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LINEAR AND NON-LINEAR OSCILLATORS
Athanasios Paraskevopoulos
Imagine a block of mass m attached to a spring. The spring is fixed at one end, and the block is free to move horizontally on a frictionless surface.
When the block is displaced from its equilibrium position and released, it oscillates back and forth
According to Hooke's Law, the restoring force exerted by the spring is proportional to the displacement
from the equilibrium position and acts in the opposite direction:

Using Newton's second law,
, where is the acceleration, we get:

Since acceleration is the second derivative of displacement with respect to time, the equation becomes:

Rearranging terms, we get:

Letting where ω is the angular frequency, the equation simplifies to

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Amplitude A: Maximum displacement from the equilibrium position.
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Angular Frequency : Related to the spring constant and mass by .
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Period T: The time for one complete oscillation, given by 
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Frequency f: The number of oscillations per unit time

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In a linear simple harmonic oscillator (SHO), the Hamiltonian energy represents the total energy of the system, which is a combination of its kinetic and potential energy.
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Position and Momentum:
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Let be the displacement of the oscillator from its equilibrium position.
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Let be the momentum of the oscillator.
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Hamiltonian ( ):
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For a linear SHO, it is given by:

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Kinetic energy depends on the mass of the object and its velocity. In physics, it's expressed as:

where:
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is the mass of the oscillator,
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*of*momentum, p = mv, be*can*energy*kinetic*re*the-as*expressed; T = p^2/(2*m)](/view.aspx?sf=227706_post/7cee68acca2a1b65b8b40007de0f78db.gif)
This equation shows that kinetic energy increases with the square of the momentum.
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Potential Energy ( )
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Potential energy in a simple harmonic oscillator arises from the restoring force of the spring. The potential energy is given by:


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is the spring constant, and
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is the displacement from the equilibrium position.
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This means that the further the mass is displaced from equilibrium, the greater the potential energy stored in the system.
Suppose you have a spring with a spring constant and a mass . If the mass is displaced by from its equilibrium position and released, it will undergo SHM
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![plot(T, x = 0 .. 1, title = "Period vs Initial Amplitude (x)", axes = boxed, labels = ["Initial Amplitude (x)", "Period (T)"])](/view.aspx?sf=227706_post/5d61c48d5e1fab84096402609571aed3.gif)
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![plot_1 := subs(A = -.104, sol); plotsresult := plot([rhs(plot_1)], t = 0 .. 2, legend = ["-0.104cos( 7.07 t)"], color = [red])](/view.aspx?sf=227706_post/afbd1dba2c641218044368a57d566497.gif)
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![energy_plot := plot([eval(T), eval(V), eval(H)], t = 0 .. 2, color = [red, blue, green], legend = ["Kinetic Energy", "Potential Energy", "Total Energy"], title = "Energy Exchange in Simple Harmonic Oscillator", labels = ["Time (s)", "Energy (Joules)"])](/view.aspx?sf=227706_post/5c30c886af36f8f8d6a8299cad6adfbd.gif)
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![directionfield := DEplot([diff(x(t), t) = v(t), diff(v(t), t) = -omega^2*x(t)], [x(t), v(t)], t = 0 .. 10, x = -2 .. 2, v = -10 .. 10, arrows = medium, title = "Direction Field for Simple Harmonic Oscillator", axes = boxed)](/view.aspx?sf=227706_post/c1cb66542f44be08cd36195e1f28032e.gif)
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![phase_plot := plot([[eval(x1, t = tval), eval(v1, t = tval), tval = 0 .. 10], [eval(x2, t = tval), eval(v2, t = tval), tval = 0 .. 10], [eval(x3, t = tval), eval(v3, t = tval), tval = 0 .. 10], [eval(x4, t = tval), eval(v4, t = tval), tval = 0 .. 10]], style = line, title = "Phase Portrait for Simple Harmonic Oscillator", labels = ["x (Displacement)", "v (Velocity)"], color = ["red", "blue", "green", "orange"], legend = ["A=1, v0=0", "A=0.5, v0=0", "A=-1, v0=0", "A=0.7, v0=0.5"], axes = boxed)](/view.aspx?sf=227706_post/99541e199598a22f70e7ce6591bee70b.gif)
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Damped Oscillators:
Consider a mass-spring-damper system, where a block of mass is attached to a spring (with spring constant ) and a damper (with damping coefficient ).
The damper provides resistance to the motion, simulating friction or air resistance.
Damped Oscillator Equation:

This is in the form of a homogeneous second order differential equation and has a solution of the form

Substituting this form gives an auxiliary equation for 

The roots of the quadratic auxiliary equation are
λ=
The three resulting cases for the damped oscillator are
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Overdamped
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Critical damping
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Underdamped
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The period of a damped oscillator is primarily determined by the damped angular frequency 
Suppose a mass is attached to a spring with and a damper with a damping coefficient 
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![odeplot(sol, [t, x(t)], 0 .. 10, color = blue)](/view.aspx?sf=227706_post/7c92bd6e3ede9de5414fba44812e0bbe.gif)
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![H := proc (t) local pos, vel, kinetic, potential; pos := rhs(sol(t)[2]); vel := rhs(sol(t)[3]); kinetic := (1/2)*m*vel^2; potential := (1/2)*k*pos^2; return kinetic+potential end proc](/view.aspx?sf=227706_post/65a36e60e0831d223dd594d2ce691880.gif)
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![timeRange := 0 .. 35; plot(H(t), t = timeRange, title = "Hamiltonian Energy vs Time", labels = ["Time (t)", "Energy (H)"])](/view.aspx?sf=227706_post/44ce607dc033a4072a51cc0c9f5ae4cf.gif)
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Mass-Spring System with a Non-Linear Spring
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Springs with large deflections are not linear and their forces can be approximated by a cubic term 
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If hardening spring (stiffer with larger deflection), if softening spring (more flexible with larger deflection).
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So, the EOM fot a mass-spring system in free vibration will be:
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The period of oscillation will not be constant and depends on the applitude of oscillations and sign of 
First case 
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![odeplot(sol, [t, x(t)], 0 .. 10, title = "Nonlinear Oscillation (Hardening Spring)", labels = ["Time (s)", "Displacement (m)"], color = blue)](/view.aspx?sf=227706_post/a52663899fad428c0f7faae6558ebe03.gif)
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![H := proc (t) local pos, vel, kinetic, potential; pos := rhs(sol(t)[2]); vel := rhs(sol(t)[3]); kinetic := (1/2)*m*vel^2; potential := (1/2)*k*pos^2+(1/4)*beta*pos^4; return kinetic+potential end proc; timeRange := 0 .. 10; plot(H(t), t = timeRange, 0 .. .1, title = "Hamiltonian Energy vs Time", labels = ["Time (t)", "Energy (H)"])](/view.aspx?sf=227706_post/5d1a4a028b7fdb78b8ed8f88711dc0d9.gif)
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In this non-linear mass-spring-damper system (without damping), the Hamiltonian is essentially the total energy of the system, as there is no energy loss.
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The Hamiltonian remains relatively constant over time, as expected in an ideal undamped system, indicating energy conservation.
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Small numerical variations may occur due to the non-linear terms and the precision of numerical methods used for solving the differential equations.
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![initial_conditions := [[x(0) = -.104, v(0) = 0]]; with(DEtools); DEplot([eq1, eq2], [x(t), v(t)], t = 0 .. 1, x = -.3 .. .3, v = -2 .. 2, initial_conditions, title = "Phase Portrait for Nonlinear Oscillator", axes = boxed, labels = ["Displacement x", "Velocity v"], linecolor = blue)](/view.aspx?sf=227706_post/b26c845442246bb2fb4e08e01a9d4671.gif)
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Second Case 
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