Physics for Dummies: At the Penalty of Sounding Real Stupid

[Crimea_River]

In your example, yes, but my air pump does not have 15 pounds of air in it when I seal the end, nor does it have 30 lbs of air in it if I double the pressure by pushing down the plunger. You can't equate lbs weight and lbs force.

 

[mikewint]

When prey tell do you use "survey" miles Mike?

Only when I pray to Baal for the patience I so sorely lack

Pbehn, not sure of your point here. Mass is invariant so the lb or Mass pound is constant anywhere in the universe. But were I to carry that cylinder if air to Mars and weigh it there I would get a different weight. So your statement is valid only at or near the Earth's surface

 

[fubar57]

The end of copy/paste....blocked

 

[pbehn]

Only when I pray to Baal for the patience I so sorely lack

Pbehn, not sure of your point here. Mass is invariant so the lb or Mass pound is constant anywhere in the universe. But were I to carry that cylinder if air to Mars and weigh it there I would get a different weight. So your statement is valid only at or near the Earth's surface

When I was ten in a classroom we didn't really discuss the possibility of being on Mars. When people land on Mars maybe this discussion could continue there. When explaining what is the situation on earth at sea level its best not to go to Mars for a demo. I am surprised that no one jumped on my lazy reference to "sea level" if the sea was level we wouldn't have a word for tides and there wouldn't be locks in the Panama canal

 

[MIflyer]

Altitude in MSL = Mean Sea Level.

I launched a satellite that used a radar altimeter to measure the shape of the Earth and height of the seas.

 

[pbehn]

Altitude in MSL = Mean Sea Level.

I launched a satellite that used a radar altimeter to measure the shape of the Earth and height of the seas.

All of these things that everyone understands as a constant, because to most people they are a constant, need precise measurement because in fact they arent. Sea level obviously changes which is why those by the sea in tidal areas have all sorts of names for them, those who live on tidal nodes dont.

 

[wuzak]

Mass and weight
In the 19th and 20th centuries, the UK used three different systems for mass and weight.

troy weight, used for precious metals;
avoirdupois weight, used for most other purposes; and
apothecaries' weight, now virtually unused since the metric system is used for all scientific purposes.
The distinction between mass and weight is not always clearly drawn. Strictly a pound is a unit of mass and it is commonly referred to as a weight. When a distinction is necessary, the term pound-force may be used to refer to a unit of force rather than mass. The troy pound (373.2417216 g) was made the primary unit of mass by the 1824 Act and its use was abolished in the UK on 1 January 1879, with only the troy ounce (31.1034768 g) and its decimal subdivisions retained. The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.

Imperial units - Wikipedia

For many hundreds of years there would have been no distinction mass and weight, since the relationship between them was not understood.

 

[michael rauls]

It matters not which pound you are using because that is the pound you use in the calculation of pound per square inch. One pound weight placed on an inch cube will exert a force of one pound on the inch square below the cube. The column of air above the earth has a weight and exerts a force of circa 15 pounds per square inch on the earths surface and because air is a gas that is the pressure we have at the surface.

Nice clear explanation.

 

[ThomasP]

For anyone interested,

Aquadraco gave a short and correct answer in his post#21.

 

[DerAdlerIstGelandet]

The second paragraph came somewhat from Wiki as I have ignored the trend to define Pound (lb) as mass so was not quite sure of the exact conversion. When I start seeing gauges labeled in "lbfsi" units I'll pay more attention.
I've also resisted pressure gauges labeled in kg/cm^2 or kg/m^2 though I can visualize a cylinder with a kg of mass sitting atop a movable piston. The kg mass at or near the Earth's surface is attracted with a force of 9.807 N of force. So the piston is applying 9.807 N of force on the confined gas. Since the derived unit Newton is such an tiny unit of force it is practically unusable in the man-sized world except in kilo- or Mega- multiples. So kg/cm^2 is preferable to 9.807 N/cm^2. I've also seen the trend to use kgf units which is the SI equivalent of calling the pound a unit of mass. So 1 kgf is the equivalent of 9.807 N of force.
When we converted a given kilogram mass into English Avoirdupois Pounds it was always clearly understood that the conversion was valid only at or near the Earth's surface. So a person with a mass of 100 kg weighed 220 Avoirdupois Pounds on Earth but 82.7 Avoirdupois pounds on Mars with its lower gravitational attraction.
The rest is from all the mostly useless data stored in me own pointy head. For the nonce I will hop into my COWc and drive down to the COWl where I'll jump into my COWb to do some fishing. I have plenty of COWw to bait on my hook hoping to catch plenty of COWf. TTFN

You can ignore defining it as mass all u want, it does not make right.

 

[mikewint]

Chris, I agree but there is a subtlety here in that definition that is causing all the problems and that I think is going to cause further problems and confusion. We never had a problem with something like PSI because it was clearly understood that P - for POUNDS represented a FORCE supplied by the billions upon billions molecular collisions with the container walls. In the same way it was visually obvious that POUNDS as WEIGHT was also a FORCE which was measured by either the compression or stretching of a spring or springs attached to a dial called a SCALE. You can directly feel the force you have to apply to compress a spring. So weighing an object is an opposition of forces. The force of gravity pulling down and the spring force pulling up. The same is true everywhere on the planet. The ground/floor HAS to push up on your feet, chairs push up on your behind, motors supply force to lift you into the air, the muscles in your legs supply force to lift you up stairs, etc.
Electronic scales have a platform that is attached to a piezoelectric crystal. The force deforms the crystal changing its electrical resistance which can be measured.
In all of the above it is easy to envision that transported to the moon, Mars, Jupiter the FORCE applied by the new gravitational field would change the force applied by gravity and thus the scale reading. You can call the scale readings by any names you want but they represent FORCE/WEIGHT

Now the QUANTITY of matter is MASS and unless you alter the object in some way THAT remains constant anywhere in the universe. So we need a device that does not rely on FORCE like a Scale. We use a BALANCE which is in essence an glorified Teeter-totter a Double Pan Balance in the lab. The unknown mass on one pan and Known masses on the other pan util we get a balance. The sum of the known masses now must equal the unknown. Once again the name you give the numbers on the known masses can be anything you want but in SI we use GRAMS a unit of MASS or QUANTITY of matter. The English/US Customary system brought in the SLUG as their unit of MASS. As long as science remained isolated everytng was fine but as science became international the two systems collided causing considerable problems

Bear with me, I'm getting there. When the old English system was in full force the US/UK part of the world resisted any Continental influences as lead by the French, i.e. that evil Metric System. [N.B. - The English refused to accept the Gregorian Calendar (it was Popish) of 1582 until 1752 at which time England and the US were ELEVEN days ahead of the rest of the western world]. So scientific measurements were conducted in the FPS system or FOOT-POUND-SECOND system in the British influenced world. As the Metric System gained traction it never supplanted the old English so Physical Laws and calculations could and were carried out in one of three systems FPS - CGS - MKS. And at times the systems crossed one into the other so we had to express Kilograms/grams into Pounds - Mililiters/Liters into Gallons/Quarts - Cubic centimeters/cubic meters into cubic inches/cubic feet. We memorized or carried around lists of "CONVERSION FACTORS like, e.g. 1 Kilogram = 2.2 Pounds BUT, BUT, BUT it was clearly understood that this was an EQUIVALENCE NOT AN EQUALITY. In other words BIRDS have WINGS and so do airplanes; Tables have LEGS and so do Horses/dogs/people BUT you would NOT for an instant suggest that they are THE SAME thing

So yes it is certainly true that a 100 kilogram MASS placed on a spring scale will cause the dial to read 220 POUNDS at or near the Earth's surface. That makes them EQUIVALENT in that situation not identical. So it is an unfortunate consequence of human resistance to change that POUNDS exist at all and that they have morphed into Pound-mass and Pound-force. The very fact that you have to hyphenate them tells you that they are NOT the same things

 

[DerAdlerIstGelandet]

No Mike. You are overcomplicating things, as usual with your copy and paste posts.

The thread starter asked why a specific PSI does not weigh the same amount in pounds. Why is that? Because in this case lbs is a unit of mass, and PSI is a unit if pressure. One does not equal the other. It is simple physics 101. That is fact.

Sometimes it is best to not search the internet for a lengthy post to copy to sound more intelligent. Sorry if this sounds harsh. I don't wish to insult you at all, but you are arguing for the sake of arguing, and its not working.

 

[fubar57]

This is perfect now

 

[Elmas]

It is a relative concept.
It depends on how many pounds is a pound...

 

[pbehn]

View attachment 564771

It is a relative concept.
It depends on how many pounds is a pound...

The curse of using an ancient name, it goes back to when currencies were in silver and a pound was a weight of silver coins.

 

[mikewint]

Chris we are certainly at an impasse here and neither of us will convince the other of error. To quote http://hyperphysics.phyastr.gsu.edu/hbase/Mechanics/slug.html
The slug is the unit of mass in the US common system of units, where the pound is the unit of force. The pound is therefore the unit of weight since weight is defined as the force of gravity on an object. While the pound force and pound weight are the widely used units for commerce in the United States, their use is strongly discouraged in scientific work. The standard units for most of scientific work are the SI units.

So Zipper's confusion with the meaning of the P in PSI would never have occurred to begin with if the silly English system had been dropped to begin with instead of trying to patch them together. Confusing EQUIVALENCE with EQUALITY, taking two vastly different physical entities, giving them the same name, thus having to hyphenate them to try to eliminate confusion simply compounds the sillyness. I'll accept POUND as both FORCE and MASS when I see my first pressure gauge labeled in: LBfSI units

Another Quote:
Science Guys | Deparment of Physics at Union University
Now, weigh the bricks on a scale, and you find that each brick weighs 10 pounds. The scales are measuring weight, that is, the force of the Earth's gravity pulling down on the mass. Weight is the force gravity exerts on something. If you determine the mass of each brick on a balance, you find that each brick has a mass of 4.54 kg. (Balances work by canceling out the effects of gravity.)
If you go to the Moon, the mass is the same but the weight is six times less because the gravity is six times weaker there. Mass and weight are related, but different. Mass refers to the amount of matter, weight to the force of gravity pulling on that matter. (In British units, a mass of one slug placed on your bathroom scale would weigh 32 pounds!)
Thus, if you place 1000g of mass on your bathroom scale, your scale would read a weight of 2.2 pounds. So, you often see written, 1000g = 2.2 pounds. This equation will irritate science purists, since we've equated apples and oranges here by setting mass equal to force. What we have just written is really an equivalence. A 1000g mass on Earth has an equivalent weight of 2.2 pounds. With a little arithmetic, you find that 454 grams has a weight of 1.0 pound (1000 / 2.2)!
Grams are different from pounds - one is mass and one is force, two different physical quantities. Yet 454 grams is equivalent to 1.0 pound here on Earth. That is, an object weighing 1.0 pounds contains 454 grams of mass.

 

[pbehn]

We never established what Zipper considers high pressure and whether the weight he was talking about was a comparison to the weight of high pressure hydraulic systems.

 

[fubar57]

Never mind that, I can't even fathom that 100,000 psi water pressure you mentioned. We used 1600 psi water wands in the paper mill to remove old pulp stock from wall and such and one guy cut through his gum boot and toe

 

[Elmas]

Yet 454 grams is equivalent to 1.0 pound here on Earth. That is, an object weighing 1.0 pounds contains 454 grams of mass.

Always?

 

[mikewint]

Always?

At or near the surface of the Earth. The 454g is constant the weight varies with the strength of the gravitational field