Wikipedia:Reference desk/Archives/Science/2013 August 17

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August 17[edit]

Early gum disease[edit]

The Reference Desk does not give medical advice. Please ask your dentist for advice on your gum disease.
The following discussion has been closed. Please do not modify it.

Hello, I went to have a cleaning at my dentists and she said I "technically have gum disease (gingivitis)" on one or two of my teeth. They are the last teeth at the back of my mouth on the top and bottom, and they are a bit tender. She said to make sure I brush and floss twice a day, along with using a mouthwash, and I plan on doing so religiously. My dentist didn't say, and I have looked on the internet for an answer, but I can't seem to find a concrete one: how long does it usually take for the gums to stop being inflamed, and the general symptoms of gingivitis to go away? Thanks. (talk) 04:21, 17 August 2013 (UTC)

using FASTA format[edit]

What's the meaning of the numbers in the 1st line of a particular sequence, like in the following example ?


Thanks, BentzyCo (talk) 16:05, 17 August 2013 (UTC)

These are accession numbers. The first one is an unique identifier for this sequence (GI number). The second one (after gb|) is an accession number. See [1] for details. (talk)

Engine braking II[edit]

I recently (August 12, 2013) posted a question on engine braking and got satisfactory answers, thanks to all who responded. Well I soon realized it provoked a new question: why is it needed at all? The negative aspects are obvious: a lot of noise, no additional energy is returned to the battery (a parasitic effect as someone, perhaps SteveBaker, said), why not to use simple mechanical brakes we all have in our cars?

Thanks, - Alex174.52.14.15 (talk) 16:58, 17 August 2013 (UTC)

Because, as our article Engine braking says (confusingly in the Legal Implications section) it "can help save wear on friction brakes." Using conventional brakes alone will result in their wearing out more quickly (and as a driver I can attest that replacing brake pads is not cheap, whereas I've never had to replace an engine before changing the car for a newer one).
In a car the additional use of engine braking is never likely to be strictly necessary as opposed to merely aiding economy, but on a heavily loaded truck going down an incline, mechanical brakes in less-than-good condition alone might prove insufficient and/or fail entirely. {The poster formerly known as} (talk) 17:28, 17 August 2013 (UTC)
The use of engine braking versus friction braking comes down to one thing:
Which is cheaper - clutch or brakes?
On a large truck, the answer turns out to be "clutch"...engine braking takes a toll on the clutch - but in a big truck, it's cheaper to replace than all of those wheels.
In your car, the clutch is much more expensive than a set of brake pads - so you should probably rely mostly on friction brakes.
That said - there is a problem with friction brakes - they dissipate the kinetic energy of the vehicle into heat in the brakes and brake pads. If they get too hot, the brake fluid will boil - then there are bubbles of steam in your brake lines and since steam is easy to compress (and brake oil isn't) - when you stomp on the brakes, nothing happens! This is called "brake fade" and it's dangerous and scarey as all hell! Hence you should ALWAYS use engine braking when coming down a long hill to avoid overheating your brakes. Race car drivers also use engine braking for the same reason - but they can use both and brake much harder than they otherwise could. (They likely replace the brakes and clutch after every race anyway).
SteveBaker (talk) 22:15, 17 August 2013 (UTC)
Engine braking won't harm or wear out the clutch any more than a simple change of gear will - the clutch doesn't slip during engine breaking. Therefore, it's always going to be cheaper to slow down with engine braking than friction. The only problem is, for rapid deceleration, it's far less effective. As a general rule, racing drivers don't rely on engine braking - the engines are too small to make much difference (in Formula One, they're 2.4 litres, which compared to brakes that can cause 5G deceleration, is nothing). Air resistance braking has far more effect at racing speeds. --Phil Holmes (talk) 13:07, 19 August 2013 (UTC)

How can I use engine braking in my small Honda sedan?

- Alex — Preceding unsigned comment added by (talk) 23:10, 17 August 2013 (UTC)

Downshift to a lower gear to slow the car down. RudolfRed (talk) 00:29, 18 August 2013 (UTC)
...and take your foot off the gas. It can seem alarming because the engine revs so high - but that's OK because without much gas in the cylinders, there is little stress. SteveBaker (talk) 01:44, 19 August 2013 (UTC)
1) Don't attempt it unless going down a long hill.
2) For a not very steep hill, just putting the car into neutral is often enough, although you might still pick up some speed, so let your speed drop before cresting the hill, and let it go up a bit by the time you get to the bottom. If you can avoid any kind of braking, this is both the most efficient use of fuel and does the least wear on the car. (Note that trucks don't attempt to keep their speed constant while going over hilly terrain.)
3) If it's a long, steep hill, then some form of braking will be needed, either friction brakes or downshifting. StuRat (talk) 07:20, 20 August 2013 (UTC)
Re: 2
"Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because
engine braking is eliminated
vehicle speed downhill will increase quickly
increased use of the footbrake can reduce its effectiveness
steering response will be affected, particularly on bends and corners
it may be more difficult to select the appropriate gear when needed."
(The Highway Code)
I have never encountered any advice involving use of neutral on hills that wasn't "never coast down a hill". (talk) 17:22, 20 August 2013 (UTC)
Vehicle speed won't increase at all on an almost flat hill, since the air resistance and rolling friction on the vehicle will keep the speed down. On a slightly steeper hill, it will increase, but if the hill isn't very long and you've let your speed decrease before you crest the hill, it won't exceed a safe speed by the bottom. I don't get why steering response is reduced or why it's any more difficult to switch into a low gear from neutral than from a higher gear. The advice on that page seems rather inappropriately "one size fits all" such as "In an emergency. Brake immediately. Try to avoid braking so harshly that you lock your wheels. Locked wheels can lead to loss of control." There are obviously emergencies where braking is the wrong thing to do, such as when a front-wheel drive vehicle on ice starts to fish-tail, where you actually want to press the accelerator a bit to pull out of it. StuRat (talk) 20:50, 20 August 2013 (UTC)

Dragonfly behaviour[edit]

I was up at a cottage (in Central Ontario) for the last week and, while there, I spent some time watching the dragonflies. At one point I saw some odd behaviour. Two fairly large dragonflies (I'm guessing common hawkers, but I'm no expert) were performing what I assumed was a mating flight, riding piggyback intermittently along the edge of a small lake (however, this indicates it's not nearly so straightforward as I thought). Afterwards, one of them began dipping itself into the water, but it was not dipping the tip of the abdomen, where any eggs would come out, but rather the thorax/legs area. It touched the water surface like so, then flew on a few inches and repeated, again and again for at least a few minutes. The abdomen was kept fairly stiff and definitely up out of the water. It was not hurt or having trouble flying (and there was a lot of vegetation it could have landed on if it was having trouble) and it performed at least a few dozen of these dip maneuvers before it flew off. It seemed to be aiming for open areas of water rather than tight against the water lilies. What was going on here? Dragonflies are (AFAIK) purely flight-based predators as adults, so it was not likely hunting anything. Given the fish population it was quite risky behaviour to engage in, so I'm assuming there's some reason behind it? Matt Deres (talk) 21:19, 17 August 2013 (UTC)

Just a guess, but could it have been checking the water temperature, looking for the proper temperature for eggs before laying them ? StuRat (talk) 07:13, 20 August 2013 (UTC)