Re: [Phys-l] [tap-l] two questions

[William Beaty <billb@eskimo.com>]

On Wed, 19 Dec 2007, Anthony Lapinski wrote:

> 1. Does anyone know how a Mexican jumping bean works? I believe there is
> an insect larva inside that can jump.

Cydia deshaisiana, see http://en.wikipedia.org/wiki/Mexican_jumping_beans

>  How, then, does the bean rise upward since the forces are internal (and
> thus momentum is conserved)?

If you hold an active "bean," you will feel distinct periodic taps or
twitches on your palm.  I recall that they don't jump well on skin.  The
beans appear to react against a solid surface, and can "jump" a few mm.

I recall a long-running debate on Newsgroups over whether the larva was
directly twitching its muscles, or if it was cocking/releasing some sort
of spring mechanism (or perhaps twanging the silk threads.)  Nobody could
find any info to give a solid answer.  I just checked a few websites and
the situation seems the same: they all ascribe jumping to the larva's
"movements."   (But are they twitches or are they twangs?!)


Video:
  http://www.youtube.com/watch?v=uX3VrkATqzY
  http://www.youtube.com/watch?v=2ykYCb_vPhg
  http://www.youtube.com/watch?v=za7vG-CsY0g
  http://www.youtube.com/watch?v=0hXhyl6AgiQ


Crackpot science experiment:  place some jumping beans in a free fall
environment (Nasa's "vomit comet") and see whether they can still propel
themselves without contacting a surface.  If they can, then violations of
momentum-conservation are possible, and Evolution has discovered how to
harness the effect!  :)



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William J. Beaty                http://staff.washington.edu/wbeaty/
beaty chem washington edu       Research Engineer
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