
Physics/Fields Colloquium 20102011
Organizing Committee: Stephen Morris (Dept. of Physics,
University of Toronto) & Mary Pugh (Toronto)
The goal of the Physics/Fields Colloquium is to feature scientists
whose work is of interest to both the physics and the mathematical
science community. The series has been running since the Spring
of 2007.
Usually there is one speaker per semester. Each speaker gives a
primary, general talk in the regular physics colloquium venue and,
whenever possible, a second, more specialised talk at the Fields
Institute.
Previous speakers have been Phil Holmes (March 2007), Jun Zhang
(October 2007), Andrea Liu (Nov 2008),
Ehud Meron (March 2009), Carson Chow (December 2009), and Jane Wang
(March 2010). Index of
200910 seminars
201011
Schedule

March
9, 2011
3:10 p.m.
Fields Institute, Room 230

Andy
Ruina (Cornell)
Some Issues in Bipedal locomotion
The coordination of robotic and natural walking and running
is often treated as a complex control problem. In contrast,
I will mention some things that can be inferred from simple
mechanical analysis. As promoted early on by Tom McMahon and
then Tad McGeer, walking gaits can be generated by machines
with no control. These machines use relatively little energy
and, like many bicycles, can have a measure of selfstability.
However, the hypothesis that natural gaits might be largely
passive is close in some ways, but not identical to, the stronger
and more biologically interesting hypothesis that natural
gaits minimize energy use. The talk includes videos of robots,
some heuristic explanations, and a few equations.
Bio: Andy Ruina got three degrees from Brown University in
Engineering. He studied friction, fracture, collisions, bicycles
and the mechanics of walking. His lab made the most energy
stingy walking robot and also a robot that walked 14.3 miles
without human contact or refueling. He spends his summers
in on an island in SouthWest Finland near Stockholm where
is wife studies wasps that lay eggs in baby butterflies. Google
ruina.

March
10, 2011
4:10 p.m.
MP 102 
Andy
Ruina (Cornell)
Rotation with zero angular momentum: Demonstrations of the
falling cat phenomenon go sour
It is well known that a system with zero angular momentum
can, by appropriate deformations, rotate while always maintaining
the condition of zero angular momentum. This effect explains
how a cat that is dropped while upside down can turn over
and of how certain gymnastic maneuvers are performed. These
rotations are taken as a demonstration of the "nonintegrability"
of a "nonholonomic" constraint. There is a simple
demonstration of this rotationwithzeroangularmomentum
effect with a rotating platform. But the demonstration often
doesn't work because most floors are not perfectly flat. I
found a simple better demonstration experiment. Unfortunately,
the experiment came out all wrong for different reasons. But
I figured out why and did a second demonstration experiment.
And that came out wrong exactly in the opposite way.
The talk presents the four puzzles: a) how can you turn while
having zero angular momentum? b) Why does a rotating platform
demonstration often not work. c) Why does a simple demonstration
not work? d) Why does almost exactly the same demonstration
not work in the opposite way?
The talk starts with various personal stories about nonholonomic
constraints and their relation to locomotion  that's bikes,
skates, driving on ice and walking  and then gets into
the 4 rotation puzzles.
Bio: Andy Ruina got three degrees from Brown University in
Engineering. He studied friction, fracture, collisions, bicycles
and the mechanics of walking. His lab made the most energy
stingy walking robot and also a robot that walked 14.3 miles
without human contact or refueling. He spends his summers
in on an island in SouthWest Finland near Stockholm where
is wife studies wasps that lay eggs in baby butterflies. Google
ruina.

Previous Seminars

November
25, 2010
4:10
MP 102

Eitan
Grinspun (Columbia University)
From Sorcery to Science: how Hollywood Physics impacts the
Sciences
Cinema uses computers to animate physics. Special effects
such as explosions and lifelike depictions of imaginary characters
are made possible by mathematical and computational models
that capture qualitative, characteristic behavior of a mechanical
system. This is scientific computing with a twist. I will
describe the process by which we derive and compute models
of physics, and show actual examples of resulting technologies
in film, consumer products, physics, and medicine.
Our research group develops scientific computing tools by
focusing on the underlying geometry of the mechanical system.
I will describe a process in which we build a discrete picture
from the ground up, mimicking the axioms, structures, and
symmetries of the smooth setting. I will survey the problems
we address using this methodology, such as computing the motion
of flexible surfaces, cloth, hair, honey, and solids experiencing
mechanical contact. Industry and academia has adopted these
methods to improve products such as Adobe Photoshop, films
such as Disney's Tangled (whose release date coincides with
this talk), train surgeons, and understand nonlinear softmatter
phenomena.
BIO
Eitan Grinspun is Associate Professor of Computer Science
at Columbia University in the City of New York. He was Professeur
d'Universite Invite at l'Universite Pierre et Marie Curie
in 2009, a Research Scientist at the Courant Institute of
Mathematical Sciences from 20032004, and a graduate student
at the California Institute of Technology from 19972003.
He was an NVIDIA Fellow in 2001, an Everhart Distinguished
Lecturer in 2003, an NSF CAREER Award recipient in 2007, and
is currently an Alfred P. Sloan Research Fellow.

November
24, 2010
3:10pm
Fields Institute, Room 230 
Eitan
Grinspun (Columbia University)
Discrete Elastic Rods and Viscous Threads
Many physical phenomena can explained by geometric principles
that underly and govern them. For example, the fascinating
curved shapes taken on by thin elastic rods (such as ropes,
knots, or DNA strands), which can be physically explained
by the nonlinear interaction between bending and twisting
potentials, have a geometric interpretation in terms of moving
adapted frames. The computation of these phenomena benefits
from explicit attention to this geometric viewpoint.
Computerbased simulations of solid rods and fluid threads
necessarily rely on reducing the continuous description to
a discrete (finite) one. Using notions of discrete framed
curves and discrete parallel transport we develop a discrete
geometric model of thin flexible rods with arbitrary cross
section and undeformed configuration. Using Raleigh's analogy
enables a timediscretized transition to a model of viscous
fluid threads. The resulting computations of elastic rods
and viscous threads are validated via comparison of buckling,
stability, and coupledmode numerical, analytical, and empirical
experiments.
This is joint work with Basile Audoly, Miklós Bergou,
Etienne Vouga and Max Wardetzky.
BIO
Eitan Grinspun is Associate Professor of Computer Science
at Columbia University in the City of New York. He was Professeur
d'Universite Invite at l'Universite Pierre et Marie Curie
in 2009, a Research Scientist at the Courant Institute of
Mathematical Sciences from 20032004, and a graduate student
at the California Institute of Technology from 19972003.
He was an NVIDIA Fellow in 2001, an Everhart Distinguished
Lecturer in 2003, an NSF CAREER Award recipient in 2007, and
is currently an Alfred P. Sloan Research Fellow.





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