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I think that the impact from hitting the water would likely cause whiplash or something. If it worked, it would be very fun, though. |
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How about a submersible ferris wheel? When passengers embark or disembark (...'ark'?!) the various chairs, the ferris wheel must be stopped, so it would be necessary to hold breath underwater then as well as during the ride if you want to live. |
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You could streamline the cars to avoid any kind of whiplash. If fact [dj_photon] suggests a nose cone, probably for this very porpoise. |
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I have had this idea for years. The submerged part of the ride would be a fiber glass tube or whatever the material is that large aquariams have and would have the track built through it. The riders are not actually submerged but the tube is. This could open the door for so many possibilities such as a deep drop into a dark abyss area or there could be scary sea monster props along the tubing. I can't wait. |
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For Steve DeGroof: I used to be on a diving team and although I never did any platform diving, my coach said teams that do it train in a pool with a bubble machine. The machine blows a huge bubble under water and the diver has to leave the platform at a specific time. Then the diver falls toward the water as the bubble floats up. The diver and the bubble meet at the surface and the diver is protected from injury in the case of hitting the water wrong. So if this is true, something similar could be used. But the original idea involved a thick (possibly solid), streamlined, metal nosecone on the front of the rollercoaster. I think it could be engineered similar to space shuttle nosecones. That would probably be stong enough. |
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for mlynn: I don't like the submerged aquarium tube idea though, sorry. It reminds me of the similar structure in the Jaws movie, which was supposed to be scary, but was more annoying than scary. The water around the tube would be more like scenery than a ride feature. There are more efficent/cost-effective ways to achieve similar scenery effects. |
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Also, if somebody DID want sharks around (not mentioned yet), I would not expect them to swim near all the noise. |
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Along the same theme as Steve DeGroof’s what about adding something to the water to help break the surface tension… I am thinking soap. Laundry detergent would reduce van der waal’s forces, probably not nearly enough, but the bubbles would be fun. |
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re: bubbles in pools, if you pump lots of bubbles in a pool, it essentially decreases the local density of the water. This has been blamed for a few ship sinkings in the bermuda triangle as subsurface methane hydrate deposits are disturbed and gasified. |
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not sure if anyone is interested in knowing, but to make something move faster in water you need to break the surface tension. that is generally what the air bubbles high divers use actually does, it breaks the surface tension of the water. It is surface tension that allows striders (A type of arachnid) to walk on water. Surface tension is what make water like concrete if you hit it straight on at high speed, you need to pierce the water to break the tension, which is why divers don't do bombs and why marines jumping out of helicopters into water jump straight with their legs down, so the narrower leading edge that hits the water hopefully pieces the water. It is also this surface tension that creates another effect, called cavitation, cavitation is where something moving through the water really fast causes the water to vapourise causing low pressure, as that low pressure meats high pressure again it cause the water vapour to impode on itself, causing shockwaves that tend to pit the side of propellers and vessels... They have come up with a way of using cavitation to their advantage and that is to put a flat nose on the vessel, as the water hits the nose it vapourises and case and pocket around the vessel enabling the vessel to move through the water more smoothly as the water tension is not holding it back. The only draw back is that you need another means of propulsion as the props don;t work if there is no water to push against, so scientists are using rockets, which funnily enough work, when there is nothing but a pocket of vapour around them. they are also using guns with bullets that have blunt ends, which using the same effect, travel in the water further... |
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One way they are working with cavitation is to also port some of the exhaust fumes out the front to help sstain the vapour pocket longer, so all in all if you can come up with some way of enabling your roller coaster to hit the water slowly with a pointed nose so it can penetrate the water, but once in the water you need to be able to excelerate to high speed and have a flat nose on the front, so you can create a cavity around the vessel and then it should be able to travel very fast for a lot less energy than if you were to travel the same speed without the cavity. |
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//to make something move faster in water you
need to break the surface tension. that is
generally what the air bubbles high divers use
actually does, it breaks the surface tension of the
water.// |
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That's true, and yet also complete bollocks. The
effects of surface tension are completely
negligible when dealing with large masses and
forces, such as a diver or a rollercoaster entering
the water. |
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Air bubbles for a high-diver do not do anything
significant to surface tension (if they did, a little
soap added to the water would be just as
effective). The air bubbles make the water locally
less dense (simply because a large proportion of it
is air), and they also make it compressible (ie, the
air bubbles are easily compressed; water isn't).
Both of these factors can greatly soften the
impact. Surface tension is irrelevant. |
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//the air bubbles are easily compressed; water isn't// That's true, but may also be complete bollocks. Compression of unconfined air is mostly insignificant except at speeds approaching the speed of sound. If you think about it, compressing a bubble to about 3/4 of its original size could raise its temperature enough to cause burns to the diver. |
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Another way of looking at it is that the force due to atmospheric pressure over the frontal area of a human is about 100,000 N; an unbalanced force only a third of that would be lethal. So a diver who bellyflopped with enough force to produce significant compression of the air bubbles would be unlikely to have complete bollocks, to say the least. |
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//Compression of unconfined air is mostly
insignificant except at speeds approaching the
speed of sound. If you think about it, compressing
a bubble to about 3/4 of its original size could
raise its temperature enough to cause burns to
the diver.// |
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We have a nested-bollocks situation here, I think. |
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Consider a bellyflop onto water. The large area of
contact means that there is huge resistance (since
the mass of water which has to move out of the
way is large). It's also pretty clear that the force
between the water and the diver's fleshy belly is
quite high: that's why it stings! |
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Now consider the idealized situation where the
diver hits a (say) 5mm thick layer of water
supported entirely by air. The inertia of the water
that has to move out of the way (the 5mm layer)
is now much less. |
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With bubbles, the situation is intermediate
between the first (regular water) and the second
(idealized) case. But, basically, the mass of water
displaced by the initial impact is reduced. High
divers use air bubbles for a very good reason (but
not, as I think we agree, surface tension). |
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Next, regarding "compressing a bubble to about
3/4 of its original size could raise its temperature
enough to cause burns to the diver." This is, with
all due respect, not just bollocks but an entire
reproductive system. Compressing air to 3/4 of its
volume will indeed raise its temperature to
something approaching 100°C; but the air bubble
is in a mass of water with a vastly greater thermal
mass. |
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I'm aware that the water surrounding the bubble would cool the air; that's why I said could, rather than will. But human flesh has a conductivity and thermal mass not radically different from that of water... |
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This is something that could be tested. Introduce some small air-filled balloons along with the bubbles. Film the impact. Carefully analyse the resulting distortion of the balloons. I predict that those close to the diver will be radically distorted in shape (allowing some water to move out of the way), but will only very slightly decrease in volume. |
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- Low density and low viscosity of air: very important. |
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- Compressibility of air: Very minor importance; possibly insignificant. |
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- Surface tension reduction: Utter bollocks. |
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//But human flesh has a conductivity and thermal mass not radically different from that of water...//
But you seem to be confusing heat and temperature... |
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Not really, I'm just wildly glossing over the details. The point is that significant compression of a pocket of air would create a significant temperature rise. If that heat is not noticed because it is immediately absorbed by the water, then by the same argument, it would be absorbed by flesh, creating at least some momentary heating of the skin surface. |
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My argument is that significant compression does not occur, so there is no need to explain where all the heat goes. I consider that the most parsimonious explanation; it is consistent with observation, and with the usual behaviour of subsonic fluids. The absence of observable heat supports my thesis (admittedly weakly) in that the assumption of significant compression requires an explanation for the missing heat, and is therefore less parsimonious. |
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To put it another way, I feel that the "compression is significant" thesis requires some supporting evidence before the simpler alternative is rejected. |
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//Compressibility of air: Very minor importance;
possibly insignificant.// |
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I disagree. Consider the case of a person falling
onto a slab of solid rubber, or onto a slab of
closed-cell foam rubber. It's pretty clear that, in
the latter case, the cells become significantly
compressed. When the diver hits aerated water,
it's safe to assume that the mass of the water will
stop it moving out of the way over a short
(milliseconds plus) timescale, and it will behave
quite a bit like foamed rubber on this timescale. |
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Put it yet another way: suppose I have a vertical
pipe full of water. The pipe is closed at the
bottom by a thin, burstable membrane. A slug of
air is travelling up the water column and is mid-
way up, when I suddenly drop a heavy piston onto
the water. The bubble will be greatly compressed
(on a short timescale), before its elastic
expansion transmits the piston's impact to the
lower part of the water column, bursting the
membrane at the bottom and allowing the water
to move out of the way. The force experienced
by the piston at the moment of contact will be
due to the inertia of only the upper (above-
bubble) part of the water column. It's all about
inertia and
elasticity. |
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As for temperature changes: suppose that the
bubbly water is 50% bubble and 50% water.
Suppose also that the bubbles are compressed to
3/4 their original volume, in a layer 10cm thick
beneath a bellyflopping diver. Suppose also that
the surface area of the diver hitting the water is 1
square metre. In this case, 0.025 cubic metres of
air is heated by (roughly) 100°C. |
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The heat capacity of air is roughly 1J/g/K. The
mass of 0.025 cubic metres of air is roughly
0.025kg (25 grams), so the 100°C rise equates to
about 2500 Joules. This is the energy which would
be released by burning about 50 milligrams (a
raindrop) of petrol. Wherever this energy goes
(and it will all go into the water, or be stored in
the compressed bubbles until they re-expand), it's
not going to parboil the diver. |
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But, [Max], compressing that air to 3/4 of its original size corresponds to a force of at least 25,000 Newtons (more if the compression is not completely adiabatic), which is borderline survivable - equivalent to momentarily bearing the weight of a small lorry. That's what I've been saying all along. Collisions that produce significant compression of air either involve confinement of the air, or extreme speeds, and are nasty in either case. |
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[Edit] OK, the lorry comparison was unfair, as it suggests a crushing force rather than an acceleration. But a 25 g acceleration is rather extreme (though mostly survivable). |
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//But, [Max], compressing that air to 3/4 of its
original size corresponds to a force of at least
25,000 Newtons// |
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True, but where did the 3/4 come from? |
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Do this thought experiment. Bellyflop onto a
bouncy castle. Does that kill you? Methinks not.
OK, you can argue that the bouncy castle has
vents to let air out. So imagine bellyflopping onto
a big sealed, flexible (but inelastic) bag of air.
Not painless, but
gentler than bellyflopping onto wet, slappy water. |
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My points were that (a) surface tension is
irrelevant (as we agree) (b) air bubbles will reduce
the mean density of the water, allowing it (ie, the
froth) to move out of the way faster than pure
water would and (c) air bubbles are compressible,
and will therefore additionally absorb some of the
abrupt shock resulting from the belly/surface
impact. |
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If you really want experimental data, I have a
jacuzzi somewhere in the south-east wing, and my
intercalary twin is a notoriously inept diver who is
always in need of finances and hence can be
bought quite cheaply. |
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"We have a nested-bollocks situation here, I think" |
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I've had some scuba dives that may as well have been roller coaster rides. Surge, currents and surf make some most interesting conditions for gymnastic scuba events. |
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Seconded.
I had to strip off all of my gear in about a fifteen foot sandy opening in a reef at 23 meters instead of 18 while the surge slammed me against either side of the coral walls.
My instructor thought I was nuts but it was the last day of my vacation and I wasn't going to get another chance so I talked him into it. |
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Quite a gymnastic scuba roller-coaster indeed... and what a rush knowing that I'd have to decompress at that depth if anything went wrong. |
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You guys are nuts. You could get killed doing stuff like
that. |
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Yeah, I should probably stick to riding motorcycles in deer-infested territory. |
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Or snowboarding, whitewater paddling, home-Halfbaking...
You know, low-risk activities. |
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