Physics for Dummies: At the Penalty of Sounding Real Stupid

[michael rauls]

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

Wow....... could you expand on that a bit?

 

[wuzak]

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.

Definition of Mass

The concept of mass is a little difficult to pin down, but basically you can think of the mass of an
object as the amount of matter contain within it. In the S.I., mass is measured in kilograms. The
kilogram is a fundamental unit of measure that does not come from any other unit of measure.

Definition of the Pound-mass
The pound mass (abbreviated as lbm or just lb) is also a fundamental unit within the Imperial
system. It is equal to exactly 0.45359237 kilograms by definition.
1 lbm ≡ 0.45359237 kg

https://durhamcollege.ca/wp-content/uploads/Pounds-Mass-and-Pounds-Force.pdf

For the last 60 or so years the pound has been defined in terms of kilograms. That is, the pound is a unit of mass.

One slug has a mass of 32.1740 lb (14.59390 kg) based on standard gravity, the international foot, and the avoirdupois pound.
Slug (unit) - Wikipedia

 

[Zipper730]

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.

You'd figure that would have some effect similar to density: I know there is an issue when free-divers go below a certain depth -- they start sinking rather than floating...

 

[pbehn]

You'd figure that would have some effect similar to density: I know there is an issue when free-divers go below a certain depth -- they start sinking rather than floating...

Is that your only input to the thread you started? I don't actually know what question you are asking.

 

[Zipper730]

Is that your only input to the thread you started? I don't actually know what question you are asking.

I was responding to messages I got alerts on in the order I got them...

 

[pbehn]

I was responding to messages I got alerts on in the order I got them...

Well respond to mine then?

 

[Crimea_River]

I'm so done with this thread.

 

[Zipper730]

Okay, so pounds of force is not the same as a pound, but instead the amount of force a pound being accelerated to 1G (32.17405 f/s)? And pressure is the amount of force applied to a given unit of area, correct?

 

[Elmas]

Question: I if I kick a (big) stone on the Earth probably I will hurt my foot. What about if I kick the same stone on the Moon?

 

[Elmas]

Please excuse me for the poor drawings.

 

[mikewint]

I know there is an issue when free-divers go below a certain depth -

A diver, free or otherwise is under 14.7 psi of pressure at the surface. Diving down to the 33 foot level increases the pressure by another 14.7 PSI. According to the gas laws a doubling of pressure causes the volume to decrease by half. For an adult total lung capacity is 4 - 6 L so let's assume 5 L at the surface, at 33 feet that drops to 2.5 L. At 99 feet add another atmosphere of pressure so volume is 1/3 of the surface volume. At 231 feet another atmosphere so capacity is 1/4 or 1.25 L. Fat is less dense than water so that also determines flotation but somewhere around the 50 -60 foot level flotation gets negative.
The same sequence of volume reduction occurs with SCUBA tanks. In my day a steel tank held 72 cu ft of air at 2250 psi. Today's standard aluminium tank holds 80 cu ft at 3000 psi. A relaxed breath is 1 cu ft per minute but exertion and excitement can double/triple that rate. So at 99 feet you have 20 minutes at best supply of air. Most compressed air divers max out at 130 - 140 feet due to the narcotic effect of the nitrogen gas. Of course as you ascend pressure is decreasing so you get your gas volume back so your total underwater maximum time is 80 minutes

 

[Zipper730]

Might as well ask another "physics for dummies" question, while I'm at it: There's this thing called the inverse square law, for every time the distance doubles, the energy is spread out more, so for twice the distance, you need four times the surface area to absorb/reflect.

That said, even the nearest stars are several light-years from Earth, and some stars we can readily see that are hundreds or even thousands of light years from Earth. Two stars that I can readily think of are Alpha and Beta Orionis, also known as Betelgeuse and Rigel, which are 700 and 870 light-years from earth, respectively. Some even further, and we can still see them, admittedly as little dots.

I'm surprised we could see anything from that kind of distance with the inverse square law in effect (and I'm not even factoring in atmospheric effects): Yet, we obviously *CAN* see the stars...


BTW: I know it's strange that some things I immediately grasp, even complicated things, and some things that are seemingly simple, fly right through me.

 

[Aquadraco]

QUOTE="I'm surprised we could see anything from that kind of distance"
For a start, it shows just how big stars really are, and how much energy they put out. Even attenuated by distance, the light they emit will essentially travel forever unless the energy is deflected or absorbed by something.
So that narrows the focus of the question - the energy will be visible as long as it's not interrupted, but why can we see it?
Presumably, even diurnal hominids need to see in very low light, and possibly use stars for night navigation, so the ability to see such small pinpricks of light evolved in our ancestors and we have inherited that ability.

 

[XBe02Drvr]

No Mike. You are overcomplicating things, as usual

MikeW, your thorough and elaborate posts are infinitely entertaining, but I learned early in my CFI career that complexity in pursuit of perfect accuracy was not conducive to understanding. The virtues of KISS took awhile to penetrate my thick skull.
Cheers,
Wes

 

[vikingBerserker]

He needs to find less complicated articles to cut and paste as his own.

 

[pbehn]

Might as well ask another "physics for dummies" question, while I'm at it: There's this thing called the inverse square law, for every time the distance doubles, the energy is spread out more, so for twice the distance, you need four times the surface area to absorb/reflect.

That said, even the nearest stars are several light-years from Earth, and some stars we can readily see that are hundreds or even thousands of light years from Earth. Two stars that I can readily think of are Alpha and Beta Orionis, also known as Betelgeuse and Rigel, which are 700 and 870 light-years from earth, respectively. Some even further, and we can still see them, admittedly as little dots.

I'm surprised we could see anything from that kind of distance with the inverse square law in effect (and I'm not even factoring in atmospheric effects): Yet, we obviously *CAN* see the stars...


BTW: I know it's strange that some things I immediately grasp, even complicated things, and some things that are seemingly simple, fly right through me.

It is just that the numbers are huge, stars, and star systems are huge, they emit a huge amount of light so even after being attenuated by a huge distance there is still enough for our eyes to sense it.

 

[Zipper730]

How much energy would you say would be emitted off the surface of Rigel. Could that be computed if I could determine a rough diameter?

 

[gumbyk]

How much energy would you say would be emitted off the surface of Rigel. Could that be computed if I could determine a rough diameter?

Not with the information we've got here, because a lot of the energy is emitted as ultra-violet.

 

[Zipper730]

Not with the information we've got here, because a lot of the energy is emitted as ultra-violet.

Luminosity figures are based on visual light I thought?

 

[gumbyk]

Luminosity figures are based on visual light I thought?

Yes, but there is a lot of the energy emitted as invisible wavelengths. Without a full spectral analysis you're only looking at a portion of the energy emitted.