Physics Question

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Nice try but no cigar.

An airplane gets lift from positive pressure on the underside of the wing. The lower pressure on top of the wing simply allows the higher pressure on the bottom to exert a net lifting force.

Same with a carburetor. The fuel is pushed out of the float bowl by positive atmospheric pressure from the vents. The venturi simply lowers the pressure of the incoming air so the the pressure differential is enough to force fuel through the jets.

This is a subtle but important distinction. Air pressure never pulls on anything. Pressure is by definition a pushing force. When the pressure on all sides of something are equal, it cancels out. When the pressure on one side is reduced, the pressure on the opposite side is what pushes the object towards the lower pressure side.
 
Negative air pressure is for real ...
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You are confusing absolute pressure with relative pressure.

'Gauge' pressure is measured relative to whatever ambient is present.
A manometer shows gauge pressure.
It has two ports.


Absolute pressure is relative to a 'perfect' vacuum.
A barometer is a decent example, the main source of error being the vapor pressure of the mercury above the column of liquid mercury.
Water or oil barometers are typically less accurate since the vapor pressure above the liquid of the column is more than mercury.
Notice there is only one port on a barometer.

Barometers using a vessel evacuated are not as accurate.
They are comparing the pressure inside the evacuated bellows with the air pressure outside.

There are semi-conductor pressure transducers that approach absolute since the vacuum trapped under the silicon strain gauge is very good, easily approaching that of free space.
 
Yes but, The American society of automotive enginiers,"A.S.E." refers to the carburator principal as negitive air pressure in the venturies allowing the atmospheric pressure in the float bowl to push the fuel out.

At one time I was certified by the A.S.E. in tune-up, and argued that point with one of the instructors, ( I lost) But, in the process I had the very detailed explination drilled home, I fully understand that it is simply a term used to discribe a condition, You should have noticed in the post where it was first mentioned I stated "Lack of, or poss. negitive air pressure", Essentially, I think it may be more appropriate than calling space a vacuum, which it is not.
 
One question left unresolved...

is whether the gun would function or cycle completely in space. I know it's not part of the original post, but several posts have mentioned that the gun wouldn't completely cycle in outer space. I think we need to nail down this one loose end, so we can all get a good nights sleep.

I held off, hoping that 1911 Tuner or Old Fluff would step in with their much-respected expertise in this matter and enlighten us mortals and physicist/rocket scientists, alike. If my memory serves me correctly, one of my above mentioned experts, stated, “if a gun runs, then there is no such thing as limp-wristing”. Limp-wristing is what you tell your wife is the reason her gun has jammed, (it’s not the crummy reloads or the cheap Wally World ammo you provided her, because she can’t hit anything, anyway); or, the new shooter, even though he usually does a hundred chin-ups a day. I’ve never been a believer in limp wristing as a cause for a malfunction in an auto loading pistol, just as a matter of experience.

So, begging the issue to be resolved, would that Glock/1911 cycle normally in space? I think it would. What say you?

Kerf
 
Claymore--

I totally understand how you are using the term "negative" air pressure. In a way you are right, and so are those saying there is no such thing. IF you define ambient air pressure as zero, then you could describe a lesser pressure with negative values. You could say, "the air pressure in this room is zero, and the air pressure in this vessel is -5 psi." When the manual you mentioned spoke of negative air pressure, it was using it to mean negative *relative* to ambient air pressure.

But, it's more accurate technically to speak of space as being a vacuum or partial vacuum or having "little or no" air pressure. A total vacuum exerts 0 psi air pressure. Can't go lower than that.

Hope this helps.

K
 
Just a little bit about the bullet burning up upon reentry. Keep in mind that the intense heat generated by an orbiting object reentering the atmosphere is due mostly to its *forward* velocity relative to the Earth's surface, not it's downward velocity. If one could merely place a bullet in space at an orbital altitude, with no forward velocity, and drop it, it's not clear to me what would happen to it as it fell back to Earth. Certainly, it's going to accelerate rapidly due to the pull of gravity, and that can be calculated, but it's also going to start to encounter ever increasing air resistance as it nears the surface. Whether it will ever reach a velocity high enough to generate enough friction to melt or vaporize is above my pay grade. Anyone want to take a stab at that?

K
 
If you are in orbit you have a high velocity.
There is no such thing as "an orbital altitude, with no forward velocity."

Even if you just fired it from the surface of the earth it would have a velocity component from the surface.
At the equator you are moving at over 1000 mph since the earth spins every 24 hours.
 
Just wait for my next thread, "If you fired a 1911 gangsta style on Mars." Thanks for the well reasoned and incredibly astute responses. I knew gun people were intelligent, but some of you guys are amazing.:)
 
If you are in orbit you have a high velocity.
There is no such thing as "an orbital altitude, with no forward velocity."
Click to expand...

I didn't say, "in orbit," I said, "at an orbital altitude." By that I meant a distance above the Earth at which objects are put in orbit, which just for this hypothetical here let's say is 200 miles, or so. So, if the term orbital altitude bothers you, reword my post to say, "...place a bullet in space at an altitude of 200 miles, with no forward velocity..."

K
 
I think it would still burn up even if fired straight down to the center of the gravity well. A falling bullet fired straight up from the earth will come back down at less than 300fps and will be tumbling, laying on its side or come down base first. We are launching a spin stabilized bullet nose first at 850 fps assuming a standard hard ball round and it is spin stabilized. Add in the acceleration from gravity and the resistance encountered as it enters the atmosphere. Its horizontal velocity will have to slow down in reference to the surface of the planet it had been aimed at and will now be coming down at an ever increasing angle, assuming we are over the equator the earth's surface is moving at 1467 fps and the atmosphere discounting winds is moving at the same rate the bullet would have the effect of its downward and horizontal speed adding to the amount of resistance its going to have from the increasingly denser mass of molecules in the atmosphere. I'm no physicist but I would think that the initial velocity + acceleration plus the ever increasing affect of the speeding atmosphere on the bullet would have to increase the temperature of the bullet from friction. I don't know how hot a 45 bullet is but a 30 caliber bullet fired at a sedate 1900 fps will have a base temp of around 500 C, a tip temp of 170 C and a groove temp of 320 C. I expect the temp of the 45 would be lower with the low pressure and short barrel but still considerable and would only get warmer with friction. Even if the jacket of the bullet didn't melt I would think the core would pretty much have to vaporize and the low pressure behind the bullet would suck it right out of the core. I admit I am only speculating but it seems logical to me. However I am the first to admit I will never be a rocket scientist, I would be the guy in Crews quarters who would have to fix the control panels after the Klingons got done making them spark and smoke again for the third time this week. I wouldn't be as fast as Spock or Scotty either because I don't have pointy ears or a Scottish accent.
 
Good to see people interested in physics.

One of my many life experiences was being a professor of physics at a major university.

The principles are basic. Students learn them around the time they are told total momentum, the vector sum of translational and angular momentum is conserved.

An object's moment of inertia is a tensor and if represented as a matrix, a handgun will have significant non-zero off diagonal elements.

On my Sig 229 with a full magazine, the center of mass is in the frame, in front of the decocker lever.

If I erect a coordinate system fixed to the gun at the center of mass, it is clear that there is no transformation that would diagonalize the moment of inertia matrix (where the eigenvector lies in real space). Nothing special in principle or mathematically, but that's why people are struggling with what the motion would look like.

I've learned not to discuss physics on gun forums, but as I said, good to see the interest.
 
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Be careful, physics can be fascinating. You'll be wanting to design rockets before you know it. :)

The mechanical theories associated with firearms were resolved hundreds of years ago, so it's difficult to get someone interested in creating simulations in response to these kinds of questions.

A junior mechanical engineer could get an animation going pretty fast. I'm sure it would be a hit on gun forums.
 
"It would be awfully silly to say that the air I'm breathing here at 5200ft altitude is -2.47 psi, wouldn't it?"

It would be silly for YOU to say that, but not for ME. Your gauge should be set to read zero at your atmospheric pressure. My gauge is set to zero for my atmospheric pressure.
 
So, if the term orbital altitude bothers you, reword my post to say, "...place a bullet in space at an altitude of 200 miles, with no forward velocity..."
Click to expand...


The prpblem is that as you move above the earth you become a free body, and all of a sudden you have a whole lot of velocity relative to the earth just by climbing to space.
The surface of the earth has a speed of over 1000 miles per hour at the equator (circumference divided by the 24 hours it takes to rotate once).

While we do not see this speed on the surface, or even in an air plane, but satellites see it (long range artillery sees it also).

If you fire a shell straight up relative to the local surface it will not return to the same spot since the earth rotates under the projectile.

Orbital mechanics is a strange world.
You move faster and slower to change altitude, and cannot change speed without also changing altitude, they are fundamentally linked by orbital mechanics.
Kepler's laws prevent you from trying to have zero velocity under orbital conditions.
 
Just a little bit about the bullet burning up upon reentry. Keep in mind that the intense heat generated by an orbiting object reentering the atmosphere is due mostly to its *forward* velocity relative to the Earth's surface, not it's downward velocity. If one could merely place a bullet in space at an orbital altitude, with no forward velocity, and drop it, it's not clear to me what would happen to it as it fell back to Earth. Certainly, it's going to accelerate rapidly due to the pull of gravity, and that can be calculated, but it's also going to start to encounter ever increasing air resistance as it nears the surface. Whether it will ever reach a velocity high enough to generate enough friction to melt or vaporize is above my pay grade. Anyone want to take a stab at that?
Click to expand...
Not sure, but Burt Rutan's Space Ship One fell back to earth from an altitude of 100 km/62 miles, and its speed was around 2170 mph, or 3182 ft/sec, which is about the same velocity as a 55-grain .223 at the muzzle. The heat pulse may be longer, so it is possible that you might have some core melting, but it won't burn up.

Dropped from 150 or 200 miles or so, your bullet would likely exceed the melting point of copper, and if you do then the bullet would indeed "burn up" (melt/ablate and resolidify as small droplets or dust).
 
If you could somehow place a bullet 200miles up in space....with no forward velocity (which is needed to orbit), it would fall directly back to earth.

And, seeing as the gravitational force working at 200miles is still about 90% of surface gravity, it would pick up speed very close to the theoretical 32 ft/sec squared.

Without significant atmospheric resistance for the first 130 miles or so...it would be moving pretty darn fast when it came in. I haven't the ability to calculate it, but I'm betting that it would melt the slug for sure.
 
ask NASA to try it on the Vomit Comet. weightlessness is maintained for almost 2 minutes. though the air in the cabin is 8psi, it is sure to drop after the first bullet passes through the fuselage. the govt loves to share; after all it is our $ making all this possible. i have even had real time on the Hubble. present them with an idea and a outline for how to do it. don't know if you don't try.

the difference between achieving orbital velocity and just say, taking an elevator there is great. and well explained by Arther C. Clark in his novel The Fountains Of Paradise
 
Has anyone mentioned TEMPERATURE yet?

From what I've heard all external parts exposed to the sun might be fine, but the internal parts would become VERY cold being shadowed.
 
The prpblem is that as you move above the earth you become a free body, and all of a sudden you have a whole lot of velocity relative to the earth just by climbing to space.
The surface of the earth has a speed of over 1000 miles per hour at the equator (circumference divided by the 24 hours it takes to rotate once).
Click to expand...

Guys, you're making my hypothetical more complex than it has to be. The question I posed was whether or not a bullet (not in orbit) falling from an altitude of 200 miles would gather enough velocity to burn up at some point during it's reentry into the atmosphere (yes, I know there's a very thin atmosphere, even at 200 miles). I noted that a satellite reentering from orbit has more "forward" velocity than vertical velocity, which is the source of most of the heat-producing friction. I also noted that the answer is complicated by the fact that the bullet will be encountering ever increasing denser air as it falls, which will reduce it's acceleration from the theoretical 32 ft/sec^2.

If it helps to clarify, assume the bullet is dropped from a 200 mile high tower built directly in line with the Earth axis at the North Pole, hence, no motion relative to the surface except downward. Also, assume a nose first stabilized bullet, just to avoid the complications of a tumbling bullet.

I don't really expect an answer, it was just one of those interesting hypotheticals that my warped mind dwells on.

K
 
would the bullet burn up--YES if released from 200 miles up
or it could be argueed--NO. if released very much lower.
 
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