Previous experiments [M. J. Coleman and A. Ruina, Phys. Rev. Lett. 80, 3658
(1998)] showed that a gravity-powered toy with no control and which has no
statically stable near-standing configurations can walk stably. We show here
that a simple rigid-body statically-unstable mathematical model based loosely
on the physical toy can predict stable limit-cycle walking motions. These
calculations add to the repertoire of rigid-body mechanism behaviors as well as
further implicating passive-dynamics as a possible contributor to stability of
animal motions.Comment: Note: only corrections so far have been fixing typo's in these
comments. 3 pages, 2 eps figures, uses epsf.tex, revtex.sty, amsfonts.sty,
aps.sty, aps10.sty, prabib.sty; Accepted for publication in Phys. Rev. E.
4/9/2001 ; information about Andy Ruina's lab (including Coleman's, Garcia's
and Ruina's other publications and associated video clips) can be found at:
http://www.tam.cornell.edu/~ruina/hplab/index.html and more about Georg
Bock's Simulation Group with whom Katja Mombaur is affiliated can be found at
http://www.iwr.uni-heidelberg.de/~agboc
