Wikipedia:Reference desk/Archives/Science/2014 March 17

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March 17

Do other armies besides the Israeli's also wear Mitznefet?

OsmanRF34 (talk) 01:01, 17 March 2014 (UTC)

Per Helmet cover "Helmet covers are used by most armies" --Jayron32 02:32, 17 March 2014 (UTC)

Note however, from what I can tell most don't appear to use anything near as floppy as the Mitznefet, so it really depends on what you mean by a Mitznefet. On the other hand, I'm not sure how strict other armies are at requiring a specific form of helmet cover [1], so it's possible provided it meets other standards and depending on their immediate superiors, some may choose to wear a Mitznefet [2] or something else you may or may not call a Mitznefet e.g. [3]. Nil Einne (talk) 05:56, 17 March 2014 (UTC)

Cockpit pressurization

One and two-seat combat jet aircraft such as the F-15, F-16, F/A-18, Eurofighter Typhoon and Sukhoi Su-27: Do they have pressurized cockpits? On one hand, their occupants wear oxygen masks, and cabin pressurization would provide unnecessary trauma in an ejection at high altitude. But on the other hand, pressurization helps guard against hypoxia, facilitates maintaining a comfortable cockpit temperature, and helps guard against barotrauma during high-speed climb and descent. Dolphin (t) 06:02, 17 March 2014 (UTC)

See the F-16.net - General F-16 forum which says of the General Dynamics F-16 Fighting Falcon: "The cockpit pressurization schedule is ambient pressure up to 8,000 feet. Between 8K-23K, cockpit pressure stays at 8K feet. Above 23K, the Environmental Control System(ECS) maintains a 5psi pressure differential between the cockpit and ambient pressure;". According to our Lockheed P-38 Lightning article, experiments with pressurised cockpits for fighter aircraft began during World War II. Alansplodge (talk) 08:56, 17 March 2014 (UTC)

Thanks Alan. Perfect! Dolphin (t) 11:52, 17 March 2014 (UTC)

You're most welcome. Alansplodge (talk) 18:57, 17 March 2014 (UTC)

Further research reveals that the wearing of oxygen masks is required because any decompression of a fast jet's small cockpit would be very rapid indeed and would not give the pilot time to put a mask on. Airliners in contrast, have a huge amount of pressurised air in the cabin, which would take time to leak out, however big the breach. See this article on hypoxia. I have added this to the Cabin pressurization article. Alansplodge (talk) 19:52, 17 March 2014 (UTC)

It's not just due to differences in cabin volume. There have been survivable incidents involving passenger jets losing very big pieces of cabin wall, which would have been accompanied by virtually instantaneous losses of cabin/cockpit pressure. (cf Aloha Airlines Flight 243, which lost an enormous piece of its cabin roof; or British Airways Flight 5390, which blew out an improperly-installed cockpit window—and half of the captain. Both incidents occurred while the aircraft involved were at altitude.)

Cruising altitudes and service ceilings for passenger jets tend to be significantly lower than for military jets, which makes a significant difference in time of useful consciousness. For example, the service ceiling for the Boeing 777 is 43,100 feet, whereas the ceiling for the F-15 is 65,000 feet. Fighter jets are often operated with a single crewperson onboard; passenger jets have at least two pilots. Further, as you noted in your earlier message, aircraft like the F-16 don't maintain full cabin pressurization (to 8,000 feet, anyway) when the aircraft is at high altitude. Finally, I would suspect that the availability of supplementary oxygen would be more important to military pilots who may be engaged in sustained high-gee manoeuvers not possible for their civilian counterparts. TenOfAllTrades(talk) 16:45, 18 March 2014 (UTC)

Many thanks. Dolphin (t) 06:15, 18 March 2014 (UTC)

Body mass index

Is this an accurate measure of body fat? Or can someone who has lots of muscle still show high BMI without being fat?--86.179.250.140 (talk) 15:21, 17 March 2014 (UTC)

According to this article: BMI. "BMO is particularly inaccurate for people who are fit or athletic".Star Lord - 星王 (talk) 15:25, 17 March 2014 (UTC)

BMI was intended as a statistical measure - not for single individuals. Over a large population, the BMI will indeed predict the average amount of body fat - but for single individuals, it's a much less useful 'rule of thumb'. So if you want to know what percentage of the population of Texas is overweight - then by all means, measure their BMI's every year and you'll get a really good idea of how the population is handling their weight. But if you want to know whether you, personally, are overweight, then BMI isn't all that much use. SteveBaker (talk) 19:08, 18 March 2014 (UTC)

That is very enlightening, Steve. I must pass that info on to the last 6 GPs I've had who've all used my BMI to convince me to lose some excess weight. No wonder it didn't work. -- Jack of Oz ^{[pleasantries]} 23:45, 19 March 2014 (UTC)

Power to weight ratio is a better measure. You then do a test on a hometrainer, after a warm up period the maximum power you can sustain for, say, 20 minutes is measured. This is divided by your weight. Fit people should be above about 3 Watt/kg. I'm above 4 Watt/kg, I weigh 57 kg and I can easily do 240 Watt for half an hour without even maxing out. Count Iblis (talk) 01:39, 20 March 2014 (UTC)

Another measure is total calorie intake divided by your weight, assuming your weight is constant. The higher this is, the more metabolically active tissue you have, which then implies that you have less fat. If you struggle with your weight while eating only small amounts of food, then your body is in a bad shape. I eat close to 4000 Kcal per day while my weigh is stable at about 57 kg, this is a very healthy ratio but still way off Michael Phelps who ate 12,000 Kcal/day, he had just 8% body fat. Count Iblis (talk) 03:39, 20 March 2014 (UTC)

Even if you don't gain weight, high calorie intake is still bad, as it means more oxidation occurs, potentially more cholesterol forms plaque in your veins, etc. In animal tests a near-starvation diet seems to extend life expectancy the most. And athletes often go to pot when they retire and still eat like they used to. StuRat (talk) 05:05, 20 March 2014 (UTC)

I don't buy the "metabolically more active tissue" argument. Thermodynamics is a harsh mistress. Those calories absolutely have to go somewhere - that's very basic physics. If you are consuming 4,000 calories and the average person only maintains a stable weight at 2500 calories - then either:

1. You have a very poor digestive system - so you are pooping out most of the calories that you ate (unlikely - but possible - and would be very evident in 'unusual' poop!).
2. Your body is doing more physical, mechanical work than most people (it would have to be a LOT more to get up to 4,000 calories - about 2 hours a day running at an 8 minute mile pace would be about what you'd need).
3. You're accumulating the calories inside your body (in which case you'd be putting on weight - and you say you're not).
4. You're generating more body heat than most people.

If it's the latter (which is what a "more active metabolism" would imply to me) - then that excess heat has to go someplace. Either you have to be wearing much less clothing and/or living in a colder environment so you can more effectively radiate that heat away than the average person - or your body temperature must be higher than an average person - and that's really not good for you - running a perpetual fever would surely be bad news! SteveBaker (talk) 19:50, 21 March 2014 (UTC)

The difference between 4000 Kcal/day and 2500 Kcal/day isn't all that big, it's just about 73 Watt over a whole day which is what you would produce if you perform about 18 Watt of work extra which is almost nothing. If half of that comes from exercise (concentrated during the hour of exercise makes that 220 Watt), I'm left with the equivalent of 9 Watt exercise all day and night long. That's what my cells in my body need to do and that leads to about 36 Watt of extra heat. But at 2500 Kcal/day you would produce 121 Watt of heat, so it is a relatively small change. Count Iblis (talk) 00:45, 22 March 2014 (UTC)

Cold Fizzy Drinks

Why is it that cold fizzy drinks tend to feel so much more refreshing than non-fizzy drinks? I am sure it's an illusion, too, because I can chug a full bottle of cola, then whilst feeling bloated and burping incessantly for the next ten minutes or so, I still feel thirsty - but not during the process of the actual drinking itself. KägeTorä - (^影_虎) (Chin Wag) 15:38, 17 March 2014 (UTC)

The aftertaste, maybe? ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:26, 17 March 2014 (UTC)

I have always assumed it was the carbonation. Flat soda, especially warm, flat sodas, are often very unpleasant. μηδείς (talk) 17:07, 17 March 2014 (UTC)

A matter of taste as well. Brits supposedly like warm ale. Yuch. Yuch to warm, flat soda as well. But McDonald's Hi-C orange drink is essentially uncarbonated, and it's excellent when served cold. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:48, 17 March 2014 (UTC)

Unrefrigerated yes, warm no. Cask ale should be served at cellar temperature which is 12-14 C (54-57 F).[4] It's not carbonated either. A growing number of British drinkers prefer cold chilled fizzy beer in the foreign style - they ask for lager. As you say, there's no accounting for taste. Alansplodge (talk) 18:16, 17 March 2014 (UTC)

As an American I'm the only one I know who thinks this, but I prefer room temperature or near room temperature soda and beer to the chilled equivalents. To me it seems like refrigerating them greatly reduces the total amount of flavor. I also kind of like the "skunky" smell that beer sometimes has (Corona in clear bottles...), which is also usually looked down on by the aficionados. Go figure. Wnt (talk) 02:31, 19 March 2014 (UTC)

Possible reasons are discussed in a Popular Science article. AndrewWTaylor (talk) 17:43, 17 March 2014 (UTC)

Please refrain from giving opinions, whether a drink is "more" or "less" refreshing is not something a Reference Desk should be suggesting. I'm deeply shocked this thread hasn't been closed down already. Sorry about that. The Rambling Man (talk) 19:17, 17 March 2014 (UTC)

I believed there is a valid scientific explanation to this. I like cold water, but I prefer cold fizzy water - I know many people do (this is why it exists). I was just wondering if anyone had any idea why this is so often preferred over normal cold water. Cola was just the example I gave. Andrew's answer above was perfect. Thanks. KägeTorä - (^影_虎) (Chin Wag) 19:40, 17 March 2014 (UTC)

Agreed, there was no reason to shut down this Q, as that source shows. StuRat (talk) 22:34, 17 March 2014 (UTC)

Resolved