From: prb@access.digex.com (Pat)
Newsgroups: sci.space
Subject: Re: Acceleration
Date: 27 Dec 1992 18:04:22 GMT
Organization: UDSI
Distribution: sci
NNTP-Posting-Host: access.digex.com

In article <1992Dec22.220405.26976@wuecl.wustl.edu> gene@wucs1.wustl.edu (_Floor_) writes:
>In article <BznC82.74x.1@cs.cmu.edu> roberts@cmr.ncsl.nist.gov (John Roberts) writes:
>] That applies to things that are somewhat resiliant (like humans with their
>] limbs not locked), because if deformation continues throughout the period
>] of acceleration, then the entire body is not really subjected to the full
>
>Hmmm...you think maybe rigidity has something to do with this?
>If something is rigid, it is much more likely to break than something
>flimsy, which will bend.  Electronics certainly canot be built in
>a manner that will bend. Any flexing of the probe would have to
>be somehow accounted for in the design.
>

Certainly electronics can be built to be flexible,  it's just what degree
of flexibilty you desire.  Flex is a stress/strain relationship.
Steel is flexible, rubber is rigid.  you just need to define these terms
first.  besides, if you build with amorphous materials,  you can get
quite a flex out of silicons.

and i believe the designers understnad the material characteristics
of their probes quite well.


>] But other than that, and factors such as prolonged stress on human hydraulic
>] systems, the greater problem can be with rapid changes in acceleration, which
>] are of course associated with short bursts of acceleration. (I believe the
>] usual term for the time derivative of acceleration is "jerk".) These rapid
>] 
>


Actually,  i think the term is Impulse.


>You're joking me if you think the Galileo probe will experience constant
>deceleration. There's going to be buffeting worse than we could imagine,
>I imagine (:-). Especially at speeds many times that of sound (which I'm
>sure will be different for the Jovian atmosphere)! So you're point is
>very applicable. Experiencing this jolting for milliseconds (as per
>a dropping watch) may not cause any damage. But if you dangled the watch
>from the ceiling and proceeded to place a jackhammer at its face,
>slamming into its face for a couple of minutes, liklihood is that
>the watch will no longer function! Ditto for an atmospheric probe.
>That thing is going to get one whale of a beating. You've helped me
>emphasize my point even more! Thanks :-)
>
>


Hopefully this kid will take a physics class.

I think he is mistaking  Work with Force and energy.

Work is force through a distance,  Energy is work*time, Force is mass*Accel
( boy i hope i got these right :-) )

It takes energy to achieve a momentum change.



A probe has high momentum hitting atmosphere.  it gets  a high acceleration,
on a small mass.  not a lot of force,  exerted through several miles of
atmosphere,  for a few minutes.

I think the kid is missing the fact that while the accelerations of dropping 
a watch and hitting it with a sledge are the same, the work products are significantly  different.  

Try this.   drop a timex.  work out the acceleration.

Now,  hang the timex from a string.   Let a pendular mass strike it, at low spe
ed.  work out the acceleration.  keep increasing the mass and speed.
continue until the timex dies.   I suspect you will be surprised at how
high you can go.  

Halting a 5 lb sledge witha watch is a major momentum change, hence mucho
work in a millisecond.  

Conducting momentum transfer via pendular masses, is much less work.
you can simulate this with that desk toy, using  pendular ball.
tape a timex on to one of the balls.  it should survive.