Wikipedia:Reference desk/Archives/Science/2013 February 14

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February 14

String vibration close to resonance.

I have a long belt-driven machine - a software-controlled stepper motor drives a toothed pulley - there is an idler pulley several feet away and a loop of 1/4" toothed belt between them.

I want to determine the tension in the belt without resorting to expensive instruments, microphones...nothing.

I know that the resonant frequency of the belt is proportional to it's length and it's tension. The length is essentially fixed and known - so the tension is the only variable.

So the plan is to have the software step the motor rapidly back and forth at a frequency that sweeps gradually from less than twice the expected resonant frequency to more than twice - and have the operator watch to see when there is a null point at the center of the belt and hit a key on the keyboard...then the software can note the frequency that it's using at that instant, figure out the tension in the belt and display it for the operator.

The fundamental frequency at the design tension is around 50Hz and the motor is easily capable of hundreds of hertz motion.

Before I go off and write software, my question is whether the expected null point will be at a sharply defined frequency...what does this look like when we're close to the right frequency but not EXACTLY there? How precise could I expect the resulting effect to be?

(A "gut feel" answer is OK here...although math would be handy!)

TIA SteveBaker (talk) 03:53, 14 February 2013 (UTC)

Whack the belt with a big wooden mallet or paddle and see if you can make it ring. If it damps out in one or two cycles, your plan won't work. If it does, rig some sort of lever on the (unpowered) drive wheel and give it a whack to see if the system rings when the force is parallel to the belt instead of at right angles. Again, if it damps out in one or two cycles, your plan won't work. If you get a nice oscillation, loosen or tighten the belt by 5% and try again. That will give you a rough curve of resonant frequency vs. belt tension.

The longer it oscillates after a whack, the sharper the peak when you run your test.

If you trust your operator to hit a key at the right time, why not trust him to put a known weight on the belt and measure the deflection with a yardstick you have mounted in an appropriate position? --Guy Macon (talk) 05:17, 14 February 2013 (UTC)

The belt does vibrate quite a bit in normal use (it's driving the head of a laser cutter which is dancing around in all sorts of complicated ways) - when those vibrations get too big (because of resonance) and if the belt isn't tight enough, it can skip a tooth - which is "A Very Bad Thing" when you a hundred watts of laser energy bouncing off of a mirror that's now misaligned! So I know the belt can be vibrated by the motor and will resonate visibly.

Well, two of the three belts on this machine are mounted in such a way that you certainly can use a weight hung off of them to get a measurement of tension. In fact, my current advice to users is: "Take a 12oz can of soda and hang it from the belt by the ring-pull using a paper-clip. If the belt deflects by more than 2cm, then it's not tight enough"! But the belt that I'm more concerned about is mounted with the pulleys laid horizontally over a flat plate - so hanging anything off of it is impossible without resorting to more complicated mechanical contrivances. Whacking parts of the system for any reason whatever is strongly discouraged! Sure, there are all sorts of mechanical arrangements that might be used (someone else is using a guitar tuner - and another person is recording audio from the sound of the belt being twanged and analysing the recording using Audacity!) - but why do that if I can do it using only software and someones' eyes? At any rate, I want to know whether my approach will work - not whether there are other techniques that might work instead. SteveBaker (talk) 14:35, 14 February 2013 (UTC)

This field of study is called vibrations analysis. StuRat (talk) 07:22, 14 February 2013 (UTC)

Yes, I know - but there is nothing there that tells me what happens when we're close to hitting the resonant frequency. I don't imagine (for example) that I have to vibrate my 50Hz-resonance system at precisely 100.000000000 Hz to get that visible stationary point on the belt - I'm reasonably sure that I'd see it at 100.000000001 Hz also. So how wide is that margin? Is there a way to know how stationary the mid point of the belt will be when I vibrate it at 99Hz and 101Hz? This would give me some idea as to the precision I might expect to get from this test without having to go to the hassle of writing the software and actually doing it.

As I said, a "gut feel" answer is good enough here!

SteveBaker (talk) 14:35, 14 February 2013 (UTC)

• I have absolutely no understanding of this (I don't understand why the speed of the motor affects resonance) but note that the article resonance does give the math for a "universal resonance curve" which is broader depending on how much damping is present. Wnt (talk) 15:36, 14 February 2013 (UTC)

The stepper motor causes resonance because it doesn't produce a continuous smooth motion - it's (more or less) a series of abrupt 'steps', controlled by computer software - the frequency of those steps can be anywhere between zero and a few thousand kilohertz - and at some movement speeds, it causes the belt to resonate enough to cause problems if the belt isn't tight enough. If the belt is too tight, it can cause premature wear on the motor bearings - so it's desirable to get the tension just right. SteveBaker (talk) 17:10, 14 February 2013 (UTC)

Hmmm - OK - so it looks like this "Q-factor" thing is what I need to consider. That's basically the ratio of the resonant frequency to the frequency range over which the belt will resonate. If the Q-factor is large, then the resonance will be over a smaller frequency range. Now all I need is an estimate of the Q-factor for my belt... SteveBaker (talk) 17:31, 14 February 2013 (UTC)

Consider using a strobe light set to the same frequency as the motor. Use the technique you described to find the resonance and then fine tune your measurement by turning the strobe light on and slowly varying the motor frequency. It becomes much easier to identify the exact spot of maximum resonance that way. Dauto (talk) 00:46, 15 February 2013 (UTC)

Again, I'm aware that there are other ways to do this - but I particularly want one that doesn't require any additional hardware whatever...so no strobe light. SteveBaker (talk) 18:17, 15 February 2013 (UTC)

Would it be possible move the idler or add an extra one nearer to the pulley that is having the problem? It seems like that would both limit the magnitude of the vibration and raise the resonant frequency, hopefully preventing the belt from slipping a tooth as easily. 38.111.64.107 (talk) 13:44, 15 February 2013 (UTC)

Well, some kind of gizmo of that nature might work - but adjusting the belt tension correctly is something we want people to do for other reasons. As I said, this is a laser cutter - and a sloppy belt results in hysteresis effects and other nastiness - while an over-tight one wears out bearings prematurely. So getting the thing properly adjusted is important - aside from the vibration issues. SteveBaker (talk) 18:17, 15 February 2013 (UTC)

Or you could just install a Tensioner. --Guy Macon (talk) 22:19, 15 February 2013 (UTC)

Egg whites and avidin

Egg whites contain avidin, a protein that binds biotin in the intestine and prevents it from being absorbed. According to Biotin_deficiency#Confirmed_causes, eating raw egg whites can cause biotin deficiency. My question is why is it raw egg white that cause biotin deficiency, what happens to avidin when egg white is heated (boiled or poached)? --PlanetEditor (talk) 06:20, 14 February 2013 (UTC)

It is denatured. Looie496 (talk) 07:09, 14 February 2013 (UTC)

One more question. If avidin, being a protein, loses its effectiveness by denaturation, does other protein (albumin for example) also lose its effectiveness due to denaturation? If this is the case, how do we get nutritionally useful protein from cooked food? --PlanetEditor (talk) 08:45, 14 February 2013 (UTC)

Even if it's called "nutritional protein", that name is a bit missleading. The proteins that you eat can only be taken up when they are split into their constituent amino acids (see, for example, Nutrition). So, all proteins that you digest are eventually disassembled into their building blocks, which are then taken up by your body to reassemble them into new proteins of your body. Cooked and uncooked can make a difference in this process, as the accessibility of the protein to your digestion can change (better or worse) according to its structure. --TheMaster17 (talk) 10:43, 14 February 2013 (UTC)

So what are you saying, denatured protein gives the same nutritional value as they provide the same amino acids? --PlanetEditor (talk) 11:16, 14 February 2013 (UTC)

Yes, as far as I know (I have never heard otherwise, and I studied digestive physiology during my undergraduate degree), I can't think of any good reason why not. I would imagine that most proteins (but not all) would be denatured by the acid environment of the stomach in any case, and most cooked foods have most of their protein denatured to some degree. Equisetum (talk | contributions) 11:29, 14 February 2013 (UTC)

According to this, denaturation of protein results in higher rate of protein absorption. Does it mean cooking increases bioavailability of protein? --PlanetEditor (talk) 12:33, 14 February 2013 (UTC)

This was something that mystified me long ago - thanks for reminding me. Why is it that egg white is not denatured in the intestine to release the biotin? Well, it turns out that (as suggested by a remarkably non-scientific source [1]) egg whites also contain a variety of trypsin inhibitors [2] (humans aren't even vulnerable to the one present at the highest level) and therefore of a raw egg white only 50% is digested. [3] So the egg seems to be making a fairly well-coordinated effort to kill habitual egg-eaters this way, which cooks circumvent. Wnt (talk) 15:29, 14 February 2013 (UTC)

Just a small Question?

It is said that both earth and the Newton's apple accelerate toward each other gravitationally but apple looks a lot to the earth due to its greater acceleration as compared to the earth toward apple which is so minuscule to be distinguished but would aforementioned apple and earth accelerate toward each other gravitationally if an apple is tossed/ accelerated in the absence of air with a force a-tad greater than gravitational, if yes then how?

Since, according to Newton's third law of motion, both earth and the Newton's apple accelerated AWAY from each other but apple would look a lot AWAY from earth due to its greater acceleration as compared to the earth AWAY from apple which is so tiny to be noticed, nonetheless, they both would experience gravitational force but lesser than the force with which apple was tossed at all-time and also gravity feebler at a distance.50.99.56.103 (talk) 07:18, 14 February 2013 (UTC)Eclectic Eccentric Khattak No.1

I can't understand your question. For example, what does "apple would look a lot AWAY from earth" mean ? The word "look" seems completely out of place. Also, use shorter sentences. If you are asking if the apple and Earth would fall towards each other more quickly in the absence of air, then the apple would, yes, but there would be no measurable difference in the Earth. StuRat (talk) 07:36, 14 February 2013 (UTC)

I struggle to understand exactly what is being asked, too. By Newton's Third Law, the throwing force that accelerates the apple upwards will be balanced by an equal and opposite downwards force on the earth, and this will give the earth a minuscule speed away from the apple. Air resistance will exert equal and opposite forces on both apple and earth, but this becomes significant only at higher speeds. Gravity is the main force (again equal and opposite) that brings the apple back to earth, and the earth back into its previous orbit. It would have to be an impossibly heavy apple to have a measurable effect on the earth. Dbfirs 08:03, 14 February 2013 (UTC)

The apple does not accelerate away from the Earth if it's tossed upwards. It moves away, but it immediately starts to slow down, which is effectively the same as accelerating toward the Earth. Rojomoke (talk) 13:45, 14 February 2013 (UTC)

Yes, I should have made that point more clearly. Once the apple leaves the hand (or whatever projected it upwards), gravity is the subsequent accelerating force back towards the earth, with air resistance helping gravity on the way up, but opposing gravity on the way down. (During the fraction of a second during which the apple is "being thrown" upwards, the apple experiences a large upwards acceleration, but this ceases once the apple leaves the hand. If the thrower was standing on a weighing scale, there would be an increase in the reading during the short "throwing" time.) Dbfirs 14:36, 14 February 2013 (UTC)

A mini rocket is attached to aforesaid apple which is responsible for the upward accealaration of an apple. The produced accelaration of an apple is always slightly greater than "g" of earth. — Preceding unsigned comment added by 162.139.255.35 (talk) 06:48, 16 February 2013 (UTC)

If your mini-rocket has unlimited fuel so that it can continue to provide an acceleration slightly greater than "g" away from the earth, then the speed of the apple will always increase and it will soon leave the earth's atmosphere, never to return. Dbfirs 13:10, 16 February 2013 (UTC)

What kind of monkey is this?

monkey! – Kerαunoςcopia^◁_galaxies 07:22, 14 February 2013 (UTC)

It looks like a Goeldi's marmoset (also known as Goeldi's monkey) (Callimico goeldii) to me. --Guy Macon (talk) 11:51, 14 February 2013 (UTC)

Our article: Goeldi's marmoset. -- ToE 06:50, 15 February 2013 (UTC)

I added it there :) – Kerαunoςcopia^◁_galaxies 01:28, 16 February 2013 (UTC)

Beautiful, thank you so much, Guy. – Kerαunoςcopia^◁_galaxies 20:05, 14 February 2013 (UTC)

Wow, cute monkey! --PlanetEditor (talk) 07:01, 15 February 2013 (UTC)

Why, thank you! I have always thought of myself as... Oh. Wait. You were talking about the Goeldi's marmoset, weren't you? --Guy Macon (talk) 00:47, 16 February 2013 (UTC)

looking for a closeup of an anglerfish lure

Hi - it's a reference for a drawing - there are plenty of anglerfish pics on the web, but none that I can find featuring the lure in hideous detail. Can anyone help please?

Thanks Adambrowne666 (talk) 10:51, 14 February 2013 (UTC)

This is specifically about lure. This image also shows the lure. --PlanetEditor (talk) 12:05, 14 February 2013 (UTC)

Perfect, thank you, Planet! Adambrowne666 (talk) 13:39, 14 February 2013 (UTC)

Wow. I had no idea they came in so many shapes. --jpgordon^{::==( o )} 21:38, 14 February 2013 (UTC)

In reality, they probably come in a much vaster variety, given that, as with most deep-sea fauna, we've likely seen only the barest fraction of angler species. And shape is only one inconstant element; there's considerable variation in the bioluminescent properties as well. Looking foreward, I wouldn't be surprised if some species are found who employ the appendage in uses quite aside from a lure, such as social/mating interactions or misdirection/defense (if indeed such haven't been documented already). Snow (talk) 04:57, 15 February 2013 (UTC)

world-wide wind speed average and standard deviation

Hi, What is the average wind-speed (over the course of a year) world-wide, and what is its standard deviation? (With this information I could calculate that for a given wind speed, say, 253.5 mph whether it is within one, two, three, four, five, etc, standard deviations of worldwide average). If you have it I would like to look at a graph of the distribution as well, to see how closely it resembles a bell curve - but this might be too much to ask! My own research has consisted of the following searches:

https://www.google.com/search?q=average+worldwide+wind+speed

https://www.google.com/search?q=what+is+the+average+wind+speed+worldwide

https://www.google.com/search?q=average+wind+speed+over+a+year

https://www.google.com/search?q=wind+speed+standard+deviation

https://www.google.com/search?q=worldwide+wind+speed

I tried to look at the top twenty links on each of the above, but saw standard deviations only for specific regions or streams - not for worldwide wind speed. Does anyone know more or have references, or maybe can see something I missed? Thanks. 91.120.48.242 (talk) 10:57, 14 February 2013 (UTC)

253.5 mph ? That's like the wind speed within an F5 tornado. StuRat (talk) 16:40, 14 February 2013 (UTC)

Just for reference as to how fast that really is, the highest ever surface wind speed (i.e. at the ground) directly measured was 231 mph in 1934 at Mount Washington, New Hampshire, and was a straight-line wind, not a tornado. There have been wind speeds measured via doppler radar in tornadoes which get up to around 300 mph, but to be a verified record there needs to be a physical anemometer reading at ground level, and this was a doppler measurement from a distance, and was aloft and not at ground level. --Jayron32 23:21, 14 February 2013 (UTC)

Both the mean and the standard deviation will depend upon what measurements you choose to include. Wind speeds on the tops of mountains are often much higher than those nearer sea level, but less frequently measured. Most quoted figures are probably averages of the measurements taken at weather stations, with an uneven area distribution. Do you require an average and standard deviation over world land area, or do you include wind-speed at sea? The answers are going to be very different, and I don't think you are likely to get one reliable figure, just some estimates. Dbfirs 09:04, 15 February 2013 (UTC)

Hi (OP), estimates are fine. Actually I would like to know the mean and standard deviation of every wind-speed (with respect to ground) at every altitude throughout the whole world, over the course of a year. Very rough estimates are fine. So, it sound slike, for example, 231 mph might be 6-10 standard deviations from the average? BUt this is a wild guess on my part. Do you guys have any guesses at all for the standard deviation of the average windspeed as I just described it? You don't have to take "every altitude" quite literally, just the air space in use. I am trying to come up with theoretical calculations only, so don't read too much (anything) into the question, I would just like to have an idea of the variance of windspeed worldwide, with respect to ground, so I can further my understanding on my own. if you can't find references either, do you have some kind of crude guess? Thanks. 178.48.114.143 (talk) 09:10, 15 February 2013 (UTC)

I could make only the same estimates that you would make from the data you have found, though someone else might find some better data. One concern over standard deviation figures is that the distribution of wind speeds will not be a Normal distribution because there are large areas of the earth (such as the Intertropical Convergence Zone where the wind speed is often close to zero, so the distribution will be very skewed (see Skewness). I wonder if a Half-normal distribution would be appropriate? Dbfirs 13:29, 15 February 2013 (UTC)

Autosomal Chromosomes =

22 pairs, that's understandable. but, are the X's of the 23's pair (XX/XY), are they also autosomal? — Preceding unsigned comment added by 79.179.134.180 (talk) 16:38, 14 February 2013 (UTC)

No - see autosome. Sex chromosomes are called allosomes. Gandalf61 (talk) 16:42, 14 February 2013 (UTC)

See pseudoautosomal region, which is shared by X and Y. But even that is, well, "pseudo" autosomal, because of the way the definitions are set. Wnt (talk) 04:33, 15 February 2013 (UTC)