We model a cord lying in a random orientation on a horizontal surface, and solve the model stochastic differential equations for the mean and variance of the squared distance between cord ends. The analytical solutions fit the simulation data and measurements made on dropped cords and have a number of applications including the shape of polymers and photon trajectories.

1.
Robert
Brown
, “
A brief account of microscopical observations made in the months on June, July, and August, 1827, on the particles contained in the pollen of plants; and on the general existence of active molecules in organic and inorganic bodies
,”
Philosophical Magazine
4
,
161
173
(
1828
).
See also
A.
Einstein
, “
Investigations on the theory of Brownian movement
,”
Ann. Phys.
17
,
549
560
(
1905
).
An English translation of Einstein’s paper appears in Einstein’s Miraculous Year, edited by J. Stachel (Princeton U.P., Princeton, NJ, 1998).
2.
For an introduction to stochastic processes in physical applications, see D. Gillespie, Markov Processes, An Introduction for Physical Scientists (Academic, New York, NY, 1992);
N. G. van Kampen, Stochastic Processes in Physics and Chemistry (North-Holland, Dordrecht, 1981), Chap. 9;
D. S. Lemons, An Introduction to Stochastic Processes in Physics (Johns Hopkins U.P., Baltimore, MD, 2002).
3.
Howard C. Berg, Random Walks in Biology (Princeton U.P., Princeton, NJ, (1993).
4.
Paul J. Flory, Statistical Mechanics of Chain Molecules (Interscience, New York, 1969), p. 10 ff.
5.
Statements of the normal sum theorem and other properties of normals can be found in Ref. 2 or 6 or any mathematical statistics text.
6.
See Ref. 2 or C. W. Gardiner, Handbook of Stochastic Methods (Springer-Verlag, New York, 1994).
7.
L.
Mahadevan
and
J. B.
Keller
, “
Coiling of flexible ropes
,”
Proc. R. Soc. London
452
,
1679
1694
(
1996
).
8.
D. S. Lemons, An Introduction to Stochastic Processes in Physics (Johns Hopkins U.P., Baltimore, MD, 2002), Sec. 10.2.
9.
D. S. Lemons and B. J. Albright, “Quiet Monte Carlo radiation transport,” J. Quant. Spectrosc. Radiat. Transf. (to be published).
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