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.-- bob, Dec 09 2018 Even more airbags, as requested https://www.youtube...watch?v=L_Gp7zSJQQwPrototype This! personal airbag test [notexactly, Dec 12 2018] For [8th of 7] https://xkcd.com/478/Stapling... [neutrinos_shadow, Dec 17 2018] // the airsack would reduce the impact of the fall by half. //
How, precisely ?
To reduce the impact, the system must dissipate energy (and consequently also momentum, though they are different things).
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" ?
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.
Now, kinetic energy is given by 0.5 * m * V^2.
So, class, what is the kinetic energy of the human, and how much energy must be dissipated to halve the impact ?
Anyone ?-- 8th of 7, Dec 09 2018 <raises hand> Ooh, ooh, me, me!-- neutrinos_shadow, Dec 11 2018 Very well, over to you.
<hands whiteboard pen to [n_s]/>-- 8th of 7, Dec 11 2018 We all know I could do a bang up job of answeringtthis but Neutrios put his hand up first.-- bob, Dec 11 2018 Yes, he did. Now sit quietly until he's done. If he gets it wrong, you can have the next go.-- 8th of 7, Dec 11 2018 // or otherwise has no pressure on the footbed, for a few seconds, //
Does this mean it will activate any time I put my feet up on a footstool?-- scad mientist, Dec 11 2018 Impressive work on that whiteboard, [neut]; the likeness is uncanny.
And let's say about 1400 joules, of which 700.
So ... ?-- pertinax, Dec 11 2018 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.-- notexactly, Dec 12 2018 Even more airbags, then.-- 8th of 7, Dec 12 2018 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...)-- neutrinos_shadow, Dec 13 2018 Well done. You are awarded a gold star.
CLUNK.
<Attaches paper star to [neut]'s forehead using a staple gun/>
So, now we have established how much energy must be dissipated.
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.
<hands whiteboard pen to [bob]/>-- 8th of 7, Dec 13 2018 // legs are usually bent to avoid breakage //
If they aren't bent before impact, they generally are afterwards, and not at the pre-installed hinge points.
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 ?-- 8th of 7, Dec 14 2018 //why not fit a pneumatic shock absorber//
You might be onto something there, how about extending the concept to some form of deployable airbrake?-- bs0u0155, Dec 14 2018 Airbrakes become much more effective at higher airspeeds; at low speeds, they have to be very large to have any effect.
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.-- 8th of 7, Dec 14 2018 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.-- Voice, Dec 16 2018 random, halfbakery