil authorities said "No, use a real balance"**, but some people won't listen.

* Which apart from the occasional rhetorical flourish is about the only place "pound of potato" shows up.

** Spring balances are now allowed but they have to be calibrated for the particular location. The calibration is needed because you want to measure mass and a spring balance does not measure mass directly.

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Re: Pound a unit of force

#647214

Postby XFool » February 15th, 2024, 8:09 pm

If I need to inflate my car tyres, 40 psi, what's that in pascals/sq cm?

Oh, never mind... ;)

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Re: Pound a unit of force

#647216

Postby servodude » February 15th, 2024, 8:27 pm

XFool wrote:If I need to inflate my car tyres, 40 psi, what's that in pascals/sq cm?

Oh, never mind... ;)


Rule of thumb is 10psi =~ 0.7 bar (for tyres' level of required accuracy)
where a bar is 100kPa
you don't need an area when taking about Pascals (they themselves are N/m^2)

so you need about 2.8 bar (or 2800 kPa)

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Re: Pound a unit of force

#647218

Postby XFool » February 15th, 2024, 8:47 pm

servodude wrote:
XFool wrote:If I need to inflate my car tyres, 40 psi, what's that in pascals/sq cm?

Oh, never mind... ;)

you don't need an area when taking about Pascals (they themselves are N/m^2)

Yes of course! :oops:

SI unit of pressure (aka force per unit area (not mass per unit area...)).


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