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Swinging Atwood's machine
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Swinging Atwood's machine : ウィキペディア英語版
Swinging Atwood's machine

The swinging Atwood's machine (SAM) is a mechanism that resembles a simple Atwood's machine except that one of the masses is allowed to swing in a two-dimensional plane, producing a dynamical system that is chaotic for some system parameters and initial conditions.
Specifically, it comprises two masses (the pendulum, mass m and counterweight, mass M) connected by an inextensible, massless string suspended on two frictionless pulleys of zero radius such that the pendulum can swing freely around its pulley without colliding with the counterweight.〔
The conventional Atwood's machine allows only "runaway" solutions (''i.e.'' either the pendulum or counterweight eventually collides with its pulley), except for M=m. However, the swinging Atwood's machine with M>m has a large parameter space of conditions that lead to a variety of motions that can be classified as terminating or non-terminating, periodic, quasiperiodic or chaotic, bounded or unbounded, singular or non-singular〔〔 due to the pendulum's reactive centrifugal force counteracting the counterweight's weight.〔 Research on the SAM started as part of a 1982 senior thesis entitled ''Smiles and Teardrops'' (referring to the shape of some trajectories of the system) by Nicholas Tufillaro at Reed College, directed by David J. Griffiths.〔
==Equations of motion==

The swinging Atwood's machine is a system with two degrees of freedom. We may derive its equations of motion using either Hamiltonian mechanics or Lagrangian mechanics. Let the swinging mass be m and the non-swinging mass be M. The kinetic energy of the system, T, is:
:
\begin
T &= \frac M v^2_M + \frac mv^2_m \\
&= \fracM \dot^2+\frac m \left(\dot^2+r^2\dot^2\right)
\end

where r is the distance of the swinging mass to its pivot, and \theta is the angle of the swinging mass relative to pointing straight downwards. The potential energy U is solely due to the acceleration due to gravity:
:
\begin
U &= Mgr - mgr \cos
\end

We may then write down the Lagrangian, \mathcal, and the Hamiltonian, \mathcal of the system:
:
\begin
\mathcal &= T-U\\
&= \fracM \dot^2+\frac m \left(\dot^2+r^2\dot^2\right) - Mgr + mgr \cos\\
\mathcal &= T+U\\
&= \fracM \dot^2+\frac m \left(\dot^2+r^2\dot^2\right) + Mgr - mgr \cos
\end

We can then express the Hamiltonian in terms of the canonical momenta, p_r, p_\theta:
:
\begin
p_r &= \frac\\
p_\theta &= \frac\\
\therefore \mathcal &= \frac + \frac + Mgr - mgr \cos
\end

Lagrange analysis can be applied to obtain two second-order coupled ordinary differential equations in r and \theta. First, the \theta equation:
:
\begin
\frac &= \frac \left(\frac &= 2mr \dot\dot + mr^2 \ddot\\
r\ddot + 2\dot\dot + g\sin &= 0
\end

And the r equation:
:
\begin
\frac &= \frac \left( \frac^2 - Mg + mg\cos &= (M+m) \ddot
\end

We simplify the equations by defining the mass ratio \mu = \frac. The above then becomes:
:(\mu+1)\ddot - r\dot^2 + g(\mu - \cos) = 0
Hamiltonian analysis may also be applied to determine four first order ODEs in terms of r, \theta and their corresponding canonical momenta p_r and p_\theta:
:
\begin
\dot&=\frac \\
\dot &= - \frac - Mg + mg\cos \\
\dot&=\frac \\
\dot &= - \frac
\end

Notice that in both of these derivations, if one sets \theta and angular velocity \dot to zero, the resulting special case is the regular non-swinging Atwood machine:
:\ddot = g \frac=g\frac
The swinging Atwood's machine has a four-dimensional phase space defined by r, \theta and their corresponding canonical momenta p_r and p_\theta. However, due to energy conservation, the phase space is constrained to three dimensions.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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