# Ponytail Motion - Western Michigan University Ponytail Motion Author: Joseph B. Keller Source: SIAM J. Appl. Math Vol. 70. No. 7, pp. 2667-2672 About Author: Joseph Bishop Keller Born Residence Nationality Fields Institutions Alma mater Known for July 31, 1923 (age 89) Paterson, New Jersey U.S. American Mathematician New York University Stanford University

New York University Geometrical Theory of Diffraction EinsteinBrillouinKeller method Notable awards National Medal of Science (USA) in Mathematical, Statistical, and Computational Sciences (1988) Wolf Prize (1997) Nemmers Prize in Mathematics(1996) This presentation will: Demonstrate the motion of a ponytail Analyze the stability/instability of this motion Key words and Quick definitions: Frequency: The number of occurrence of a repeating event(or cycles) per unit time. Angular Frequency: Angular frequency(or angular speed) is the magnitude of angular velocity Amplitude: The measure of change in a periodic variable over a single period (or peak deviation from zero). Oscillation: A repetitive variation typically in time of some measure about a central value or between two or more states. Resonance:

The tendency of a system to oscillate with greater amplitude at some frequencies than at others. Parametric Resonance: The phenomenon of resonance that deals with the instability conditions. Linearization: Finding a linear approximation to a function at a given point. Exponential Growth: When the growth rate of the value of a mathematical function is proportional to functions current value. Excitation: An elevation in energy level above an arbitrary baseline energy state. Parametric Excitation: The method of exciting and maintaining oscillations in a dynamic system in which excitation results from a periodic variation in energy storage element in a system e.g. excitation of swing due to properly time bending of the knees. Equilibrium Point: The point is an equilibrium point for the differential equation if Perturbation: A small change/disturbance in the physical state (or initial/existing condition) of a system. Lateral Perturbation: A change which occurs in the state side by side as the physical state changes with time. How to establish the equations of motion?? Equations of motion are formulated as a system of second-order

ODEs that may be converted to a system of first-order equations whose dependent variables are positions and velocities of the object. Generic form for such systems: x f ( t , x ), t 0 , x ( 0 ) x 0 --------(a) where: x 0 is a specified initial condition for the system the component of x are the positions and velocities of the object f (t , x ) includes the external forces and torques of the system Example: The equation of motion of pendulum Replace ( x1 , x 2 ) x 2 For linearization replace ( sin x x ) g sin l g sin x1 l

b lm c x2 lm How we discuss Stability and Instability? Stability: A solution (t ) to the system (a) is said to be stable if every solution (t ) of the system close to (t ) at initial time t 0 remains close to all future time. In mathematical terms, for each choice of 0 such that ( t ) ( t ) whenever 0 there is a (0) (0) Instability: If for at least one solution (t ) does not remain close, then (t ) is said to be unstable Eigenvalues can also be helpful:

Let A Df (c ) be the matrix of first-order partial derivative of f (x ) (Jacobian Matrix)evaluated at c, then: Every solution(or equilibrium) is stable if all eigenvalues of A has negative real parts. Every solution(or equilibrium) is unstable if at least one eigenvalue of A has positive real part. Hills equation (G. W. Hill 1886) can also be helpful: The hills equation is a second order linear ODE, d 2 y dt 2 f (t ) y 0 where f ( t ) is a periodic function. If Hills equation has the solution that grows exponentially with time then motion will be considered as unstable. Few reminders:

The ponytail of the running jogger sways from side to side The swaying (lateral motion) is an example of parametric excitation Joggers head generally moves up and down The vertical motion of hanging ponytail is unstable to lateral perturbations Suggested ways to study this motion:

Either consider the ponytail as a rigid pendulum Or consider the ponytail as a flexible string Or consider the ponytail as an inextensible rod with small bending stiffness 1 Case: Ponytail as a rigid rod st As runner moves along +z-axis, her head moves up and down along y-axis. One end of ponytail is attached to joggers head at x o , y a ( t ), z Ut the position Consider, L = Length of ponytail (a uniform rigid rod) U = runners speed along z-axis = Position of the ponytail in the plane z=Ut a(t) = periodic vertical displacement along y-axis A = amplitude of oscillation

A cos t Then it is a simple pendulum having one end point fixed with vertical acceleration added to the acceleration due to gravity. 2 tt L ( g a tt ) sin 0 -----(1) With the vertical acceleration a tt of the end point added to the acceleration of gravity g, eq.(1) has two solutions in the interval 0 < < 2 :

1. 0 0 , means pendulum hanging straight down 2. 0 , means pendulum balanced pointing upward The Stability/instability of either solution determined by the equation for perturbation (t ) obtained by linearizing about 0 : 2 ---------(2) tt ( g a tt ) 0 L Which shows that system oscillates between limits [ 0 ,0 ] : , When att 0 : the solution for is sinusoidal for 0 0 the solution for is exponentially growing or decaying for 0 means the hanging pendulum is stable and the balanced pendulum is unstable when att 0 (but a periodic function of t, eq.(1) is called Hills equation):

Recall: Equation of motion for a simple pendulum: Hills equation: And Where tt 2 ( g a tt ) sin 0 L d 2 y dt 2 f (t ) y 0

If Hills equation has the solution in the interval of that grows exponentially with time then motion will be considered as unstable. is the result of equation of motion in dimensionless parameter. Mathematically: For any periodic function a (t ) with frequency there are infinite many intervals of throughout which Hills equation has solutions that grows exponentially with t. In Ponytail situation: When the solution lies in one of these intervals, the hanging pendulum becomes unstable or we observe the swaying of ponytail. Few interesting calculations: For a ponytail of length L=25cm has natural frequency ( 2 980 / 25 )1 / 2 8 . 85 radians / sec 1 . 41 cycles / sec must have the frequency of motion of joggers head twice the natural frequency means 17 . 71 radians / sec 2 . 82 cycles / sec A cycle correspond to a step with one leg means 2.82 cycles / sec 169 cycles / min 169 steps / min

Summary of case 1 A ponytail of length 25 cm can be expected to sway at a typical running cadence which is 160 steps/min according to website RunGearRun.com 2nd Case: Ponytail as a flexible string Let the ponytail hanging in the plane z Ut having: L = Length of ponytail as inextensible flexible string = constant density of string T = tension in the string g = (0,-g) = acceleration Let x ( s , t ) ( x ( s , t ), y ( s , t )) be the position at time t of the point at arc-length distance s from the top of string then: x (T x ) g it satisfies equation of motion: 0

--------(6) tt s s 2 s One solution of eq.(3)-eq.(6) represents vertically hanging string moving up and down is: x ( s, t ) [0, a(t ) s] ---------(7) 0 and the corresponding tension is: T 0 ( s, t ) ( g att )( L s) ---------(8) Again for checking Stability/Instability we need0 to see the linearized problem for perturbation in 0 x T, Which

T linearizing around the solution and by and will become: x x tt (T x 0s T 0 x s ) s Ts y (T 0 x s ) s Equation of motion: The condition that s is arc-length requires: Position in the plane at the end s=0: Tension vanishes at the end s=L: 0 s x x s y s 0 x (0, t ) 0 T ( L, t ) 0

0

--------(19) with boundary condition v(0) 0. tt 1 tt s s tt tt tt tt 1 1 s s

s ) x s ] s ---(15) ---(16) ---(17) The only solution for eq.(18) which is regular at s=L is a constant multiple of Bessels v ( s ) J 0 [ 2 1 / 2 ( L s )1 / 2 ] function J 0 : --------(20) we call this solution for nth mode v ( s ) and substitute in eq.(16), then the desired result x ( s , t ) u ( t , n ) J 0 [( 1 s / L ) 1 / 2 j n ] --------(21) will be: n The amplitude u ( t , n ) in eq.(21) satisfies eq.(19), which is Hills equation with Mathematically: For any periodic function a (t ) with frequency there are infinite many intervals of n g / 2 throughout which Hills equation has solutions that grows exponentially with t. In Our situation: when solution lies in one of these intervals, the vertical motion of the

flexible string becomes unstable to the lateral perturbations or we observe the swaying of ponytail. Few interesting calculations: For the lowest mode n 1 and j1 2 . 4 , the mode frequency is j1 / 2 ( g / L )1 / 2 1 .2 ( g / L )1 / 2 For a ponytail of length L=25 cm when is around twice the lowest mode frequency i.e., 2 (1 . 2 )( 980 / 25 )1 / 2 15 . 0 radians / sec 2 . 39 cycles / sec . 143 . 5 steps / min . Summary of case 2 143.5 steps/min is slightly less than the cadence required for swaying the joggers ponytail having length 25cm but still the ponytail can be expected to sway. A more realistic model Ponytail as a inextensible flexible rod When runner is not moving the ponytail will extend away from head and hang downward in its characteristic shape. e.g. Cantilever Beam When runner is moving and her head is bobbing up and down and ponytail oscillate in yz-plane, the instability of this motion

would determine when swaying occurs and would determine the swaying mode shape. The equation of motion with the addition of bending term Bx ssss x tt ( g a tt )[( L s ) x s ] s B x ssss will become: Since it is of fourth order, so it needs four boundary condition: x ( 0 , t ) 0 , x s ( 0 , t ) 0 Two conditions for the ponytail clamped at the top x ss ( L , t ) 0 , x sss ( L , t ) 0 Two conditions for the ponytail free at the bottom References: 1] J. J. Stoker, Nonlinear Vibrations, Interscience, New York, 1950.  W. Magnus and S. Winkler, Hills Equation, Interscience, New York, 1966.  A. Belmonte, M. J. Shelley, S. T. Eldakar, and C. H. Wiggins, Dynamic patterns and self-knotting of a driven hanging chain, Phys. Rev. Lett., 87 (2001), pp. 114301114304.  A. Stephenson, On a new type of dynamical stability, Mem. Proc. Manch. Lit. Phil. Soc., 52 (1908), pp. 110.  D. J. Acheson, A pendulum theorem, Proc. Roy. Soc. London Ser. A, 443 (1993), pp. 239245.  D. J. Acheson and T. Mullin, Upside-down pendulums, Nature, 366 (1993), pp. 215216.  G. H. Handelman and J. B. Keller, Small vibrations of a slightly stiff pendulum, in Proceedings of the 4th U.S. National Congress on Applied Mechanics, Amer. Soc. Mech. Eng., New York, 1963, pp. 195202.

 A. R. Champneys and W. B. Fraser, The Indian rope trick for a parametrically excited flexible rod: Linearized analysis, R. Soc. Lond. Proc. Ser. A Math. Phys. Eng. Sci., 456 (2000), pp. 553570. Note: Graphics and images used in this presentation are easily available on Google images section. Note: Quick definitions used in this presentation are taken from mathematics section based websites. Presented by: Adnan Ahmed Thank you Few additional: Newtonian Equation of Motion: Hamiltonian Equation of Motion: Bessels Differential Equation: Bessels functions are canonical solutions y(x) of Bessels differential equation Where is the gamma function, a shifted generalization of the factorial function to non integer values. Parametric Excitation:(Journal of Applied Physics, vol 22, num. 1, Jan 1951) If a parameter of an oscillatory system is varied periodically between certain limits, the system become excited, i.e., start oscillating with frequency equal to one-half of that with which the parameter varies. The term parametric

excitation is used to designate this phenomenon. The stability of upside down pendulums: Theorem: Let N pendulums hang down, one from another, under gravity g, each having one degree of freedom, the uppermost being suspended from a pivot point O. Let (max) and (min) denote the largest and the smallest of the natural frequencies of small oscillation about this equilibrium state. Now turn the whole system upside-down. The resulting configuration of the pendulums can be stabilized (according to linear theory, atleast) if we subject the pivot point O to vertical oscillations of suitable amplitude and frequency . When the stability criterion is ------ (1) NOTE: when several pendulums are involved is typically much greater than . The condition is then necessary for the stability of the inverted state. So eq(1) then gives the whole stability region in the - plane.