Physics for Dummies: At the Penalty of Sounding Real Stupid

[Aquadraco]

How is it you can have a compressed air tank for paint-balling that has an absurdly high PSI (pounds per square inch) and doesn't weigh several tons? I figure with PSI being pounds per square inch...

This is the situation as I understand it - 1 cubic metre of dry air at sea level has a volume of 1000 litres and weighs 1.292 kg.
If you take that 1000 litres and cram it into a 1 litre container, it should exert a pressure on the walls of the container of around 1000 bar, or 14,700psi. But even though you have changed the volume and therefore the pressure, the actual mass of the air still weighs 1.292 kg - it just takes up less space than before.
Warning - my mathematics may be wonky. Your mileage may vary.

 

[michael rauls]

M michael rauls That would require the pressure to be exerted in all directions (including up) evenly. That said, how come to put a given PSI of water inside a vessel, it weighs more? It seems there's a conceptual error here...

P pbehn The term is figurative, I know it's evolved.

Im not sure I understand your question but just to answer literally as best I can if you increase the preasure in an inclosed container you do so by cramming more say air for example in said container thereby increasing the wieght, although with air or any gas for that matter the increase in wieght would be very slight indeed.

 

[wuzak]

Chris LBS or Pounds is a unit of FORCE. There is an English unit of mass called a SLUG though it is seldom used today.
At or near the Earth's surface gravity exerts a FORCE on all massed objects which accelerate the object towards the Earths center of mass. That FORCE varies inversely with the square of the distance between the two centers of mass. Commonly we call that FORCE the object weight which thus varies with distance from the Earth's center of mass. An objects MASS is constant anywhere in the universe. Kilogram is the SI unit of MASS while the NEWTON is the SI unit of FORCE. So a KILOGRAM of MASS weighs 9.8 NEWTONS at or near the Earth's surface or in the English system 2.2 POUNDS.

"The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound (symbol lb) has been defined as exactly 0.45359237 kg.[7][8]

In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.[9]

The yard or the metre shall be the unit of measurement of length and the pound or the kilogram shall be the unit of measurement of mass by reference to which any measurement involving a measurement of length or mass shall be made in the United Kingdom; and- (a) the yard shall be 0.9144 metre exactly; (b) the pound shall be 0.45359237 kilogram exactly.

— Weights and Measures Act, 1963, Section 1(1)"

Pound (lb) is a mass in current usage, pound-force (lbf) is a force.

The pound-force is the weight of 1 pound mass in standard gravity. That means that in most static cases pound mass (lb) and pound-force (lbf) have the same number, which is what tends to lead to confusion.

Pounds-per-square inch (psi) is actually pound-force per square inch.

The metric equivalent is used by some countries - kg for mass and kgf for force. 1 kgf = 9.81N.

 

[wuzak]

M michael rauls That would require the pressure to be exerted in all directions (including up) evenly. That said, how come to put a given PSI of water inside a vessel, it weighs more? It seems there's a conceptual error here...

Weighs more than what?

 

[DerAdlerIstGelandet]

"The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound (symbol lb) has been defined as exactly 0.45359237 kg.[7][8]

In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.[9]

The yard or the metre shall be the unit of measurement of length and the pound or the kilogram shall be the unit of measurement of mass by reference to which any measurement involving a measurement of length or mass shall be made in the United Kingdom; and- (a) the yard shall be 0.9144 metre exactly; (b) the pound shall be 0.45359237 kilogram exactly.

— Weights and Measures Act, 1963, Section 1(1)"

Pound (lb) is a mass in current usage, pound-force (lbf) is a force.

The pound-force is the weight of 1 pound mass in standard gravity. That means that in most static cases pound mass (lb) and pound-force (lbf) have the same number, which is what tends to lead to confusion.

Pounds-per-square inch (psi) is actually pound-force per square inch.

The metric equivalent is used by some countries - kg for mass and kgf for force. 1 kgf = 9.81N.

Mike read this...

 

[michael rauls]

Weighs more than what?

I think he meant more than before increasing the preasure but I'm not sure.

 

[Crimea_River]

M michael rauls That would require the pressure to be exerted in all directions (including up) evenly. That said, how come to put a given PSI of water inside a vessel, it weighs more? It seems there's a conceptual error here...

Pressure is exerted in all directions.

"Putting a given psi of water inside a vessel" does not make it weigh more. First of all, you can't "put a psi" in anything because psi is not a weight or volume. You can put a pound of water in a vessel or a gallon of water in a vessel but to say you put a psi of water into a container is meaningless. Now, take a simple bicycle hand pump and pull the handle all the way out. It sucks in air right? That air has a weight, though very small, and also a pressure which is equal to the atmospheric pressure all around you. Now, plug the valve end of the pump and push the plunger down. The weight of the air has not changed because no more air has been added or removed because it's in a closed system. However, the pressure has increased because you have the same amount (weight) of air now pushed into a smaller volume, so fewer square inches of surface. Therefore, the pounds per square inch has gone up, not because the pounds have gone up but because the square inches have gone down.

 

[pbehn]

How is it you can have a compressed air tank for paint-balling that has an absurdly high PSI (pounds per square inch) and doesn't weigh several tons? I figure with PSI being pounds per square inch...

What do you call absurdly high? I have seen hydraulic presses working at over 1000 Bar. Commercial water jet cutters use water pressures up to 100,000 psi (680MPa).

 

[MIflyer]

I did a study on composite pressure vessels for the Kennedy Space Center - such as are used for paintball guns. Their concern was that theoretically the pressure vessels will last for thousands of years, but in fact the things do fail, so how likely are they to do that and what happens if they do?

First thing I found out was that the major manufacturers were not talking. "You are asking us to air our dirty laundry." was the response of one. They refused to provide any info on known failures of their products because it would make them look bad. But I found out that composite pressure vessels are used to hold Compressed Natural Gas for vehicles powered by that fuel, and the Clean Vehicle Education Foundation kept records on use and of mishaps, I go a bunch of data from them. It is hard to cover something up when there is an explosion in a parking lot.

It turns out that CNG tanks do fail and can do so catastrophically. In the vast majority of cases the tanks have been damaged by something, and it usually is not obvious that it is getting ready to cut loose. And in the case of vehicle accidents the CNG vessels are actually safer than a tank of gasoline. In the event of fire they have devices designed to vent their contents, which always rises, usually upwards where it is not much of a hazard. There have been cases where a CNG tank vented due to, say, and electrical fire fire, while the vehicles were in the garage, and the vented gas was ignited by a sparking light and ... wooof! No damage to speak of. If it had been a tank of gasoline it would likely have burnt the whole building down. And the CNG tanks are made so strong that in the event of a crash they do not rupture except in cases where the impact was so severe no one did or could survive the accident anyway.

On the other hand, you DO NOT screw around with those CNG tanks! If one gets damaged, for example by having battery acid spill on it, you get rid of it. If it has an expiration date, you don't use it after that. And you do not buy a used one on ebay and install it as an extra tank on your vehicle.

The other thing is weight. A composite pressure tank capable of holding the equivalent of 10 gallons of gasoline weighs about 100 pounds, empty. Full, it weighs about150 pounds; that is a lot more than 10 gal of gasoline does (about 60 pounds).

 

[vikingBerserker]

Mike, Seriously?

 

[mikewint]

Mike read this...

Chris, I did and I am aware of the definition. The linking of a variable unit like force with a non-variable unit of mass, i.e., the Pound and the Kilogram was made necessary by the US refusal to adopt the SI system thus a multi-definition POUND needed to be defined in terms of a fixed definition KILOGRAM used by the rest of the world. However take note of the word in front of Pound. It is specified as a Avoirdupois Pound. Avoirdupois is an Anglo-Norman-French pastiche which translates as "Goods of Weight". As the name implies Avoirdupois Pounds were employed to weigh merchandise for sale.
To make this point very clear to students we had the old riddle question: "Which weighs more a Pound of Feathers or a Pound of Gold?" (Answer Feathers by a significant amount). Most today think that this is a trick density question and say that they are equal but in fact they are not. The question was used to drive home the difference between the Avoirdupois Pound (merchandise) and the Troy Pound (precious metals/diamonds/drugs).
So today in an effort to define a Cow in terms of Fish we enter into the new world order of Pound definitions:

The pound of force or pound-force (symbol: lbf) is a unit of force used in some systems of measurement including English Engineering units and the old "Foot–pound–second system" (we also had and used at one time: a CGS system or Centigram-Gram-Second system and a MKS system or Meter-Kilogram-Second system). So in this new "simplified" system we have the "Pound-force" which is easily confused with "foot-pound", a unit of energy, or "pound-foot", a unit of torque( already easily confused), that is now written as "lbf⋅ft"; AND all of these should not be confused with the pound-mass (symbol: lb), often simply called "pound" (to add to the confusion), which is now taken as a unit of mass simply because it was defined in terms of a real existing mass unit, the Kilogram..
When a Pound was simply a POUND and a unit of FORCE the SLUG was the MASS unit so no confusion no disambiguation required. This new definition of lbf can be rephrased in terms of the old unit slug. A slug has a mass of 32.174049 lb. The new pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s^2

All of the questions above have revolved around the term PSI or Pounds per Square inch. Note Zippers original confusion in which Pounds as force as in PSI are confused with Avoirdupois Pounds as weight.
It is a simple concept: MASS is QUANTITY of matter and all MASSES are attracted to and by all other MASSES present in the universe. The attraction applies a FORCE to the MASSES which produces an ACCELERATED MOTION bringing the masses together. The MASS and the FORCE are directly proportionate to each other thus all MASSES accelerate towards each other at the same rate.
The Earth's gravitational field causes all MASSES (at or near the surface and neglect air resistance) to accelerate at 32.17 ft/s^2 or 9.81 m/S^2).
Thus an objects WEIGHT is a measure of the objects Gravitational field. Since different objects have different gravitational field strengths an OBJECTS WEIGHT is a variable quantity.
A column of air 1 inch by 1 inch by 100 miles or so is attracted by the Earth's gravitational field by a force giving the column a weight at or near the surface of 14.696 Avoirdupois Pounds. Since this Force is spread over a one square inch area we have PRESSURE defined as FORCE per unit of AREA thus 14.696 Avoirdupois Pounds per square inch of Area. Simply put PSI or I suspect to make things "simpler" 14.696 lbfsi.

 

[DerAdlerIstGelandet]

Mike please source your copy and paste posts...

 

[mikewint]

Mike please source your copy and paste posts...

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

 

[Crimea_River]

Ask a simple question and you get a rocket science answer.

 

[mikewint]

Ask a simple question and you get a rocket science answer.

Not all things in heaven and Earth Horatio are explainable in 5 one-syllable words

 

[pbehn]

Ask a simple question and you get a rocket science answer.

I don't remember my teachers explanation being so complicated when I was 10 years old.

 

[Crimea_River]

But if someone is trying to get a basic understanding, then a basic (5 letters) explanation would suffice. Hopefully my analogy in post 27 makes sense, Zipper. And yes, I know, "pounds" and "pounds force" are different but I'm trying to convey with my simple example that the weight, or shall we say MASS, of the air does not by itself increase with pressure.

 

[mikewint]

Andy agreement in principle but simple answers leave lots of room for faulty perception and ideas to take root. Zipper's initial post is a perfect example. Pound in PSI is Force while a Pound of butter is weight. I've heard many people say "I put 30 pounds of air in my tires" and my grill utilizes a 20 pound propane cylinder. The words are the same but mean vastly different things and we trust that the person we are speaking to are applying the same definition as we are. "I live 10 miles from you" OK so what mile are you using? Statute - Survey - Nautical - Metric - Geographic?
Slippery things are words tis how wars start

And Pbend I assumed I was writing to non-10yo

 

[fubar57]

Wow....unbelievable. When prey tell do you use "survey" miles Mike?

 

[pbehn]

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.