Wikipedia:Reference desk/Archives/Science/2012 March 13

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March 13

penicillin

is it true that penicillin by inter-muscular injection is less likely to cause an allergic reaction than oral Penicillin? — Preceding unsigned comment added by 64.38.197.218 (talk) 03:54, 13 March 2012 (UTC)

There is nothing in the current Penicillin article to support or refute the claim. I unfortunately don't have the skill to do a search of the relevant literature. Roger (talk) 14:45, 13 March 2012 (UTC)

"Topical application is the most likely to elicit sensitization followed by parenteral administration, especially intramuscular injections, whereas the oral route is the safest probably because larger doses are delivered parenterally or intravenously over a shorter period of time. In general, drugs used continuously for extended periods of time have less chance to trigger an adverse reaction but multiple intermittent therapy courses increase the risk..."^[1] Wnt (talk) 23:30, 14 March 2012 (UTC)

Aerodynamics

If an aircraft wing works by reducing pressure on the top surface Cf the bottom surface, then how come planes can fly upside down and stil have lift from the wings?--92.25.105.29 (talk) 15:36, 13 March 2012 (UTC)

Most planes can't. Those are special stunt planes with different wing designs. Not quite sure how they work, but I suspect they allow the plane to change the angle of attack on the wings, so they strike the air at a different angle when upside-down. There is the less efficient method of deriving lift by merely ramming the air at an angle, and perhaps this is what they use. StuRat (talk) 15:51, 13 March 2012 (UTC)

No. That is at best horribly misleading. If you look at the plot of lift vs angle of attack for any normal wing section it is reasonably linear around 0, therefore the airfoil will generate lift when inverted. Most a/c are not stressed (at the design stage) to fly inverted, so the manual says don't do it. But -1 g is fairly small departure from the design loads and if you gently flew into it (preferably via half a barrel roll, as that is the gentlest way in) I would expect almost any aircraft to be able to fly inverted, briefly. The reason I say briefly is that the airfoil will be generating a lot of drag, which leads into the more detauiled response below. Greglocock (talk) 01:03, 15 March 2012 (UTC)

What part is misleading ? Your "more drag" is basically the same as my "less efficient". StuRat (talk) 01:35, 15 March 2012 (UTC)

Here is the Straight Dope explanation. Basically, the pilot increases the angle of attack by keeping the nose up and the tail down (relative to the ground), and it needs a light but strong plane with powerful engines. Gandalf61 (talk) 15:57, 13 March 2012 (UTC)

Indeed. see Angle of attack. --Tagishsimon (talk) 15:58, 13 March 2012 (UTC)

(edit conflict) The lift created by airplane wings is not entirely due to one side of the wing being shaped different than the other. Depending on who you ask, this may be a very minor component of the lift generated - some planes do have symmetrical wings, and manage to fly just fine. The oft-cited "Bernoulli’s principle" explanation for lift is simplistic, and may be misleading for many people. Other people prefer the "Newton explanation", where lift is explained in terms of deflecting air, rather than in terms of a pressure difference. Neither explanation is "wrong", but they can both be misleading, if applied incorrectly. At the end of the day, if you really want to understand lift, you need to understand the Navier–Stokes equations. They say (only joking a little) that even most people with PhDs in fluid dynamics don't really understand the Navier-Stokes equations. Anyways, take a look at Lift (force) for some further treatment of the subject. For a more humorous take, see [2] Buddy431 (talk) 16:03, 13 March 2012 (UTC)

This is a complex question. I'd suggest that a good place to start would be our Airfoil article. Both the shape of the wing and the angle of relative airflow are factors in determining in which direction the 'lift' force is applied. Though some stunt aircraft may have symmetrical airfoils, not all do, and the capacity to maintain inverted flight may be more a question of having an engine that will work properly under negative G. AndyTheGrump (talk) 16:08, 13 March 2012 (UTC)

Also, I'd recommend playing with NASA's Foilsim simulation to get a feel for what is actually going on [3]. AndyTheGrump (talk) 16:13, 13 March 2012 (UTC)

The fundamental answer is that wings don't work by reducing pressure due to the profile shape. In fact, wings on stunt planes meant for frequent upside down flying are often completely symmetrical. Flight is really achieved by angle of attack. You can make a plane fly with a flat piece of material for a wing. Air molecules hit the underside of the wing, are deflected downwards, and due to force and counterforce, the wing pushes up. For this and many more things you think you know check out List of common misconceptions. 88.112.59.31 (talk) 16:37, 13 March 2012 (UTC)

That may be a 'fundamental answer' - but only in as much as it is fundamentally wrong. I suggest that you also should read the articles linked above. AndyTheGrump (talk) 16:49, 13 March 2012 (UTC)

A side issue to the aerodynamics, but flyers have told me that the plane needs some special feature in the carb to allow upside-down flight. Fuel injection might help with that, but there could still be issues with the fuel inlet in the fuel tank not finding the fuel when the plane is upside down, whatever its angle of attack. Edison (talk) 18:33, 13 March 2012 (UTC)

I spent many years academically studying fluid motion, but it was not until I first controlled a sideslip in a Citabria that I felt like I "understood" aerodynamic lift. In reality, if you want an intuition of how an aircraft flies, you may gain more by spending time on a sailboat or flying a kite, and observing how flowing air interacts with rigid and semi-rigid surfaces. The mathematical formulations of aerodynamics are great if you are performance-tuning an airfoil, but they don't do a great job elucidating the conceptual underpinnings of modern aviation aerodynamics. It may help the OP to state, for the record, that even a Citabria does not fly very well when the wings are upside down (though in this configuration, its name reads as a slight variation on "acrobatic"). Technically stated, its drag coefficient and lift-to-drag ratio are much worse in the inverted configuration. Anyway, the aircraft I am training on is rated for only a few moments of inverted flight; (it is carburated, but even worse, its fuel system is gravity-fed; so in inverted configuration, fuel cannot flow "uphill" to the engine). The more advanced Super Dec has a fuel pump and injectors, and can sustain inverted flight for several minutes (though, few pilots can sustain this). Our club requires pilots to wear parachutes when flying aerobatic maneuvers; and while this seems "cool!" at first, it actually becomes quite worrying when you consider the implications. Nimur (talk) 19:01, 13 March 2012 (UTC)

And, for the record, neither the 7ECA nor the Super Dec have symmetrical airfoils. They are usually outfitted with the NACA 4412 or 1412 airfoil design. You can even graphically experiment with the airflow on these airfoils 4412 using Wolfram Alpha. In fact, a perfectly symmetric wing has poor stalling characteristics. Nimur (talk) 19:13, 13 March 2012 (UTC)

I wonder if anyone has considered a system where the cockpit rotates so the pilot is always "up" (according to the current G-forces, not the horizon). Just a bottom-weighted cylindrical cockpit floating in a fluid in a slightly larger cylinder would do it, using all fly-by-wire technology (or fly-by-wireless, if there is such a term). This would fix the problem of blood rushing to the pilot's head, and the fluid might also tend to damp out noise and vibration. StuRat (talk) 19:08, 13 March 2012 (UTC)

Well, it appears engineers have implemented that idea into cereal bowls, so there's that :) 20.137.18.53 (talk) 20:25, 13 March 2012 (UTC)

An essential read is this paper about theoretical work done by NASA scientist Mary Shafer and others. Roger (talk) 06:49, 14 March 2012 (UTC)

We talk about the top surface and the bottom surface of a wing but these are just simple, convenient expressions that serve the purpose most of the time. The top and bottom surfaces are most definitely not selected relative to the fuselage, or relative to the Earth's surface. When an aircraft is flying upside down it is necessary for the lift to continue to support the weight of the aircraft so the surface that is normally the top surface becomes the bottom surface, and vice versa. By using the pitch control (or joystick or control column) the pilot can determine the angle of attack on the wing and force the aircraft to fly right-way-up or inverted. Dolphin (t) 21:44, 15 March 2012 (UTC)

Deflecting a bullet with a laser

It seems the energy of a bullet fired from an AK47 is 1.4 kilojoules. If you were to fire a 1.4 kilojoule laser at that bullet, would that be enough to deflect it? ScienceApe (talk) 18:54, 13 March 2012 (UTC)

I suppose a powerful enough laser could vaporize or break up the bullet. But, if you can manage to hit a bullet in flight with a laser, you can probably just hit it with another bullet, too. StuRat (talk) 19:01, 13 March 2012 (UTC)

It strikes me (punny) that there would be advantages to using lasers to hit bullets as opposed to other bullets. The laser's speed would solve a lot of practical problems involved in intercepting a bullet, no? --Mr.98 (talk) 19:29, 13 March 2012 (UTC)

Aren't lasers specified by wattage (power) not joules (energy)? Just a nitpick. If you want to suppose that the laser beam imparts K joules to the bullet in 1/nth of a second, then your laser would have to be a Kn watt laser. As always, I expect to be corrected by someone smarter. 20.137.18.53 (talk) 19:17, 13 March 2012 (UTC)

Well power is just energy per second. If it fires a single pulse you just measure it in joules. A 1.4 kilowatt laser outputs 1.4 kilojoules per second. ScienceApe (talk) 19:20, 13 March 2012 (UTC)

Oh, if it were only that simple! Unfortunately, that is not how laser people usually report their laser power! You're talking about average power (either DC average, or RMS average); but lasers report peak power, because it makes them seem more powerful! I'll find a reliable source to cite when I get back from lunch; but you can check vendor websites for details. Nimur (talk) 19:56, 13 March 2012 (UTC)

Some more details at pulsed laser in our article. For the advanced, reader, Measuring Beam Quality, from an emeritus Stanford professor of lasers. Awesome website overall, including an archive of "History of Lasers" presentations, loads of animations, online PDF laser textbooks... Nimur (talk) 00:05, 14 March 2012 (UTC)

Is your hypothetical laser tracking the bullet for 1 second? The heat capacity of the bullet's material is probably relevant. 20.137.18.53 (talk) 19:49, 13 March 2012 (UTC)

We're really talking two different kinds of energy: radiant energy versus inertia. The laser has the same energy, but in the form of light and heat; it has very little force per F=ma. What little deflection you could get would mostly be through ionization, or the heat deforming the bullet and altering its aerodynamics. Laser propulsion hasn't really caught on for just that reason: you'll eventually make an object move in a vaccuum, but it takes forever. Doing so in an atmosphere is a nightmare. — The Hand That Feeds You:^Bite 19:59, 13 March 2012 (UTC)

Inertia is a property of matter, not a form of energy. I think what you mean to say is that laboratory-grade laser light cannot impart a large amount of force or momentum to a macroscopic object. In theory, laser light is capable of imparting both force and momentum to an object, thereby changing the target's kinetic energy and trajectory; but this would require a laser beam of much greater intensity than we are able to build using current technology.

A laser's energy is not usually described as being "in the form of light and heat;" it is a highly collimated, highly monochromatic, propagating electromagnetic field disturbance. This energy can be transferred to matter through many physical processes: the photoelectric effect; dielectric heating; joule heating; and so on. The laser light also is able to exert radiation pressure, and therefore exert force on the target object. If you work out the math for any of these processes, you'll see that the effects of each of these energy and momentum transfers are very small when you plug in reasonable values for laser light intensity - even for extraordinarily powerful lasers. Nimur (talk) 23:34, 13 March 2012 (UTC)

The laser pulse may have a comparable amount of energy to the bullet, but momentum has to be conserved in a collision, too, and light has very little momentum. So providing a deflection directly via an ordinary reflection wouldn't work too well. What might have a chance of working under the right circumstances would be using laser ablation to evaporate a bit of the bullet. The momentum carried away by the ablated material would then give the bullet some momentum in the opposite direction. That's the basic idea used in some directed-energy weapons, and in a laser broom. Red Act (talk) 20:20, 13 March 2012 (UTC)

A laser certainly can apply a force to the bullet (see light pressure), but your problem is hitting it with enough light fast enough. As others have pointed out, lasers are measured in terms of power, not energy. As you said, a 1.4kW laser will emit 1.4kJ in a second, but a second is far too long - the bullet will have already hit you by then. The muzzle velocity of an AK47 is (according to the infobox in that article), 715m/s. So, it will have already travelled 715m by the time your laser has emitted 1.4kJ. I'm not sure of the best way to calculate it, but the power of laser you would need to actually deflect a bullet a useful amount would be enormous. You would also have a real challenge aiming it and tracking the bullet as it moves. You would be better off using the laser to blind the gunman - a 1.4kW laser to the eye would blind someone very quickly. --Tango (talk) 00:23, 14 March 2012 (UTC)

If you change "bullets" to "artillery shells", and "deflect" to "heat until it explodes", then that's very doable. See the Tactical High Energy Laser. But in addition to all the technical difficulties pointed out above, I think Tango hit the nail on the head. If you have a system that can track and zap a moving bullet over short distances with an uberlaser, you are better off just building a killbot that aims for the gunman. Someguy1221 (talk) 00:30, 14 March 2012 (UTC)

I'm guessing that the strongest pulsed laser would have no difficulty dealing with a simple bullet:

Highest-energy laser pulse: 150 thousand joules, in a single 10-nsec pulse. The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in Livermore, California, achieved this result in 2005. The energy contained in the 150 kJ pulse is equivalent to a 1-ton automobile traveling at about 60 miles per hour. . The bullet only travels 7 microns during that pulse.. 84.197.178.75 (talk) 12:37, 14 March 2012 (UTC)

How massive a bullet or projectile could be melted or vaporized by such a pulse? A difficulty would be how much of the energy would be absorbed by the projectile. Reagan's "star wars" notion would have shot down ballistic missiles with x-ray lasers. It was claimed that a device the size of an office desk could "shoot down the entire Soviet land-based missile force." Later the US shifted to a different quixotic plan to hit warheads with solid objects. The kinetic interceptors occasionally worked in carefully staged tests when no countermeasures were employed (such as decoys). The Tactical High Energy Laser , Boeing YAL-1 and Advanced Tactical Laser articles seem relevant to the question. THEL supposedly can destroy mortar rounds and artillery shells, but the project is on hold for some reason, such as being expensive and very big and heavy, requiring several trailers full of equipment. The airborne laser weapons also supposedly suffered from a poor weight to output problem. The other programs also seem to be on hold (though if I were really developing such a weapon I would not necessarily advertise the fact). Weight and the need for large power input would not seem to be much of a problem for ships, compared to traditional large gun turrets and magazines. Edison (talk) 16:16, 14 March 2012 (UTC)

The whole point of the doomsday machine...is lost if you keep it a secret! Airborne Laser is still in development (though the 747 is grounded); you can read about it and other projects of the Air Force Research Laboratory's Directed Energy Directorate at their main website, http://www.kirtland.af.mil/afrl_de - laser enthusiasts on Wikipedia may recognize their laboratory's homepage splash picture, which is also featured on our main article, Laser. Nimur (talk) 18:20, 14 March 2012 (UTC)

Let's assume the bullet is made of iron. Iron has a heat capacity of 0.450 J/g/K (at 25C, anyway - let's assume it's constant in order to make the maths easy, it probably won't vary much). The boiling point of iron is 2862C and let's say it is at 25C to start with, so we need to heat it up by 2837 degrees. That would take 1277 J/g. In addition, you need to allow for the heat of fusion (248 J/g) and the heat of vaporisation (6069 J/g). In total, then it takes about 7,600 J to vaporise 1g of iron. 150kJ would be enough to vaporise about 20g. That's assuming all the energy is absorbed, which it wouldn't be. So, somewhere between 0g (if the laser is completely reflected) and 20g... a small bullet might get vaporised, then. An artillery shell wouldn't be. --Tango (talk) 02:40, 15 March 2012 (UTC)

But, unlike a bullet, you don't need to vaporize an artillery shell - you just need to heat and prematurely detonate it. Similarly, a ballistic missile need not be destroyed; it just needs a laser strike powerful enough to zap a tiny scoring in its heat shield, or an thermoelectrostatic discharge to its controller electronics. Nimur (talk) 18:23, 15 March 2012 (UTC)

Which country has the most rivers? The most river span?

The United States has more than 250,000 rivers, totaling about 3.5 million miles of river span....

I've been trying to find the same info for other countries, but I'm having trouble. I mostly need to know if the U.S. has more rivers and river mileage than any other nation, and, if so, what are the stats for the second place country.

If someone could email me at jerjacques at gmail dot com with any leads, I would be very grateful.

Thank you, JSJ — Preceding unsigned comment added by Jerjacques111 (talk • contribs) 20:07, 13 March 2012 (UTC)

What is the formal definition of a river? Without knowing that, there is no clear answer. By any reasonable definition, though, I would expect Russia, Canada, and Brazil to have more than the US. Looie496 (talk) 20:32, 13 March 2012 (UTC)

Hydrogeology and hydrology are the formal study of surface water. There are specialists in these fields who quantify parameters about the surface water in various regions. Typically, it's more useful to discuss the average precipitation over a large geographic area; rather than the specific number of rivers. The total amount of water that flows out of a region - called the total river discharge, is easy to measure for a drainage basin. However, trying to count the number of rivers is actually quite difficult - because where the water flows is going to depend on the ground and subsurface geology, and may even vary from day to day, season to season (for example, consider a wadi - is it a river? ...Perhaps only during the spring rain season? How large must a tributary become before you count it in its own right?) In light of this, you might start by looking at List of drainage basins by area; watershed basins by country..., and see where the related links take you. You may also find these websites interesting: The Environmental Fluid Mechanics research program, and for a more commercial tack, Schlumberger Water Services, who make their money by quantitatively analyzing surface and groundwater. Nimur (talk) 23:45, 13 March 2012 (UTC)

It is hard to define precisely enough what constitutes a river. One closely related thing that is relatively easy to measure is the total amount of annual rainfall received by a country (since all water that falls down as rain, less some small fraction lost to evaporation, ends up in the rivers.) By that measure, Brazil is the leader by a large margin, followed by Russia, USA, China, and Indonesia.--Itinerant1 (talk) 06:12, 14 March 2012 (UTC)

Todd River near Alice Springs

In Australia we're pretty generous when naming rivers. Here's the Todd River near Alice Springs. It even has its own regatta. HiLo48 (talk) 06:54, 14 March 2012 (UTC)

The article Stream says that rivers and creeks (sometimes called brooks and other names) are both streams. I have seen some large creeks and some small rivers in the US, and I would judge there to be some overlap in their width, depth, and flow volume. The overlap is very large if the extremes of seasonal flow are considered. Is there an operational definition, or some bright-line standard endorsed by some international scientific or geographic or hydrological body? What are some very "small" (variously measured) rivers and some very "large" (variously measured) creeks? It also seems difficult to determine how some small a water channel must be to qualify as a stream, since I know some small "branches" which drain a valley and have a definite watercourse with water flowing all year, fed by runoff from a "hollow" and exiting into a creek which locals would never call a creek or stream, because they are narrow and shallow (the watercourse, not the local folk). Edison (talk) 15:46, 14 March 2012 (UTC)

A "creek" in the England is generally a tidal inlet from the sea; Barking Creek is an example. It just shows the difficulty of trying to pin down an exact number of features going only on what local people call them. Alansplodge (talk) 09:12, 16 March 2012 (UTC)