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July 29

Attracting a float in a pool

Suppose there are a couple of people around a float in a pool and the object is to get the float to move to you without touching it, any ideas what the best way of doing it would be? As far as I can see doing practically anything makes it move away and just doing nothing is the best strategy!, but I'm sure there must be something useful one can do. Dmcq (talk) 12:44, 29 July 2016 (UTC)[reply]

I would guess that if you face the float and row water behind you in a butterfly-like motion, it should bring the float closer. Intuitively you are creating fluid motion in the right direction; less intuitively, an analogy (that is probably deeply flawed anyways) is that you create a "hole" of water in front of you and a "heap" behind you, and so the "hole" will suck water in the front, going by the field lines of a dipole. Even if my reasoning is wrong, in all likelihood this has an effect, positive or negative, and reversing the motion would reverse the effect (for symmetry reasons).

This being said, throwing water directly at the float will likely push it away orders of magnitude more (compared to the amount of water displaced). Of course, any drop that you throw at the float will hit it with a velocity that pushes it away from you. It does not seem unrealistic that, as you perform a vigorous butterfly stroke, you throw enough drops towards it that it more than counteracts the "global effect" of creating a dipole-like motion. The effect of waves is also unclear - just for the lol, I will mention the Casimir effect (which likely does not apply since the damping is too strong).

The best strategy (I assume it is a game) in theory is probably to do some sort of stroke but slowly to limit the drops that you throw. That does not look very fun, and in practice, the best idea is to get the opponents to do the drop-throwing thing. I would try to find a motion that (1) can be done fast, (2) does not throw many drops toward the float. Find this, and you can impress your friends! Tigraan^{Click here to contact me} 13:44, 29 July 2016 (UTC)[reply]

Thanks. I think I got a bit of motion towards me by pushing water under the float towards or past the opposite side. It is very frustrating how one can try and get a vortex near one or try dragging water back and the float just goes away. Dmcq (talk) 14:21, 29 July 2016 (UTC)[reply]

This is sort of vaguely related to the reynolds number, right? Sort of like how you can't easily catch a dandilion's pappus in the air. The problem is that vorticity at all scales prevents much laminar flow. So: I'd try messing with a 4x4' sheet of plywood or stiff plastic. Get a lot of water moving slowly, watch the vortices. Likewise, it should probably be easier to get a ping-ping ball to float toward you, compared to a full-body raft (or maybe that last bit is backwards, I'm groggy, the point is there's a length scale that goes into effective reynolds number, and different sized floats will behave differently in the same water) SemanticMantis (talk) 14:39, 29 July 2016 (UTC)[reply]

Don't think it is any good trying to paddle the water in-front towards oneself with one's hands as the water will fill the void sideways. Instead, stand in front and move the water sideways (with both hands in unison) . Flotsam and jetsam will then drift towards you from both the font and rear. Kids instinctively learn to do this in the bath tub.--Aspro (talk) 14:48, 29 July 2016 (UTC)[reply]

Thanks, I'll try that next time I'm swimming :) I guess this is a bit like where people compete trying to call a dog or cat to them! Dmcq (talk) 16:27, 29 July 2016 (UTC)[reply]

One thing to keep in mind is that any water you move away from one spot, say by pushing it behind you, will find it's way back, sooner or later. So, you may find the float moves towards you initially, but then, as the water flows back in front of you, the float moves back away. The sure-fire cure for this is to move water from the side of the float near you, to the side of the float away from you. This would be easy with a sump pump and hose, but difficult with your hands, as throwing handfuls of water over the float would result in splashing it with water, moving it away from you. But, if you were very close to the float, but not allowed to touch it, cupping hands full of water in front and dropping them behind it might work. This could be an entertaining game, sort of "water curling". Long arms would be an advantage.

Another approach you could take is moving water at depth, underneath the float. That should then cause surface water to move in the opposite direction. Actually getting water to move in one direction, at depth, might be difficult. Again assuming electrical pumps are out, how about a manual pump, like a bellows ? I can't picture this method being all that effective, though, in a large pool, as the amount of water moved is tiny and the volume is then distributed over that entire half of the pool. You would do better to aim the nozzle at the far side of the float, toward you, if you could reach that far. StuRat (talk) 18:38, 30 July 2016 (UTC)[reply]

There is a video here on how to paddle a coracle. Learning to drive a coracle One can see how they start off trying to paddles the water towards themselves but as I pointed out above, this is of little help. One can see that they get more experienced, they find they only need only swish the paddle from side to side in a figure-of-eight fashion. The coracle moves forwards and the flotsam and jetsam move towards the coracle. Trying to move the water towards one with a paddle or hand only works in a narrow channels. When a kid, a bathtub seems huge and a side to side motion gives instant feed back that ones toy boat is coming closer faster, than trying to drag the water towards oneself. --Aspro (talk) 10:42, 31 July 2016 (UTC)[reply]

I'm a kayak coach and can confirm that pulling the water towards yourself (a draw stroke) certainly is effective, but lacks the fine control available with "swish[ing] the paddle from side to side in a figure-of-eight fashion" (a sculling draw stroke). The problem with a coracle is that it is very short in every axis (well, circular actually), making the control with an ordinary draw stroke very difficult (but not impossible) and the sculling draw is the easiest with that particular craft. An ordinary draw stroke would be easier with a long boat like a sea kayak and it's the stroke we teach beginners in any craft. My Scouts were perfectly able to move their kayaks sideways using a draw stroke about half-an-hour ago. Alansplodge (talk) 14:37, 31 July 2016 (UTC)[reply]

Think you're getting a bit confused. A draw stroke is using the 'resistance' of the paddle through the water to pull the vessel towards the oar. If it was drawing the water towards the vessel, the vessel would end up going in the opposite direction! Some water may come your way but most flows in from the sides due to the hydrodynamics. These things can be counter intuitive to some. Consider a scout attempting to do his first eskimo roll . Muscle memory may we have him lifting the paddle in to the air, because when he is the right way up it is not normal to lift the oar above oneself to paddle fresh air but upside down one has to reverse some of those actions. Try doing a draw stroke in a coracle and you will need a longer paddle which keeps hitting the river bed and is less comfortable to use when it is being held out in-front. The OP's question is how to move a 'floating' object towards one and a draw stroke is not effective. Run this by your scouts and maybe they can teach their Akela a thing or two :-) My swimming coach (a Middlesex Champion circa 1960's) mentioned that in order to pass his coaching exams he had to repeat what he knew was wrong about how to create thrust but he had to write it in order to satisfy the examiners. So don't believe everything you have been taught. Been there – done it – and nearly drowned several times. --Aspro (talk) 23:14, 31 July 2016 (UTC)[reply]

Whether the boat moves towards the paddle or the paddle towards the boat doesn't seem germane to me, equal and opposite reaction n'all that. (actually there's a lot more movement in the paddle than in the boat, so that's the way that we usually describe the action to beginners, who probably aren't physicists). I was disputing your claim (which you just restated) that you can't propel a coracle using a draw stroke, which you certainly can as I have done it. Alansplodge (talk) 19:59, 1 August 2016 (UTC) AKA "Akela"[reply]

I did not say that that you can't propel a coracle using a draw stroke, I said is is uncomfortable (compared to the figure of eight motion). Of course there is going to be more relative moment of the paddle compared to the boat and the physics behind it is good to know. For instance, if a scout was on a lock side, helping to guide in a barge and the barge was coming in a bit too fast, he may be tempted to stop it, not realizing that the barge has a lot of kinetic energy which is sufficient to crush his hands or foot that he puts in the way. Try explaining that mishap to his parents. There was a resent case of a school girl mixing up plaster of paris in a bucket and it set with both of her hand in it. As you know, plaster of paris generates heat on re-hydrating and the resulting burns required amputation of some of her fingers. Schoolgirl 16 lost eight fingers in plaster of Paris accident during art lesson. Cost -nineteen thousand quid, a few fingers and an art teacher's teaching career. The OP's question is about 'attracting a float'; not sitting in a float and manoeuvring it. A d-r-a-w s-t-r-o-k-e is not effective for this. If that doesn't seem germane to you then you should remember that you have a duty and responsibility to join up the dots. So, as you may now have guessed, non of my grand children will be getting any lessons from a particular kayak coach. Not even on how to boil an egg in water!--Aspro (talk) 00:02, 2 August 2016 (UTC)[reply]

Suit yourself. You did say it would be "of little help" which I don't believe to be true. We may have drifted off topic somewhat. Alansplodge (talk) 20:12, 2 August 2016 (UTC)[reply]

Is vision permanently corrected after laser eyes surgery?

Or will eyes deteriorate as normal, just from a starting point of perfect vision? 2.102.187.157 (talk) 12:52, 29 July 2016 (UTC)[reply]

The latter. Laser eye surgery (e.g. LASIK) aims to correct current eyesight problems but doesn't prevent the natural aging that results in most people needing glasses later in life. Some people who have had laser surgery in the early days of the technique even choose to repeat it later in life after their eyesight deteriorates due to natural aging. However, I do know one woman who was advised that she had a thin cornea and consequently wouldn't be eligible for a second procedure. Dragons flight (talk) 13:03, 29 July 2016 (UTC)[reply]

And yet our lasik article says

For most patients, LASIK provides a permanent alternative to eyeglasses or contact lenses.[2]

although the reference given is apparently to a newspaper article, and clicking it gives an error message. Note also that it does nothing for the loss of reading ability in middle age, so someone who has had lasik will probably need reading glasses eventually. Loraof (talk) 14:58, 29 July 2016 (UTC)[reply]

Anecdote is not evidence but both people I know who had LASIK now wear glasses for reading. Greglocock (talk) 15:30, 29 July 2016 (UTC)[reply]

Iirc, age related eyesight issues are due to muscle decline, not changes in eye shape that result in early life eye issues. Laser surgery cannot fix muscles. For us myopic folks, it's supposed to be a permanent fix assuming your vision correction is stable (mine isn't sadly). EvergreenFir (talk) 16:06, 29 July 2016 (UTC)[reply]

Right, but myopia may delay the onset of presbyopia because your eyes are set up to focus excessively close. So having corrective surgery for a myopic may make presbyopia more likely or at an earlier age. See our articles and [1] for some brief discussion of this. BTW that later ref also has some details on how many people need followup surgery and when it's less likely to succeed. (Note also that stability is one key point of difference where local regulations and specialist views will affect when the surgery will be offered.) Nil Einne (talk) 22:18, 29 July 2016 (UTC)[reply]

Not having surgery will make it more likely that you can do without glasses later in life as Nil Einne points out above . Currently I don't even need a magnification glass, all I need to do is take off my glasses allowing me to see an object up close just a few centimeters from my eye. Why destroy this supervision ability using laser surgery? Also note that laser surgery will only correct your eyesight to within some tolerance, which may not be good enough e.g. for doing (naked eye) amateur astronomy. You may then need glasses of lower diopter, say -1/2 with a small cylindrical correction for optimal viewing. To actually improve the quality of vision requires using precision glasses that accurately correct for not just the diopter and cylinder, but also higher order aberrations. Carl Zeiss Vision has developed a new system to produce such precision glasses, these allow for perfect sharp vision at night when the pupil is large and vision tends to deteriorate due to these higher order aberrations. Even people who have perfect vision when measured at daytime will not have have perfect vision at night, so they can also drastically improve their vision using such glasses. Count Iblis (talk) 22:17, 29 July 2016 (UTC)[reply]

For me, I knew what I was getting into when I had lasik surgery about 20 years ago. I expected before the surgery that it would reverse the situations in which I need glasses, and that's why I wanted it. I didn't like to wear glasses while driving a car, piloting an airplane, riding my bike, or doing any other life-endangering outdoor activity in which I must rely on glasses to see. I didn't (and still don't) give a damn if I need glasses while sedentary: reading, using the computer, etc. 20 years later, that's still true, I can still see distances clearly, but my ability to focus close-range is deteriorating (presbyiopia). So to answer the OP's question, in my case my distant vision was permanently corrected, but my near-vision prescriptions are changing. ~Amatulić (talk) 23:52, 29 July 2016 (UTC)[reply]

The most general answer is that there's no procedure that zaps your eyes and magically makes them perfect forever. There are different procedures that can all be considered "laser eye surgery". The most commonly-performed procedures reshape the cornea to compensate for refractive error. This doesn't stop or prevent any changes to the eyes; it just substitutes for corrective lenses. There are many things other than refractive error that can impair vision. A few can be treated with other laser procedures, though there's no medical procedure that's perfect. --71.110.8.102 (talk) 00:26, 30 July 2016 (UTC)[reply]

Hitting walls

Does hitting walls with bare fists or wrapped in towel cloth cause any damage to wrist and other joints in long run ? — Preceding unsigned comment added by 182.18.177.78 (talk) 16:02, 29 July 2016 (UTC)[reply]

Maybe, maybe not. Answering that question for a specific person and context would constitute medical advice, which we cannot give. You may be interested in reading about repetitive strain injury, sports injury, tendinitis, tendinosis, and maybe bits of sports training, punching or boxing. SemanticMantis (talk) 16:28, 29 July 2016 (UTC)[reply]

I think the basic answer is Yes:any impact will cause a certain degree of damage - though whether or not you become aware of the damage caused will depend on the strength of you hand and wrist, and how hard you actually hit the wall. Low levels of damage may have no effect that you are aware of, and should heal easily. Only if the level of damage passes a certain threshold (specific to you) might the damage be severe enough for you to suffer. Wymspen (talk) 16:52, 29 July 2016 (UTC)[reply]

If done right, the body will be able to repair it and make itself stronger. This sort of training is necessary to be able to do things shown here and here. Hitting a wall causes a rebound of the shock wave into your hand, as pointed out here this can cause more damage compared to breaking an object. Count Iblis (talk) 20:51, 30 July 2016 (UTC)[reply]

Caitlyn Jenner: Why is there no baldness in older transgendered people, when men transition to being women?

Every so often, we see Caitlyn Jenner (Bruce Jenner) in the news. I often see photographs that look like this (more or less): [2]. I am struck by the very full head of hair. I assume that is "real hair" and not a wig; am I correct? Also, I see that Jenner is age 66. So, my question is: at that age, why is there no hint of any baldness (when he transitioned to being a woman)? And, if that's the case, doesn't whatever they do with Jenner offer the cure for male baldness? I never see transsexuals (man to woman) who are bald, even in their older age. Is this due to wigs? Or what else is going on? Thanks. Joseph A. Spadaro (talk) 16:56, 29 July 2016 (UTC)[reply]

It's probably a wig, though a very well done and natural looking one. --OuroborosCobra (talk) 17:02, 29 July 2016 (UTC)[reply]

Your horribly problematic language aside, male pattern baldness (and even baldness in women) is linked to testosterone levels iirc. Hormone replacement therapy trans folks often use would likely affect baldness. It would be inappropriate to speculate on Caitlyn Jenner's medical history though. EvergreenFir (talk) 17:09, 29 July 2016 (UTC)[reply]

I thought the OP's language was a little clumsy, but also showed at least some small effort to get it "right"/inoffensive. It seems you think that he did not succeed. So, in the name of edification, can you explain to us which bits are horribly problematic? Not really trying to fight or argue, just understand, thanks. SemanticMantis (talk) 20:47, 29 July 2016 (UTC)[reply]

For the record, though, many people of her age, of both genders, wear wigs; c.f. John Travolta with and without his wig. Whether Caitlyn does, or does not, wear a wig or hair extensions or something like that isn't something we should speculate on, and also is not out of the ordinary. At the high school I teach at, more than half of the female students, and a non-zero number of male students, wear some kind of hair extensions, weaves, or something similar. --Jayron 32 17:31, 29 July 2016 (UTC)[reply]

Google Images results would suggest that it's her own hair. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:37, 29 July 2016 (UTC)[reply]

Or that she has very good wigs. Images are a poor judge. --OuroborosCobra (talk) 18:14, 31 July 2016 (UTC)[reply]

I'm going by pictures of Bruce when he was letting his hair grow long, comparing with Caitlyn's hair now. To me they look similar. Though you're right that a visual check is not 100 percent proof. ←Baseball Bugs ^{What's up, Doc?} carrots→ 18:44, 31 July 2016 (UTC)[reply]

I assume, not jokingly, that castration is a good measure against male-pattern baldness. Although Caitlyn Jenner has not undergone sex reassignment surgery, this could still partially explain the alleged non-existence of bald trans-women. Long-term hormone replacement could also be beneficial against male-pattern baldness, since this is caused by the action of androgenic hormones on hair follicles. Add to this that trans-women would be more prone to undergo hair transplantation to look more feminine. Hofhof (talk) 13:10, 30 July 2016 (UTC)[reply]

Thanks. Joseph A. Spadaro (talk) 04:29, 2 August 2016 (UTC)[reply]