Science fleeting thoughts

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pogrmman
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Re: Science fleeting thoughts

Postby pogrmman » Wed Aug 22, 2018 5:18 pm UTC

I’m wondering a bit about the physics and biology of hand slings. Using them is one of my hobbies, and I’m always really impressed at how fast I can adapt to a different sling or a different style or different ammunition.

From what physics knowledge I’ve got, it seems like you’d need quite accurate release timing for a given throw, and a different slings, styles, and ammunition would need different timing. I’m wondering about actually how accurate release timing needs to be and how it must vary based on projectile mass, sling mass, sling length, and throwing style (not to mention other stuff like stiffness/stretchiness of the sling). I’m also wondering how I’m able to adapt so fast — it’s literally 2 or 3 throws, and it’s like I’ve recalibrated my throwing to the new style and or sling. It’s also always interested me how I can tell when to release — especially considering what a small window the release needs to be in to get a projectile to go forward (I mean, my arm is ~1m long, as is the sling. So the ammunition is whirling around a ~1.5m circle and going quite fast. And yet, I’m able to release with enough consistency to nail a 1.5m x 1.5m target 30m away maybe ~60-70% of the time using random rocks I pick up.)

Any thoughts?

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Re: Science fleeting thoughts

Postby Soupspoon » Wed Aug 22, 2018 5:32 pm UTC

You have very reliably learnt to feel the action of release? By detecting the push-back before/during/after release and 'knowing' what should work you unconsciously microadjust your action to account for distortion, once you get the initial general feel of the differences, and having trained nerve/muscle-memory to shortcut the more conscious relearning mechanisms of a relative amateur.

(And, for the general feel, it's probably like having to adapt to walking whilst carrying an awkward load, the different characteristics of your bodily system (including the different 'bodily extension') may cause you a few steps of re-learning the basic movements needed but you quickly establish a sweet-spot that means you don't walk sideways/aim off, or rather that you change so that you may still do so but by 'trying' to go in the other direction from the forced-error you come back to the original intention.)

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Re: Science fleeting thoughts

Postby Sableagle » Wed Aug 22, 2018 6:05 pm UTC

I'm not a slinger, so I don't know quite how one aims it. Is it learned tmiing or do you let go when your hand's pulling at right angles to the line from stone to target or what?

Wikipedia mentions some amusing inscriptions like "Catch!" and "Ouch!" on cast sling bullets, but not speeds. For speed, I found a forum thread:

http://myarmoury.com/talk/viewtopic.16063.html

... that mentions 150 km/h. That's 41.6667 m/s. If it's going round a a.5 PI m circle, that's a 4.71m circumference, so 8.84 revolutions per second or 55.55555555556 radians per second. Nice number. Very recursive. Lots of fives. I like it.

The same thread talks about a slinging competition in which:
The target used is 1.2 metres (4 ft.) wide with a 50 cm (1.6 ft.) hole in the centre fitted with an iron plate for a bullseye. Men sling at 19.5 metres (64 ft.) and 29.25 metres (96 ft.).

That bullseye plate at long range subtends an angle of 0.00855 radians at the slinger, so that's a 0.154 millisecond release window.

:shock:

Impressive.

Most impressive.
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Re: Science fleeting thoughts

Postby gmalivuk » Wed Aug 22, 2018 9:46 pm UTC

At 30m, 0.5m is 1/60 of a radian.

Still impressive, but only half as impressive as you said.

Edit: Also, from videos of the competitions described there, the swing is much slower than 8 revolutions per second.
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Re: Science fleeting thoughts

Postby pogrmman » Wed Aug 22, 2018 11:27 pm UTC

Let's see -- I'm not actually sure how I aim. It has to do with the angle of my hand and knowing when to release, but I sort of do the proper adjustments automatically. I definitely have learned to "feel" when to release as mentioned earlier.

Regarding velocity, it's not all that hard for me to throw a rock the size of a large egg (usually limestone, so ~100g or so) about 100m with a sling if I'm going for distance (so release way earlier than I normally do at maybe ~45 degrees) -- which puts the velocity at something like 30 m/s (given no air resistance, which I'd imagine is a big factor). So, 40 m/s is a reasonable number for the launch velocity (especially given air resistance and the fact that those 100m throws aren't anywhere near the furthest I can toss and that I'm not a great slinger).

The throwing style I mostly use -- figure 8 (not my video) -- has only a single rotation around the head. The radius of that rotation is probably ~1.5 meters (my forearm plus the folded sling), making it more like 4.25 revolutions per second.

The competition you talk about is in the Balearic Isles and is generally regarded to be pretty much the standard. On that kind of target, I can hit the target about 70% of the time at 20m, with hitting the bullseye maybe 15% of the time. But, I've never tested that with consistently shaped ammunition -- only with random rocks ranging from maybe nickel sized to fist sized. And I don't consistently practice -- I maybe practice a couple times a week for an hour or so, with long, long breaks between "active" times and times when I don't practice at all.

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Re: Science fleeting thoughts

Postby Eebster the Great » Thu Aug 23, 2018 1:02 am UTC

A 50 cm bullseye at 20 m subtends an angle of about 0.025 rad (thank you, small angle approximation), so if the stone is moving at 4.25 rotations per second = 26.7 rad/s, that gives you about a 0.94 ms release window. Since you obviously don't get the bullseye every time, your actual timing is not quite that precise, but it should be within an order of magnitude. Millisecond-precision timing is very impressive, and this type of thing is also necessary for throwing accurately at long distances. It is something we as humans seem to be almost uniquely good at.

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Re: Science fleeting thoughts

Postby jaap » Thu Aug 23, 2018 6:22 am UTC

Eebster the Great wrote:Millisecond-precision timing is very impressive, and this type of thing is also necessary for throwing accurately at long distances. It is something we as humans seem to be almost uniquely good at.

Humans are also very good at recognising and following patterns. I reckon that by getting a regular swing going, it is easier to mentally home in on the point of the swing where the release should be, like a musician joining in with the rest of a band. If you had to release in the first or second swing, it would be much more difficult.

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Re: Science fleeting thoughts

Postby Sableagle » Thu Aug 23, 2018 8:52 am UTC

gmalivuk wrote:At 30m, 0.5m is 1/60 of a radian.

Still impressive, but only half as impressive as you said.

Edit: Also, from videos of the competitions described there, the swing is much slower than 8 revolutions per second.


Sorry, yes. 0.017 radians.

For small enough angles, just treating the target width as the opposite and range as adjacent is close enough, but I prefer to be accurate, so I halved the width, calculated half the angle, doubled it, realised the calculator was in degrees, started again and forgot to double it the second time through.

You've also got to get elevation right, of course.
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Eebster the Great
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Re: Science fleeting thoughts

Postby Eebster the Great » Thu Aug 23, 2018 5:57 pm UTC

Well 2*arctan(1/120) = arctan(1/60) = 1/60, to within about a microradian, or 0.2 seconds of arc. Good enough for me.

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Re: Science fleeting thoughts

Postby Soupspoon » Wed Aug 29, 2018 12:32 pm UTC

Is a "Contraception App" really contraception if it just tells you when to use a contraception*? Whether it's accurate or not (and however much it is guessing or not) when it tells you to use contraception you're then supposed to use one of the forms of contraception that it seems to be claiming to be better than, thus already reducing its efficacy to the level of that thing you haven't been using when it doesn't ask you to.

* Including attempted abstinence, one presumes.

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Re: Science fleeting thoughts

Postby Sableagle » Wed Oct 17, 2018 7:53 pm UTC

"To bring light back to the gardens of Moria, the Dwarves crafted great mirrors, and placed them atop the summit of Zirak-Zigil."

Assuming they can manage airflow well enough to keep the air cave-system klar rather than forge-ceiling schmutzig, what's the curve like on the graph of whether it's more efficient to do that or to cover the summit in PV panels and run LED lights in your garden?
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Re: Science fleeting thoughts

Postby Soupspoon » Wed Oct 17, 2018 10:21 pm UTC

I always imagined it was something like a Light Pipe. Or a set of such shafts, specifically with mirror-assisted capture either as a heliostat or a static 'whole-sky concentrator' to act as a sort of fish-eye lens across the track of the sun mapping to the shaft.

(Possibly crystal-filled, if they had the means to work or even grow such a thing, for the total-internal-refraction method. But I imagine highly polished metal lining would suffice for the mirrored version.)

I don't know if JRR would have known about/imagined full tubes of this kind, as seen these days, but Deck- and Pavement-Lights aren't a new thing.

The rest is just a matter of having as much practical collection area atop as effective dispersal area below (the refraction/reflection within the shaft(s) smudges out the bright-spot, I imagine) and daylightesque illumination, not much more/not much less, could 'feed' the gardens.

The big issue might be the energy concentration through the tube(s) themselves, at least in whatever absorption bands the system suffers with. Maybe they have a mithril alloy ("It could be beaten like copper, and polished like glass; and the Dwarves could make of it a metal, light and yet harder than tempered steel. Its beauty was like to that of common silver, but the beauty of mithril did not tarnish or grow dim.") that suited the purposes. Though I could imagine a warming of the air that would turn the tubing into a powerfully-draughting chimney (it could aid with the ventilation?) if not capped with baffled and/or sealed vanes/windows.


I have had in mind for a while now a lunar-base design where the agriculture is undertaken in underground areas (dug out voids, dug-down-to natural voids, cut'n'covered trenches in regolith or surface/shallow-trench-set modules where regolith is piled atop) with light-tubes set down to them. The tubes would be kinked (like a periscope, but halved and remounted eyepiece to forward lens) to stop direct radiation, the mirror/prism system designed to pass 'safe' radiation around the kink, and at the top either a heliostat or broad capture device. Given the 28-day solar cycle, WRT the Moon, it would have to be augmented for lunar night (backup illumination) and if the moon-daily cycle were too long in terms of daylight a grid/diffraction-grating could block (or pass around amongst multiple light-tube heads, destined to illuminate different 'fields' below) the normal light on a 12ish hours on 12ish hours off cycle.

Or a PVPanel->Cells->Timer->LED system could be used, but I'd been imaging this more as a passive system (for the setting I'd been thinking of) that at least during the two weeks of natural daylight would be self-regulating. Either arctic/genetically-made-arctic-like plants could be used that fully tolerate the extreme day/night lengths or rapid-growth-cycling plants could be raised within the 14 days of illumination and the system (hydroponics/aeroponics?) left fallow in the fortnight gap between harvest and reseeding. Might not work well with an aquaculture system of fish+plants.

But I had gone so far as to imagine some passively automated system (bimetallic levers tuned to a secondary solar-heat detector?) to cap superfluous tube-tops. During lunar-day the caps would rotate over tube-tops, covering and uncovering them on a 24-hour 'winding' cycle. Dumb-as-a-brick hardy PV/storage systems atop the caps would make some inefficient use of the incident light not destined to go down that tube. Passing into lunar-night, the mechanism whirs back and the (independently detected) lack of solar power triggers LEDs or other means of electoluminescence set within the underside of the 'caps' to send a smattering of the 'surrogate' solar light down the various shafts in lieu of the Real McCoy.

(But that was intended as a high-science/zeerusted setting where lack of technical maintenance was no bar to some 'reasonable' continuing functionality of a well-planned Moonbase. Also considered was a whole equatorial-ring complex where the entire lit half of the complex would directly supply as much or as little of the whole ring as needed, whether or not the locations needing energy had the Sun currently risen above their own set of PV panels. Obviously needs something like an HT-cabling running round the whole circumference along with the continuous/intermittent PV topping.)


Back to Moria (and away from recalling my prior less-than-Fleeting Thoughts about the other thing), some of the Lunar Base problems are moot to their design. I think that suitably-designed mirror-collectors, mirrored tube-linings and perhaps some method of further diffusing the semi-collimated beams punching through the ceiling back up onto a suitable 'rock sky' surface could (within the realms of the arcane workings of Middle Earth and its 'physical'-cum-magical everyday accessible properties) suffice to create a subterranean Eden. Give or take the active labour of dwarven mechanics to keep the system going prior to the desertion of the underground kingdom.


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