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[Re-written as an 'invention', but really a
question!]
Those two well-known superheroes -
Very Fast Falling Man and HalfBakerMan,
are standing on top of a 500m building,
on a calm windless day. On a whim,
HalfBakerMan grins at VFF man, says
"race you to the ground!" and steps off
the
edge. HBMan wastes no time - he
drags his handy Negative Glider to the
roof's edge, climbs in, and lets it tip over
the edge in pursuit of the falling VFF
man.
VFF man, seeing HBman in hot pursuit,
morphs into the most streamlined
possible shape, and whistles flechette-
like [thanks Klaatu] downward. HB man
in his (quite streamlined) negative glider
is following but losing ground.
Suddenly, HBman's glider executes a sort
of swooping motion away from the
building, and rapidly diminishes into the
distance. "Aha!" cries VFF man "he's
doomed! He has foolishly traded some
of his vertical speed for horizontal
motion, and is now sure to lose!"
Then to VFF man's dismay, he watches as
HBman's glider executes a nifty half-
barrel roll. A split second later, HBman
yanks back on the stick with all his
might, snapping his glider into a steep
dive. In an instant, he has lost his
horizontal motion but is now hurtling
toward the ground at an alarming rate.
The gentle puff of dust, followed a few
moments later by the sound of a
splintery thud, tells VFFman that he has,
once again, been outwitted by that
devilish brainbox, HBman.
"Gosh" thinks VFFman "How did he do
that? What possible combination of
angles and aerodynamics could he
possibly have used? Did he cheat, I
wonder?"
Tune in next week to find out if HBman is
a no-good cheating dog, or if some trick
of aeroynamics and angles made it
possible for his negative glider to beat
VFF man to the ground.
Overbakery link
http://groups.yahoo.com/group/overbaked/
[gnomethang, Mar 13 2005]
Positive Slingshot Shot
Positive_20slingshot_20shot
Going up? [baconbrain, Mar 16 2005]
Thermodynamics
http://farside.ph.u...ectures/node47.html
For those of you who are interested [scubadooper, Mar 16 2005]
Vaporator
Vaporator_20airflow_20smoother
Could water droplets decrease air resistance even further? [bungston, Oct 21 2005]
[link]
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During WWI and now, with the current-day "beehive round", fleshettes have been used effectively. I have read that during WWI, boxes of sharpened darts were dropped over enemy lines from aircraft and that the darts could penetrate from head to foot despite helmets. |
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I guess that you are asking for a rocket-shaped aircraft aimed at the ground? Or, am I missing the point somewhere? |
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Flechettes, shirley! - French for darts or little arrows. I suspect that the answer is that the fastest shape for any given mass is the one that offers the least air resistance - the teardrop is pretty good but I don't know if the design has been improved upon. |
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No, this is different. A flechette is just a
streamlined shape designed to fall with
minimal air resistance. I'm asking for
something which uses aerodynamic lift
in a sort of upside-down way to
accelerate its descent (like a glider uses
lift directed upwards to slow its
descent). So, I'm asking for something
that would fall faster than streamlining
alone would allow. |
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(I'm sure it can't be done, but I can't
think of a concise reason why not, and
it bugs me). |
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Sorry! - that was what I was trying to say!. When falling, you have gravity pulling you down (which is the same for everything!) and air resistance trying to slow you down. There is no 'negative' buoyancy that can be created. As you said, it's just a thought but the fact remains that unless you power your device then the limit for terminal velocity is the limit on air resistance and associated drag. |
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If you have an upside down wing,
and somehow fling it horizontally
within an atmosphere,
it will fall faster than an object of
the same general dimensions that
is also flug horizontally in the
same manor. It will not fall faster
than an object accelerated by
gravity in a vacuum. Note that
race cars use upside down wings,
aka spoilers, to keep them from
flying out of control, so the effect
is not unexploited. |
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The answer's no. The negative lift produced is proportional to the horizontal speed, and hence is effectively limitless (unlike weight, which is fixed). Hence you can accelerate downwards faster than gravity if you give it a high enough horizontal starting velocity (although it will fall more slowly as it trades this energy for downwards motion). |
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Drop it vertically, and the only energy it has is potential energy mgh, so the kinetic energy it can get is strictly limited, and hence max velocity is limited. |
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Hmmm lots of intersting
responses but do they add up to the
answer I was looking for? :- |
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[Gnomethang] - I appreciate the
balance between G and air resistance,
but that doesn't account for the
possibilities of aerodynamic lift, if
directed downward. (By your argument,
the model glider would take the same
time to descend whether it was 'gliding'
or just sinking vertically in the same
orientation, whereas in fact the gliding
path will slow its descent due to the lift
caused by the wings moving forward,
errrr, if you follow!). As [JakePatterson]
points out, a forward-moving upside-
down wing would fall faster than a wing
falling down-but-not-horizontally,
which is of course true (the wing's "lift"
is acting downwards). |
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[David Scothern] points out that you can
accelerate downwards faster than
gravity given sufficient forward velocity
(which also makes sense - you can get
negative G's in a glider if you push the
stick forward at high speed). He also
points out, though, that with an initial
vertical drop, any horizontal
acceleration is going to be derived from
its gravity-driven vertical acceleration (I
mean you're stealing from one to
provide the other). |
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HOWever, it's still not clear to me that,
with the right aerodynamics, you
couldn't drop vertically, swoop out
horizontally (like the regular glider, and
yes, losing vertical speed), and use the
horizontal movement to provide a
down-thrust (I mean downward ;'lift')
which would more than make up for the
loss of vertical velocity caused by the
'swoop'. |
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(OK, sorry to go on. I 'know' intuitively
that this shouldn't be possible, but I still
can't see which laws of physics make it
impossible, and it still bugs me! I'm
just muttering out loud, so feel free to
ignore and thanks to all for your
thoughts!). |
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[Base] I think that Dave sorted it out quite well. The upside down wing will only work on a self powered machine due to the fact that it needs rather large horizontal velocity to allow the 'down' bit to work. We all agree that this will be self defeating in the long run since the horizontal velocity means that the damn thing isn't dropping - more to the point, when it does drop, it will drop slower than a teardrop shape due to the above.
Having said all this, we are not in the right forum. May I suggest that we remove this and repair to the Overbakery? (Link) |
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Good lord [gnomethang] - I had no idea
there was such a thing as an
Overbakery! Still not utterly
convinced but I didn't want to drag this
out, so I'll just keep mulling.... |
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I'll delete this tomorrow if OK - just
wanted to leave this up long enough to
say thanks to David S, Jake P and
yourself. |
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I think that a device such as Basepair describes could be made. The secret is that it changes shape. Imagine a falling mass with large wings angled such that as it falls, air resistance turns it - not unlike a helicopter elm seed. It will fall slowly since it is storing gravitational pull as rotation. |
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Then it pulls its wings in to become very short. Or it drops them completely. Like a skater pulling into a tight spin, the speed of rotation will increase as the area rotating decreases. If it deploys a propeller at the rear, it can convert this rotation into forward propulsion and fall faster than gravity would otherwise pull it. |
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Is this a self powered machine? Is shapechange cheating? I think this is fine HB material, by the way. If [B] had proposed to invent this thing and suggested a design, the discussion could continue exactly as it has. It would be nice to leave this around long enough for [Worldgineer] and some of the other folks who like this stuff to groove on it some. |
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The conversation is handling its elf quite nicely, but I just wanted to show up and play with all the others, forgive me....... |
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"I've fallen, and I can't pick up more speed" |
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Another idea re the negative glider. If there were a strong lateral wind, I wonder if the glider could be constructed with a sail such that sidewinds accelerate it downward. Terminal velocity for a random shape is about 120 mph. I am pretty sure that winds that strong or stronger exist at high altitudes. |
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The amount of gravitational potential energy an object has is equal to its weight (in newtons) times the distance it can fall. The amount of kinetic energy an object has is equal to half its mass (in kilograms) times the square of its velocity. |
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An object weighing one newton positioned where it can fall 4.9 meters will have 4.9 joules of potential energy and zero joules of kinetic energy. When the object hits bottom, absent any air resistance, it will have 0 joules of potential energy and be travelling at 9.8 meters/second. Because the mass of an object weighing one newton is (1/9.8)kg, such an object will have a kinetic energy of 0.5*(1/9.8)kg*((9.8m/s)^2), which when all is said and done translates into 4.9 joules. |
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For an eblect to exceed the speed it would have reached falling in a vacuum, it must have an energy input somewhere. |
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Based on some comments above, I'll
leave this here for now (unless anyone
else strongly thinks otherwise?) but will
re-phrase it as an invention, useless
though it may be. |
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I guess in-flight shape changes are
allowed. And I agree that it would be
impossible to achieve a fall-rate greater
than that ina vacuum (since you'd
violate energy conservation). But the
question revolves around air resistance
and the use of aerodynamics - can a
negative glider fall faster (overall) than
an optimally streamlined shape in air.
As has
been pointed out, any 'downthrust'
must come from a horizontal motion,
which can only be achieved in the first
place by 'stealing' from the vertical
downward motion. So, the question is,
is there an absolute law that says that
you can't gain more from the
downthrust than you "stole" to achieve
the horizontal (or rotational, in
Bungston's anno) motion? As long as
we don't accelerate faster (overall) than
in vacuo, I don't think we're breaking
any conservation laws (???). |
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Here is a third idea for how a glider might accelerate its fall. It can do so by decreasing air pressure in front of it. The Bernoulli effect states that air moving across a surface decreases air pressure - this is the principle which airplane wings use to create lift. The faster the speed, the lower the pressure. |
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If the front of the falling bomb spun very quickly, it seems to me that the relative airspeed of air traversing the bomb front would be greated: the air already rushing past as the bomb falls, plus the relative difference between the spinning front and the rushing air. The air moving past the back of the bomb moves relatively more slowly: just rushing past. |
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I propose that the relative difference will serve to decrease the pressure in front of the bomb, thus "pulling" it down faster. |
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From what I understand, air friction has two components: (1) friction caused by a surface moving perpendicular to itself and thus displacing air; (2) friction caused by a surface moving parallel to itself and thus 'sliding' on the air.<p> |
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It would seem like an object which had spiral fins might be able to achieve a reduction in the first type of friction component (which, from what I understand, increases as the square of velocity) in exchange for in an increase in the second (which increases linearly with velocity, up to a point). Not sure whether anything works out to be a practical benefit, though. |
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It would take some serious math to figure that out. |
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[Bungston] - I had never thought of a
rotating object as generating Bernoulli-
style lift (in this case, directed
downwards), but I can't see why it
wouldn't.
Does anyone know whether it would?
Or if not, why not? |
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If such rotation would generate the
expected Bernoulli-derived force, then
the other question is what drives the
rotation of the nose-cone. The device
has to be unpowered, so are you
imagining the spin to be driven turbine-
like by the fall? And, if so, would the
drag generated by the turbine outweigh
the Bernoulli force? |
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If a spinning disc or nose-cone
generates lift, then I've learned
something new - thank you! |
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[Supercat] - If I've read correctly, you're
evisaging a spiral-finned 'bomb' which
would sort of screw itself through the
air? I agree the maths would be
nightmare. I suspect that the forces
needed to accelerate the bomb into a
spin (which presumably have to be
robbed from its downward speed)
would cancel the gain, but I don't know. |
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I'm not sure that the fins (or the
'screwing' motion) would reduce the
amount of air displacement, though. |
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Thanks for thinking of me, [bung] - I immensely enjoy ideas like this. |
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(responding to the original idea) Let's walk through what would happen to an upside-down glider. First, you're right, inverted lift would pull you strongly toward the ground. Air is moving past your wing in a nearly horizontal direction, creating a vacuum at the top side of your wing (well, normally top side - the side currently closer to the ground), pulling you downward. As you accelerate downward, however, the direction of air movement shifts. In normal flight you aren't climbing at a high rate - you're just battling gravity and air is moving horizontally over your wing. However, in upside-down gliding you'd quickly have air moving faster in the vertical direction than horizontally. Any vacuum you've created on the top side of your wing will be filled with this air. |
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(Re: spinning bomb) I don't really see how this case would be any different than the non-spinning version, but it's possible that I'm missing something. |
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I've got to put my money on VFF Man tasting pavement first. |
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HB Man's glider isn't going to be able to gain any horizontal velicity without fist trading downward velocity. i.e. You can't 'swoop' without slowing your downward fall. |
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So if a swoop converts downward motion for horizontal motion, an anit-swoop, will simply do the opposite, another conversion. |
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I was thinking that if air-resistance is what creates a terminal velocity - to exceed the terminal velocity you need to either
a) reduce the air-viscosity or
b) use some additional form of accelaration
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A super-heating laser might rarify the air in front of you thinning it out, reducing the pressure against which you are falling - though, it might equally expand the gasses in an explosive way, providing more resistance. |
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A spinning bomb will not travel more quickly unless energy is imparted upon it at the start of the fall by some external force. |
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Think of a screw. Put it, point down, on a block of wood. Does pushing down on it cause it to automatically rotate and bury itself in the wood? No, it must be torqued. |
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A spinning bomb would be the same. Spinning a vaned bomb *prior to drop* would cause it to descend more quickly than a rotationally-static bomb. Letting a vaned and unvaned bomb drop at the same time would cause the vaned bomb to drop more slowly, because energy would be disappated as friction rotates the bomb. |
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A spinning vaned glider would cause you to fall faster than gravity would allow, because you're pushing yourself through a fluid using some extra force that a falling body does not have, in addition to gravity. |
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I don't get why there's so much discussion about this. |
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F=ma, without applying an outside force you're restricted to falling at the rate of gravity. All forces created by any glider are derived from gravity - it's only by falling through the airstream and converting that into a horizontal movement that the glider generates lift. Any attempt to generate an additional force in a direction towards the ground will slow the fall of the object. |
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Rotating the airstream produces a force perpendicular to the axis of rotation, so rotating the nose cone will not 'suck' the object along. |
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If you want to fall faster than VFF man, strap a jet pack to your back! |
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<edit> Marked-for-Deletion amounts to a perpetual motion machine </edit> |
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I've read through about half of the annotations (does that make me a half-half-baker?) but all the "this might work" methods amount to bad science. |
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If you're starting from a zero speed drop, the entire trip down is trading gravitational potential energy (GPE)for kinetic energy (KE), and dissipating some of this energy via drag. |
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Anything other than a purely vertical drop will trade this GPE for some other form of KE. This will be a lossy process. Again, any process done to convert this non-vertical KE back into vertical KE will be lossy. |
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Anything you do to rob energy from the flow will produce a net loss in final energy. That's about as succinct as I can put it. |
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Sorry, I have to call this bad science. |
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/Rotating the airstream produces a force perpendicular to the axis of rotation, so rotating the nose cone will not 'suck' the object along./
- OK, that answers the spinning nose cone idea. |
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/Anything other than a purely vertical drop will trade this GPE for some other form of KE. This will be a lossy process. Again, any process done to convert this non-vertical KE back into vertical KE will be lossy./
This answers the shapeshifting glider idea: with the entire trip down considered as a whole, this thing will be slower than something which just drops. Essentially this is just a back door to powered flight. |
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I still have not seen my sail idea shredded. It is again, essentially powered flight, but does not require any moving parts, stored energy or the like, and takes advantage of common, real world conditions. I know that sailboats and especially iceboats can sail several times faster than the wind but I am fuzzy about how this works. Could a sail be used to accelerate the glider downwards? It would increase air resistance, but this is true of any sail. |
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FreeFall: It is certainly clear that an object falling through still air cannot hit the ground with a velocity faster than it would have if it were falling in a vacuum unless it expends energy from some internal source. What isn't quite so clear is whether an object might be constructed to hit the ground faster than would a teardrop-shaped object of the same mass and density. |
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//I still have not seen my sail idea shredded// Sorry we missed it. What allows sailboats to work is a speed difference between different media (air and water). Even if there is a strong wind, a sail would soon be moving at the same speed as the wind. At that point, the sail would feel no wind. Think of a kite - the only reason it flies is because you're holding it on a string, connecting it to the other-speed-medium (the ground). If you let go, it will travel along with the wind and simply fall from the sky. |
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[worldengineer] Could you have the sail-faller run on vertical rails on the side of the building? It's horisontal motion is then zero, alowing it to use the horizontaly moving wind. |
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Yes, it is possible to have a device hit the ground faster, but not sooner. Have it store the energy as (say) spinning, and then use that energy to hurl downward. |
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[World] you're right, but sorry it's not the best explanation. |
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What makes a sailboat able to travel faster than the wind is the fact that it's keel (or daggerboard) provides a propulsive force as it travels through the water (which as [World] said is moving at a different speed and direction to the air.) That motion through the water is generated by the movement of air over the sail. The fact that the boat is moving generates an apparent wind direction and speed that is a combination of the actually wind and a component equal to the motion of the boat in the opposite direction to that of the boats travel. If the boat is traveling across the wind (on a broad reach) the component due to the boats movement can be considerable. |
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[supercat] without applying an outside force you won't get anything to fall faster than an optimised shape |
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//rails// Ha! That would do it. |
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/sails/ so as I proposed it, the thing would be a like a kite with no string. Putting it on rails down a building makes the building analogous to the water, and so it could sail down the building. |
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I wonder if a gyroscope and its resistance to position change could be substituted for the substrate in such a vehicle? Could you sail a falling gyroscope into the ground? |
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//[supercat] without applying an outside force you won't get anything to fall faster than an optimised shape// |
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That may be true, but a conservation-of-energy argument would not imply it. |
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Actually, I would think it at least theoretically possible to have a device which would attempt to turn some of the kinetic energy of the object into some other form during much of the fall, and then convert it back into kinetic energy immediately before landing. Although in most cases the energy losses in this process would outweigh any "improvements", that would not necessarily be true for an object that was dropped from sufficient height. |
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In particular, if an object is dropped from a height much higher than what would be needed for a streamlined object to reach terminal velocity, it would be possible for the device to capture energy for awhile, then switch to a streamlined shape, get close to terminal velocity by free-falling, and then use some of the captured energy to propel itself down faster than what would otherwise be terminal velocity. |
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/supercat/ - this is similar to my rotating glider that extends a propeller to use the captured energy. Do you think that a device such as you mention could beat an optimized teardrop to the ground? |
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///supercat/ - this is similar to my rotating glider that extends a propeller to use the captured energy. Do you think that a device such as you mention could beat an optimized teardrop to the ground?// |
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There are two things one may be trying to do to "fall faster" than a streamlined shape: |
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-1- Reach the ground sooner |
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-2- Hit the ground at a higher velocity. |
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I don't think it's possible to beat a streamlined shape by criterion #1. #2, however, would probably be possible in at least some cases using the method I described. |
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Just to clarify, I'm agreeing with the
[slight] majority that this is *probably*
impossible, but am puzzled as to
exactly why, and think it's interesting to
find the absolute reason (if any). See
below - I don't see a simple way to
prove impossibility using conservation
laws... |
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[zen tom] "So if a swoop converts
downward motion for horizontal
motion, an anti-swoop, will simply do
the opposite, another conversion." |
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- I agree qualitatively, but it is not
clear to me that the two conversions
will be *equal* and opposite,
necessarily. For one thing, air
resistance is more-than-proportional
to relative air speed, so there should be
some room for juggling there, given
that the two maneuvers could take
place at different points in the fall and
at different speeds, perhaps. Re. using
laser to 'thin' the air ahead, this would
only be allowable if it derived its power
from the fall itself (eg, some sort of
turbine generator), in which case I
presume (but don't know) that the
additional drag from the turbine would
outweight the benefit of the laser. |
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[shapu] re screws: "does pushing down
on it cause it to automatically rotate
and bury itself in the wood" well, yes it
will if the pitch of the thread is OK
(there are 'nailable screws' which to
exactly this, with no applied torque).
However, I do agree that without an
external torque, the turning has to be
driven by the falling and so (I think)
would not be any net advantage. |
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[scubadooper] "amounts to a
perpetual motion machine":
- if this were all in a vacuum, I would
agree absolutely. In vacuo, there is a
straight conversion of potential to
kinetic energy, and all objects will fall at
the same rate, end of story. HOWever,
this is *not* in a vacuum. All falling
bodies in air lose some energy to air
resistance (and hence land later than
they would in vacuo), but I don't think
there is any conservation law that
dictates how much energy has to be
lost. So, the question is whether you
can design something which will lose
less energy to air resistance than an
optimally streamlined shape. So, it's
*not* excluded immediately on
conservation/perpetual-motion
grounds. |
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[freefall] "Sorry, I have to call this bad
science" - I agree that we are trading
horizontal and vertical motions back
and forth, and that each 'trade' will be
lossy (though, in a thought-experiment,
perhaps arbitrarily-little-lossy). But,
see comments to [zen tom] above. I
think you're probably right, but don't
see a rigorous argument (yet) that
shuffling and trading may allow you to
dodge some of the losses due to air
resistance. |
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[UnaBubba] "He tells me that HBM gets
there first, if he knows how to optimise
the "lift" of his glider." Aha! A ray of
hope for HBman! But (playing devil's
advocate) is this true if they both start
with zero horizontal (and indeed
vertical) speed, such that any horizontal
speed has to be 'robbed' from vertical
speed? |
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[Supercat] " don't think it's possible to
beat a streamlined shape by criterion
#1 {faster overall fall}. #2{faster impact
speed}, however, would probably be
possible in at least some cases using
the method I described." I agree that it
would be possible to hit the ground
faster than VFFman (store the energy of
the early part of the fall in any of
several ways, and use this later to
accelerate against air-resistance during
the latter part of the fall), but the real
challenge is #1 (quicker overall). |
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Thank you all for your comments - this
is interesting! |
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HBM will be faster than VFFM, I agree. |
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It has some to do with sailboats but not with if they can sail faster than the wind. HBM falls faster using the same principle a boat uses to sail upwind. |
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Reminds me of thoughtexperiment: Drop a bullet at the same time as you fire a similar bullet horizontally. Which is the first to the groud? |
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<idiots question> that terminal velocity thing... isn't that just applied to objects falling due to gravity alone, as opposed to bullets, or aircraft harnessing aerodynamic thrust? <idiots question> |
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But [Bubs] those forces must be perpendicular to one another at best - which means there should be no advantage to the vertical component. Due to the imperfect conversion of force, the forces will form an angle of less than 90deg, slowing the descent. |
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Also, there are more than 2 forces at work here:
1) Gravity (downward direction)
2) Wind Resistance (in a direction opposite to the vector of motion)
3) Induced 'Lift' (A force perpendicular to motion)
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You have to trade off any increase in 3 by a similar increase in 2. |
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[etherman] I'd associate terminal velocity with sky-divers. It's the point where the forces from wind (air)resistance balance out the accerating force of gravity. Bullets start slowing down from the moment they leave the barrel, so terminal velocity doesn't really apply for them. |
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That may be so <zen tom> but I'm gonna get the video out tonight and get a second opinion fron Prof. Wesley Snipes, if you don't mind. |
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Etherman - Zen Tom is right. Objects
falling in a vacuum will accelerate
smoothly ad infinitum (subject to
relativity!), and hence have no terminal
(ie limiting) velocity. Objects falling in
air (or water etc) eventually reach a
speed where resistance balances
gravity, and hence don't get any faster
than this. (Alternative definition -
terminal velocity is the speed that kills
you when you stop suddenly :-) ) |
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It's the second law of thermodynamics that prevents this. |
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You can't have a shape more optimised than an optimised shape. |
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Gliders derive lift by converting gravitational forces into forward motion and hence air flow over the wing therefore lift. Any attempt to use such lift to reach the ground before VFFM hits the problem of the 2nd law. |
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If the fall were of exceptional height, then the possiblity of storing energy and converting it back to propulsive force, will still fail due to the second law. However, as has been stated it would be possible to hit the ground at a higher velocity than VFFM. |
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I am going to use two words that will betray me as a physics-naive fan of the mobile perpetuum. Wait for it. Before I get there, compliments to [scubadooper] for concise and informative posts. |
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A falling object has a certain amount of gravitational potential energy. At terminal velocity, the acceleration by gravity is balanced by air resistance and the object falls no faster. Windmill-type attempts to capture energy increase air resistance, and are "lossy", so when the fall is considered as a whole, the energy-capturing faller falls more slowly. |
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Air resistance slows the fall by converting gravitational potential energy into heat: of the air and of the object. Get ready for it: Stirling Engine! Yes - the waste heat capturing engine. Friction from passing air causes big heat differentials in a falling object. This would be ideal for a stirling engine which would turn a propeller. |
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The question: can waste heat be recaptured and turned into acceleration without slowing the overall fall. [Scuba]? |
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Wonderful, [bungs]. You should end up with a little tiny push that will put HBMan ahead by a nose. If nothing else you've shown the 'bakers that claim a fundemental law is controlling this are likely wrong (unless there's something I'm missing). |
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If nothing else, just make your perfectly aerodynamic bomb more thermally conductive (try copper, or even diamond), which will condense air a little bit in front while expanding it a little bit behind (compared to the less-conductive design). That should speed you up a little. |
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Bungston! HBman is deeply in your
debt! I suspect that recapturing some
of the 'wasted' heat would indeed give
him the edge. |
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As regards objections based on the
second law and others, I am in a bind.
On the one hand, gut feeling suggests
that it *shouldn't* be possible to fall
faster than the teardrop (except by
using your cunning Stirling engine), but
as [Worldgineer] points out, there isn't
an obvious (to a non-physicist like me)
way to exclude it from simple laws. I
guess the complexity and ambiguity
arises from the complexities of air
resistance (*for example* - I presume
there is no 'simple' calculation that tells
you the fall-rate of the teardrop based
only on thermodynamics, and probably
no rigorous mathematical proof that a
teardrop is the most streamlined shape
for freefalling). |
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Anyway, I think the emergence of your
sterling Stirling idea, and [Worldgineers]
conductive heat-recovery scheme, make
it all worth while :-) |
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I just got done with a rambling anno over on [Basepair]'s Positive Slingshot Shot. I should have read this first. Oh, well. |
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[bungston] way up there you wrote //Like a skater pulling into a tight spin, the speed of rotation will increase as the area rotating decreases. If it deploys a propeller at the rear, it can convert this rotation into forward propulsion // Sorry, it takes work to do the pulling-in. Further on, Bernoulli is about half-baked his ownself, but that's a whole 'nother kettle of worms. The heat-engine bit is dashed clever, and something new for me to think about. Thanks. |
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[Basepair] The idea/invention may be a little bit less than halfbaked, but the discussions are great! |
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It's cheating a bit, but if HBMan could drop a large (teardop shaped?) object a second or so before he dives after it - he might get some benefits from the object's slipstream which, if he timed it right, might give him a slight advantage... |
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Maybe thats HBman's super power. He has massive tear ducts. |
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...and a picture uf L'il Bruddor. |
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[Bungston] that is brilliant! |
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You could use the stolen energy to drive a tiny propellor at the end of the tear drop where the air flow has seperated, thus delaying seperation and making you hit the ground before VFFM! |
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Unabubba, I'd have to disagree with your aeronautical engineer friend. I am also an aeronautical engineer. |
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As many have stated, you can:
a) hit the ground faster
or:
b) hit the ground sooner |
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If you trade off velocity for stored energy early on, let yourself accelerate to terminal velocity during most of the fall, then re-trade this stored energy for additional velocity just prior to impact, you can hit the ground FASTER. However, in the process of collecting the energy early on, you have slowed down, allowing VFFM to pull ahead. VFFM will still hit first. |
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Lift is perpendicular to velocity. Drag is parallel to velocity. Whenever you're creating lift, you're creating drag, and thus wasting energy. |
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Now, if you change the conditions so that you have a purely vertical race constrained by rails, and allow that the two contestants will be exposed to wind (a source of energy), HBM can indeed take advantage of this wind to beat VFFM to the ground. |
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Oh, and gyroscopes do not exhibit positional rigidity. They exhibit orientational rigidity. |
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UnaBubba: "I had intended having lunch
with Pat, on Sunday, to beat more info
out of him." - and I thought thought
experiments were cheap and safe! But
thanks to both you and he for the
thoughts... |
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Note to self: Be cautious about witholding info from [Unabubba], and also about lunching with same. |
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Two new ideas to get to the ground faster:
1: Heat. Hot air is thinner than cold. Perhaps it offers less drag? I propose that if two ideally aeodynamic projectiles were dropped, one glowing hot and the other ice cold, the hot one would beat the cold one down. |
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2: Vapor. Aerosolized liquid apparently cuts back turbulence (link). If the front of the projectile spewed a vapor, it might speed its descent. If incorporating a spewer is just another way to get to self propulsion, what about a projectile made of ice or some other frozen liquid? The friction of descent would melt the nose, allowing a stream of smoothing vapor to flow back over the projectile. |
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The only way to fall fast enough to overtake a perfect aerodynamic shape in a reasonable amount of time is for your object to, somehow, have a lesser drag coefficient than the perfect object. Which is patently ridiculous, since you're trying to improve on something perfect. |
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On the other hand, if we're trying to improve on a real teardrop-shaped object, we could simply add very tiny grooves along our own object to coax the air into flowing more orderly behind it and hopefully reduce the vorteces that rob it of energy. |
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Energy saved in the back there might be greater than energy lost to friction. If it's equal this would likely serve to enhance the stirling engine idea, anyway... |
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