Quick question on work and energy

  • #26
jbriggs444
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If you have limted space, yes.

The vertical distance covered bringing a falling object to a stop with a uniform upward vertical acceleration will obviously be lower than the vertical distance covered using a profile that is limited to the same maximum acceleration magnitude but which involves any non-zero horizontal acceleration.

The horizontal distance covered bringing such an object to a stop will be zero using a uniform upward vertical acceleration (assuming the object started with zero horizontal velocity).

So... what sort of space limitations are you considering?
 
  • #27
A.T.
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As to the previous posts regarding the bag of liquid, could this be practically utilized as a sort of 'covering'? That is, could a thick shell be made where the body is submerged in liquid, and this would absorb all the acceleration?
James Essig posted an interesting article in another thread:

http://www.esa.int/gsp/ACT/doc/MAD/pub/ACT-RPR-MAD-2007-SuperAstronaut.pdf

When floating in a liquid, and using liquid ventilation mice can stand 3800g for 15min "without any physical impairment" (no loss of
consciousness etc.).
 
  • #28
James Essig posted an interesting article in another thread:

http://www.esa.int/gsp/ACT/doc/MAD/pub/ACT-RPR-MAD-2007-SuperAstronaut.pdf

When floating in a liquid, and using liquid ventilation mice can stand 3800g for 15min "without any physical impairment" (no loss of
consciousness etc.).

Very interesting. 3800g would indicate that with half an inch of deceleration distance, the maximum height one could fall with this setup would then be about 48 meters - not too shabby.

I wonder what a shear thinning fluid would do here? Not out of any application to the subject at hand, just out of curiosity. Would you be able to develop a system where the solid material is placed as a fully bodysuit, and upon landing this shifts to a liquid of appropriate density?
 
  • #29
A.T.
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Very interesting. 3800g would indicate that with half an inch of deceleration distance, the maximum height one could fall with this setup would then be about 48 meters - not too shabby.
Keep in mind that the acceleration for these mice was probably increased relatively slowly. During impacts you have a short peak, which might produce other adverse effects, like shock waves propagating through the liquid, and the body.
 
  • #30
Keep in mind that the acceleration for these mice was probably increased relatively slowly. During impacts you have a short peak, which might produce other adverse effects, like shock waves propagating through the liquid, and the body.

Alright. So, how does one avoid the propagation of shock waves through the body? Literature search hasn't done much for me, or maybe my Google-fu has dropped considerably since the last time I researched this.

If I'm reading this thread right, so far there are established methods to limit the effects of deceleration, stress, and kinetic energy on a body. Therefore, all that seems to be left are shock waves...
 
  • #31
A.T.
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If I'm reading this thread right, so far there are established methods to limit the effects of deceleration, stress, and kinetic energy on a body. Therefore, all that seems to be left are shock waves...
That's a bit backwards. The method limits the stresses themselves (not the effects of stresses) during high acceleration (but low jerk). Shock waves (due to high jerk) are one way to dissipate the kinetic energy of the object. The method is usually to dissipate the kinetic energy by destroying some softer crumple zone, to protect the important parts in a harder shell.
 
  • #32
Back to the question of deceleration: My goal here is to make the vertical distance of deceleration as small as possible without harming the body. To that end, would it be possible to do the following:

-Attach a bunch of tiny, hollow cylindrical 'wheels' to the bottom of the system. Through these are connected a bunch of axial rods which are rigidly attached to the system, and the whole setup is encased in a semi-viscous fluid. The rods are, through some means, made to stay on the side of the tube until the moment of impact. Upon impact, the rods begin rapidly rotating around the inside of the tubes, converting the deceleration into rotational force and slowing down via means of the viscous fluid until they are once again at rest.

Is this a viable means of safely removing deceleration from a system? I'm mainly just trying to convert that idea about the ski slopes from earlier into something mechanical. If this isn't a good idea, does anyone have a better one?
 
  • #33
olgerm
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I thought of this recently, and want to know if there's something I'm missing.

So, say a very fragile object decelerates to a stop from a given velocity. For example, a free-falling object hitting the ground.

But, all the kinetic energy is absorbed through use of some clever mechanics (assume magic for this part).

If no energy was transmitted to the object, and (therefore) no work was done on it, would the fragile object still be harmed?
The "magic" gets that energy. If the magic is sand and falling object is ball, then energy kinetic energy turns to thermal energy of ball and sand. Work is done during the impact.
 
  • #34
CWatters
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Back to the question of deceleration: My goal here is to make the vertical distance of deceleration as small as possible without harming the body.

First you have to determine what you mean by "harming the body" and what the body can withstand before that happens.

Once you have worked out what you mean by that you need to devise a scheme that ensures the body doesn't experience anything worse. For example it might be the case that constant deceleration gives the shortest stopping distance without harm, or some other profile might prove to be better. Then you need to design the system to achieve that.

I've no idea if your proposed system achieves your objective as neither are well specified. There is a lot of science and engineering in springs and damping systems.
 

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