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EFFICIENCY & EQUILIBRIUM Lecture 1 : Geometry – Nature’s Poetry. Wayne Lawton Department of Mathematics National University of Singapore [email protected] http://www.math.nus.edu.sg/~matwml. SPS2171 Presentation 21/01/2008. What do you see in this soap bubble ?.

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Efficiency equilibrium lecture 1 geometry nature s poetry

EFFICIENCY & EQUILIBRIUM Lecture 1 : Geometry – Nature’s Poetry

Wayne Lawton

Department of Mathematics

National University of Singapore

[email protected]

http://www.math.nus.edu.sg/~matwml

SPS2171 Presentation 21/01/2008











Equilibrium
Equilibrium

ball is in a (stable) equilribium when the net force on

it (gravity + constraint) = zero (hence it is stationary)

height

constraint

equilibria


Efficiency principles
Efficiency Principles

describe the behavior of physical, chemical, biological, and social systems

systems behave so as to minimize “resources”

within constraints

balls move to minimize their local height

soap surfaces move to minimize their local area

bees and cells build (hives and skeletons) to minimize material


Principle of minimum distance

A thrifty bee located at point A wants to get a drink (on the planar surface S of a pool) before flying to point B, what point P on the surface S should it fly to

Principle of Minimum Distance

plane S

reflection of B through S


Principle of minimum distance1

Our thrifty bee will fly in straight towards to the pools surface at P, drink, then fly straight to B.

Principle of Minimum Distance

1. line segments AP and PB lie in a normal plane to S

2. incidence angle APA’ = reflection angle BPB’

plane S

reflection of B through S

330 BC our enlightened bee is reincarnated as Euclid, writes The Elements, and (reputedly) in the Catopricastates that rules 1 & 2 govern the refection of light


Principle of minimum distance2
Principle of Minimum Distance pools surface at P, drink, then fly straight to B.

Euclid’s laws of reflection were well understood by Archimedes who, in 214BC used parabolic mirrors

to ignite Roman warships during the 2nd Punic War.

About 100AD Heron states his law that “light must always take the shortest path” (hmm – what took them 400 years to discover something obvious to a bee?)


Conic sections

Appolonius 262-190BC studied conic sections, pools surface at P, drink, then fly straight to B.

curves (ellipses, parabolas, and hyberbolas)formed

by intersecting planes with conical surfaces.

Conic Sections

Their optical properties were long understood.

1605 Johannes Kepler discovers his three laws, the first states that planets move in ellipses. This leads Newton to develop his theory of gravitation which he combines with his calculus to show that all celestial bodies move in conic orbits (nonbound objects move in parabolas and hyperbolas).


Principle of minimum action
Principle of Minimum Action pools surface at P, drink, then fly straight to B.

1710 Gottfried Wilhelm Leibniz publishes “On the Kindness of God, the Freedom of Man, and the Origin of Evil” in which he develops the principle that our world is organized to be

the best of all possible worlds

He also states in a letter the following principle :

nature always minimizes action


Principle of minimum action1
Principle of Minimum Action pools surface at P, drink, then fly straight to B.

This principle, mistakenly attributed to Maupertuis, implies that a particle moving (only) under its own inertia and constrained to move on a surface, moves with constant speed along a path that minimizes the distance between any two points on the path.

These paths are called geodesics. For motion on

a plane they are straight lines, for motion on a spherical surface they are arcs of great circles.

Euler, Lagrange, and Hamilton developed the calculus of variations and classical mechanics from this principle which underlines Einstein’s relativity.


Steiner s problem and 120 degree angles

Choose P to minimize dist (AP)+dist (BP)+dist (CP) pools surface at P, drink, then fly straight to B.

Steiner’s Problem and 120 Degree Angles

pages 92-100 in The Parsimonius Universe,

Steffan Hildebrandt and Anthony Tromba


Tutorial problems

1. Light rays change direction when they traverse interfaces between substances such as air and water. Explain this phenomena using a minimum principle.

Tutorial Problems

2. Make a conical surface out of paper, place two dots on it and then draw a geodesic connecting them. Test your drawing by cutting and flattening the cone.

3. Build a physical devise to choose point P in Steiner’s Problem. Hint: use 3 rings fastened on the edges of a table at vertices A,B,C; 3 equal weights attached to 3 strings that are tied at a single knot.

4. Investigate & Explain : mean curvature, minimal surfaces, surface tension, 1st Fields Medal problem.


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