Ponytail Motion

Author: Joseph B. KellerSource: SIAM J. Appl. Math

Vol. 70. No. 7, pp. 2667-2672

About Author:

Joseph Bishop KellerBorn July 31, 1923 (age 89)

Paterson, New JerseyResidence U.S.Nationality AmericanFields MathematicianInstitutions New York University

Stanford UniversityAlma mater New York UniversityKnown for Geometrical Theory of Diffraction

Einstein–Brillouin–Keller methodNotable 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 function’s 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 ODE’s 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: --------(a)where:• is a specified initial condition for the system• the component of are the positions and velocities of the object• includes the external forces and torques of the system

Example: The equation of motion of pendulumReplace ( )For linearization replace ( )

0)0(,0),,( xxtxtfx

0xx

),( xtf

lmb

lg

sin

212 sin xlmcx

lgx

21 , xx

xx sin

How we discuss Stability and Instability?

Stability:A solution to the system (a) is said to be stable if every

solution of the system close to at initial time remains close to all future time.

In mathematical terms, for each choice of there is a such that whenever

Instability:If for at least one solution does not remain close, then is said to be unstable

)(t)(t)(t 0t

00 )()( tt )0()0(

)(t )(t

Eigenvalues can also be helpful:Let be the matrix of first-order partial derivative of (Jacobian Matrix)evaluated at c, then: Every solution(or equilibrium) is stable if all eigenvalues of has negative real parts. Every solution(or equilibrium) is unstable if at least one eigenvalue of has positive real part.

Hill’s equation (G. W. Hill 1886) can also be helpful:The hill’s equation is a second order linear ODE,

where is a periodic function. If Hill’s equation has the solution that grows exponentially with

time then motion will be considered as unstable.

)(cDfA )(xf

A

A

0)(2

2

ytfdtyd

)(tf

Few reminders:

The ponytail of the running jogger

sways from side to side

Jogger’s head generally

moves up and down

The vertical motion of hanging ponytail is unstable to lateral

perturbations

The swaying (lateral motion) is

an example of parametric excitation

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

1st Case: Ponytail as a rigid rodAs runner moves along +z-axis, her head moves up and down along y-axis. One end of ponytail is attached to jogger’s head at the position Consider,L = Length of ponytail (a uniform rigid rod)U = runner’s speed along z-axis = Position of the ponytail in the plane z=Uta(t) = periodic vertical displacement along y-axis = amplitude of oscillation

Then it is a simple pendulum having one end point fixed with vertical acceleration added to the acceleration due to gravity. -----(1)

0sin)(2 tttt agL

Utztayox ),(,

tA cosA

With the vertical acceleration of the end point added to the acceleration of gravity g, eq.(1) has two solutions in the interval 0 < < 2 :1. , means pendulum hanging straight down2. , means pendulum balanced pointing upward

The Stability/instability of either solution determined by the equation for perturbation obtained by linearizing about :

---------(2)

Which shows that system oscillates between limits : ,

When : the solution for is sinusoidal for the solution for is exponentially growing or decaying for means the hanging pendulum is stable and the balanced pendulum is unstable

tta

00

0

)(t

0

0)(2 tttt ag

L

0tta 00

0

],[ 00

when (but a periodic function of t, eq.(1) is called Hill’s equation):

Recall:Equation of motion for a simple pendulum:

Hill’s equation:

And If Hill’s equation has the solution in the interval of that grows exponentially with time then motion will be considered as unstable.

Where is the result of equation of motion in dimensionless parameter.

Mathematically:For any periodic function with frequency there are infinite many intervals of

throughout which Hill’s 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.

0tta

0sin)(2 tttt agL

0)(2

2

ytfdtyd

)(ta

Few interesting calculations:

For a ponytail of length L=25cm has natural frequency

must have the frequency of motion of jogger’s head twice the natural frequency means

A cycle correspond to a step with one leg means

Summary of case 1A 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

sec/41.1sec/85.8)25/9802( 2/1 cyclesradians

sec/82.2sec/71.17 cyclesradians

min/169min/169sec/82.2 stepscyclescycles

2nd Case: Ponytail as a flexible stringLet the ponytail hanging in the plane having:L = Length of ponytail as inextensible flexible string = constant density of stringT = tension in the string = (0,-g) = acceleration Let be the position at time t of the point at arc-length distance s from the top of string then: it satisfies equation of motion: 0<s<L --------(3)The condition that s is arc-length requires: 0<s<L --------(4) Position in the plane at the end s=0: --------(5) Tension vanishes at the end s=L: --------(6)

One solution of eq.(3)-eq.(6) represents vertically hanging string moving up and down is: ---------(7)

and the corresponding tension is: ---------(8)

)),(),,((),( tsytsxtsx

g

gxTx sstt )(

12 sx

))(,0(),0( tatx

0),( tLT

))((),(0 sLagtsT tt

])(,0[),(0 statsx

Utz

Again for checking Stability/Instability we need to see the linearized problem for perturbation in and by linearizing around the solution and , Which will become:

Equation of motion: 0<s<L ---------(9)The condition that s is arc-length requires: 0<s<L -------(10) Position in the plane at the end s=0: --------(11) Tension vanishes at the end s=L: --------(12)

Integrate eq.(10) w.r.t s and using the y-component of eq.(11) gives: ------(13)When eq.(13) used in y-component of of eq.(9) gives: or ------(14)

Solution for lateral displacement:

when eq.(8) is used for , the x-component of eq.(9) becomes ---(15) we are interested in solution having product form ---(16) substitution of eq.(13) into eq.(14) gives ---(17) from eq.(14) we get two equation : --------(18) and --------(19) with boundary condition .

x T0x 0T

sssssstt xTyTxTxTx )()( 000

00 sss yxx

0),0( tx0),( tLT

0T sstttt xsLagx ]))[((

)()(),( svtutsx 111 ])[()( vvsLuagu sstttt

0])[( vvsL ss

0)( uagu tttt

0)0( v

The only solution for eq.(18) which is regular at s=L is a constant multiple of Bessel’s function : --------(20) we call this solution for nth mode and substitute in eq.(16), then the desired result will be: --------(21) The amplitude in eq.(21) satisfies eq.(19), which is Hill’s equation with

Mathematically: For any periodic function with frequency there are infinite many intervals of

throughout which Hill’s 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.

0J ])(2[)( 2/12/10 sLJsv

])/1[(),(),( 2/10 nn jLsJtutsx )(svn

),( ntu

2/ gn)(ta

Few interesting calculations:

For the lowest mode and , the mode frequency is

For a ponytail of length L=25 cm when is around twice the lowest mode frequency i.e.,

Summary of case 2

143.5 steps/min is slightly less than the cadence required for swaying the jogger’s ponytail having length 25cm but still the ponytail can be expected to sway.

4.21 j2/12/1

1 )/(2.1)/(2/ LgLgj 1n

.min/5.143.sec/39.2sec/0.15)25/980)(2.1(2 2/1 stepscyclesradians

A more realistic modelPonytail as a inextensible flexible rod

When runner is not moving the ponytail will extend awayfrom 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 will become:

Since it is of fourth order, so it needs four boundary condition:Two conditions for the ponytail clamped at the top ,Two conditions for the ponytail free at the bottom ,

ssssxB sssssstttt xBxsLagx ]))[((

0),0( tx 0),0( txs0),( tLxss 0),( tLxsss

References:

1] J. J. Stoker, Nonlinear Vibrations, Interscience, New York, 1950.[2] W. Magnus and S. Winkler, Hill’s Equation, Interscience, New York, 1966.[3] 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. 114301–114304.[4] A. Stephenson, On a new type of dynamical stability, Mem. Proc. Manch. Lit. Phil. Soc., 52 (1908), pp. 1–10.[5] D. J. Acheson, A pendulum theorem, Proc. Roy. Soc. London Ser. A, 443 (1993), pp. 239–245.[6] D. J. Acheson and T. Mullin, Upside-down pendulums, Nature, 366 (1993), pp. 215–216.[7] 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. 195–202.[8] 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. 553–570.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.

Thank you

Presented by: Adnan Ahmed

Few additional:

Newtonian Equation of Motion:

Hamiltonian Equation of Motion:

Bessel’s Differential Equation:

Bessel’s functions are canonical solutions y(x) of Bessel’s differential equation

Where Γ 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 ω(max) and ω(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 Є 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 Є- plane.