h a l f b a k e r yCompound disinterest.
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Shoes with self-filling air sacks between the heel and the
footbed. noIn normal wear the weight of the wearer would
ensure the air bladder would be empty, and the shoes
would
function as regular shoes do. however if a person jumps or
falls, or otherwise has no pressure on the footbed, for
a
few
seconds, springs within the bladder would cause the airsack
to fill. then the person does fall/land the airsack would
reduce the impact of the fall by half.
Even more airbags, as requested
https://www.youtube...watch?v=L_Gp7zSJQQw
Prototype This! personal airbag test [notexactly, Dec 12 2018]
For [8th of 7]
https://xkcd.com/478/
Stapling... [neutrinos_shadow, Dec 17 2018]
[link]
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// the airsack would reduce the impact of the fall by half. // |
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To reduce the impact, the system must dissipate energy (and consequently also momentum, though they are different things). |
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Since the gas in the inflatable sack must obey PV=RT, and V is diminished by compression, P and T must increase in proportion. This indeed absorbs some energy, but "half" ? |
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Taking a 70kg human as the starting point, and applying V^2 = U^2 + 2*F*S, rearranged to give V for zero intital velocity and a 2 metre drop i.e. V = SQRT (2 * 9.8 * 2) gives a V on impact of 6.26 m/s. |
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Now, kinetic energy is given by 0.5 * m * V^2. |
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So, class, what is the kinetic energy of the human, and how much energy must be dissipated to halve the impact ? |
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<raises hand> Ooh, ooh, me, me! |
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<hands whiteboard pen to [n_s]/> |
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We all know I could do a bang up job of
answeringtthis but Neutrios put his hand up first. |
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Yes, he did. Now sit quietly until he's done. If he gets it wrong, you can have the next go. |
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// or otherwise has no pressure on the footbed, for a few
seconds, // |
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Does this mean it will activate any time I put my feet up on
a footstool? |
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Impressive work on that whiteboard, [neut]; the likeness is
uncanny. |
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And let's say about 1400 joules, of which 700. |
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Airbags will (at least in theory) reduce the peak acceleration by
extending the duration of the landing, but that neglects the
impulse absorption and balancing functions of the legs. The legs
already serve the purpose of spreading out the impulse in time to
reduce the peak acceleration; I don't know if the airbags will
interfere with that by changing the legs' perception of the
beginning of the landing. What I do expect for sure is that the
airbags will make balancing during the landing far more difficult.
You will probably end up rolling off the side of the airbags, and the
side of your body doesn't have the same ability as your legs to
safely absorb impacts. |
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Sorry I'm late back...
I don't know where you got the "F" and "S" from; if you use
"a" for acceleration and "d" for distance, you get (assuming
u=0) KE = mad, which is much easier to remember.
So, 70kg of human accelerating at 9.8m/s2 for 2 metres is
1,372J.
(Lucky this isn't a whiteboard, you probably wouldn't be able
to read my handwriting...) |
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Well done. You are awarded a gold star. |
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<Attaches paper star to [neut]'s forehead using a staple gun/> |
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So, now we have established how much energy must be dissipated. |
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If you look up the specific heat of atmospheric air, and then apply the General Gas Law, it's possible to estimate values for delta-P and delta-T. |
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<hands whiteboard pen to [bob]/> |
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// legs are usually bent to avoid breakage // |
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If they aren't bent before impact, they generally are afterwards, and not at the pre-installed hinge points. |
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So rather than putting the energy absorbing element on the shoes, why not fit a pneumatic shock absorber along the back of the leg between pelvis and ankle, taking a portion of the load from the muscles and bones ? |
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//why not fit a pneumatic shock absorber// |
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You might be onto something there, how about extending
the concept to some form of deployable airbrake? |
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Airbrakes become much more effective at higher airspeeds; at low speeds, they have to be very large to have any effect. |
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A reaction system, perhaps using water expelled through a nozzle by a pyrotechnic gas generator, would give a "zero-zero" capability, and might possibly be almost acceptably safe. |
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To soften a fall, simply liquefy the surface beneath the faller. If it's already liquid, reduce its surface tension. If that's not enough you're probably falling through air and you'll be better served by an ablative heat shield. |
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