Physlib

Physlib.ClassicalMechanics.DampedHarmonicOscillator.Solution

Solutions to the damped harmonic oscillator

i. Overview

In this module we define the solution to the damped harmonic oscillator for given initial conditions and prove that it satisfies the equation of motion. The solution selects the appropriate closed form from the sign of the discriminant: trigonometric for the underdamped case, polynomial for the critically damped case, and hyperbolic for the overdamped case.

ii. Key results

- `InitialConditions` is a structure for the initial position and velocity. - `trajectory` selects the appropriate regime-specific trajectory from the sign of the discriminant. - `trajectory_equationOfMotion_of_underdamped`, `trajectory_equationOfMotion_of_criticallyDamped`, and `trajectory_equationOfMotion_of_overdamped` prove the selected trajectory satisfies the equation of motion in each damping regime. - `trajectory_equationOfMotion` proves that the selected trajectory satisfies the equation of motion.

iii. Table of contents

- A. The initial conditions - B. Trajectories associated with the initial conditions - B.1. Regime-specific base trajectories - B.2. The selected trajectory - B.3. Shared calculus lemmas - B.4. Derivatives of the base trajectories - C. Trajectories and equation of motion - C.1. The selected trajectory satisfies the equation of motion - C.2. Uniqueness of the solutions

iv. References

References for the damped harmonic oscillator include: - Landau & Lifshitz, Mechanics, page 76, section 25. - Goldstein, Classical Mechanics, Chapter 2.

v. TODOs

A. The initial conditions

We define the type of initial conditions for the damped harmonic oscillator. The initial conditions are the position and velocity at time `0`.

B. Trajectories associated with the initial conditions

For each damping regime, we give an explicit formula for the trajectory with the specified initial conditions.

B.1. Regime-specific base trajectories

B.2. The selected trajectory

B.3. Shared calculus lemmas

The three solution formulas all have the form `exp (-a * t) • y t`. The following private lemmas compute the first and second derivatives of that expression and package the common equation-of-motion argument.

B.4. Derivatives of the base trajectories

The remaining private lemmas compute the velocity and acceleration of the trigonometric, polynomial, and hyperbolic base trajectories before the exponential decay factor is applied.

C. Trajectories and equation of motion

The regime-specific trajectories satisfy the equation of motion for the damped harmonic oscillator.

C.1. The selected trajectory satisfies the equation of motion

C.2. Uniqueness of the solutions

Future work: prove that, in each damping regime, the selected explicit branch is the unique solution of the damped equation of motion with the given initial conditions.

15 declarations

definition

Oscillatory part of the underdamped trajectory x0cos(ωt)+v0+γx0ωsin(ωt)x_0 \cos(\omega t) + \frac{v_0 + \gamma x_0}{\omega} \sin(\omega t)

Given a set of initial conditions ICIC with initial position x0x_0 and initial velocity v0v_0, and a system with angular frequency ω\omega and decay rate γ\gamma, this function maps a time tt to the oscillatory component of the underdamped harmonic oscillator trajectory: tx0cos(ωt)+v0+γx0ωsin(ωt) t \mapsto x_0 \cos(\omega t) + \frac{v_0 + \gamma x_0}{\omega} \sin(\omega t) This represents the part of the solution that is subsequently multiplied by the exponential factor eγte^{-\gamma t} to form the complete trajectory. The values are elements of the 1-dimensional Euclidean space R1\mathbb{R}^1.

definition

Polynomial part of the critically damped trajectory x0+t(v0+γx0)x_0 + t(v_0 + \gamma x_0)

Given a set of initial conditions ICIC with initial position x0x_0 and initial velocity v0v_0, and a system decay rate γ\gamma, this function maps a time tt to the polynomial component of the critically damped oscillator trajectory: tx0+t(v0+γx0) t \mapsto x_0 + t(v_0 + \gamma x_0) This represents the part of the solution that is subsequently multiplied by the exponential factor eγte^{-\gamma t} to form the complete trajectory.

definition

Hyperbolic part of the overdamped trajectory cosh(ωt)x0+sinh(ωt)ω(v0+γx0)\cosh(\omega t) x_0 + \frac{\sinh(\omega t)}{\omega} (v_0 + \gamma x_0)

Given initial conditions ICIC with initial position x0x_0 and initial velocity v0v_0, a system decay rate γ\gamma, and an angular frequency ω\omega, this function maps a time tt to the hyperbolic component of the overdamped oscillator trajectory: tcosh(ωt)x0+sinh(ωt)ω(v0+γx0) t \mapsto \cosh(\omega t) x_0 + \frac{\sinh(\omega t)}{\omega} (v_0 + \gamma x_0) This represents the part of the solution that is subsequently multiplied by the exponential factor eγte^{-\gamma t} to form the complete trajectory in the overdamped regime.

definition

Trajectory of a damped harmonic oscillator for given initial conditions ICIC

Given initial conditions ICIC (representing initial position x0x_0 and initial velocity v0v_0 at t=0t=0), the function `trajectory` maps a time tt to the position of the damped harmonic oscillator in a 1-dimensional Euclidean space R1\mathbb{R}^1. The solution is defined piecewise according to the damping regime of the system: - If the system is **underdamped**, the trajectory is eγtxunder(t)e^{-\gamma t} \cdot x_{\text{under}}(t), where xunder(t)x_{\text{under}}(t) is the trigonometric base solution. - If the system is **critically damped**, the trajectory is eγtxcrit(t)e^{-\gamma t} \cdot x_{\text{crit}}(t), where xcrit(t)x_{\text{crit}}(t) is the polynomial base solution. - Otherwise (the **overdamped** case), the trajectory is eγtxover(t)e^{-\gamma t} \cdot x_{\text{over}}(t), where xover(t)x_{\text{over}}(t) is the hyperbolic base solution. Here, γ\gamma denotes the decay rate of the oscillator.

theorem

Underdamped trajectory equals eγtxunder(t)e^{-\gamma t} \cdot x_{\text{under}}(t)

Consider a damped harmonic oscillator system SS with initial conditions ICIC (consisting of initial position x0x_0 and initial velocity v0v_0). If the system is underdamped (i.e., its discriminant γ24mk<0\gamma^2 - 4mk < 0), then the trajectory x(t)x(t) is given by the product of an exponential decay factor and a trigonometric base solution: x(t)=eγtxunder(t) x(t) = e^{-\gamma t} \cdot x_{\text{under}}(t) where γ\gamma is the decay rate of the system and xunder(t)x_{\text{under}}(t) is the trigonometric base solution x0cos(ωt)+v0+γx0ωsin(ωt)x_0 \cos(\omega t) + \frac{v_0 + \gamma x_0}{\omega} \sin(\omega t).

theorem

Trajectory of a Critically Damped Oscillator equals eγtxcrit(t)e^{-\gamma t} \cdot x_{\text{crit}}(t)

In a critically damped harmonic oscillator system SS, for any given initial conditions ICIC, the trajectory x(t)x(t) is given by the product of an exponential decay factor and a polynomial base solution: x(t)=eγtxcrit(t) x(t) = e^{-\gamma t} \cdot x_{\text{crit}}(t) where γ\gamma is the decay rate of the system and xcrit(t)x_{\text{crit}}(t) is the polynomial base solution x0+t(v0+γx0)x_0 + t(v_0 + \gamma x_0) determined by the initial position x0x_0 and initial velocity v0v_0.

theorem

Overdamped Trajectory is eγtxover(t)e^{-\gamma t} \cdot x_{\text{over}}(t)

For a damped harmonic oscillator in the overdamped regime (where the discriminant γ24mk>0\gamma^2 - 4mk > 0), the trajectory x(t)x(t) for a given set of initial conditions ICIC is the product of an exponential decay factor and the hyperbolic base solution. Specifically, for any time tt, the trajectory is given by: x(t)=eγtxover(t) x(t) = e^{-\gamma t} \cdot x_{\text{over}}(t) where γ\gamma is the decay rate of the oscillator and xover(t)x_{\text{over}}(t) is the hyperbolic base solution defined as cosh(ωt)x0+sinh(ωt)ω(v0+γx0)\cosh(\omega t) x_0 + \frac{\sinh(\omega t)}{\omega} (v_0 + \gamma x_0) for initial position x0x_0 and initial velocity v0v_0.

theorem

The Trajectory of a Damped Harmonic Oscillator is CC^\infty

For any initial conditions ICIC of a damped harmonic oscillator system SS, the resulting trajectory x(t)x(t) is infinitely differentiable with respect to time, i.e., xC(R,R1)x \in C^\infty(\mathbb{R}, \mathbb{R}^1).

theorem

The trajectory at t=0t=0 is the initial position x0x_0

For a damped harmonic oscillator with given initial conditions ICIC, let x(t)x(t) denote the trajectory of the system as a function of time. The position of the oscillator at time t=0t=0 is equal to the initial position x0x_0 specified in the initial conditions: x(0)=x0 x(0) = x_0

theorem

The trajectory of a critically damped harmonic oscillator satisfies the equation of motion

Consider a damped harmonic oscillator system with mass mm, damping coefficient γ\gamma, and spring constant kk. For any given initial conditions ICIC (specifying position and velocity at t=0t=0), if the system is critically damped (i.e., its discriminant γ24mk=0\gamma^2 - 4mk = 0), then the trajectory x(t)x(t) associated with ICIC satisfies the damped equation of motion: mx¨(t)+γx˙(t)+kx(t)=0m \ddot{x}(t) + \gamma \dot{x}(t) + k x(t) = 0 for all times tt.

theorem

The underdamped trajectory satisfies the equation of motion mx¨+γx˙+kx=0m \ddot{x} + \gamma \dot{x} + kx = 0

Consider a damped harmonic oscillator system SS with mass mm, damping coefficient γ\gamma, and spring constant kk. If the system is underdamped (that is, the discriminant satisfies γ2<4mk\gamma^2 < 4mk), then for any initial conditions ICIC (initial position x0x_0 and initial velocity v0v_0), the resulting trajectory x(t)x(t) satisfies the equation of motion: mx¨(t)+γx˙(t)+kx(t)=0m \ddot{x}(t) + \gamma \dot{x}(t) + k x(t) = 0 where x˙(t)\dot{x}(t) and x¨(t)\ddot{x}(t) are the first and second time derivatives of the position x(t)x(t) with respect to time tt.

theorem

The overdamped trajectory satisfies the equation of motion

For a damped harmonic oscillator with mass mm, damping coefficient γ\gamma, and spring constant kk, if the system is overdamped (i.e., γ2>4mk\gamma^2 > 4mk), then the trajectory x(t)x(t) corresponding to the initial conditions ICIC satisfies the equation of motion: mx¨(t)+γx˙(t)+kx(t)=0m \ddot{x}(t) + \gamma \dot{x}(t) + k x(t) = 0 where x˙(t)\dot{x}(t) and x¨(t)\ddot{x}(t) are the first and second time derivatives of the position, respectively.

theorem

The trajectory of a damped harmonic oscillator satisfies the equation of motion mx¨+γx˙+kx=0m \ddot{x} + \gamma \dot{x} + kx = 0

For a damped harmonic oscillator with mass mm, damping coefficient γ\gamma, and spring constant kk, the trajectory x(t)x(t) defined for any given initial conditions ICIC satisfies the equation of motion: mx¨(t)+γx˙(t)+kx(t)=0m \ddot{x}(t) + \gamma \dot{x}(t) + k x(t) = 0 where x˙(t)\dot{x}(t) and x¨(t)\ddot{x}(t) denote the first and second time derivatives of the position x(t)x(t), respectively. This holds regardless of whether the system is underdamped, critically damped, or overdamped.

theorem

Uniqueness of Solutions to the Damped Harmonic Oscillator Equation of Motion

Let x,y:TimeR1x, y : \text{Time} \to \mathbb{R}^1 be two infinitely differentiable (CC^\infty) functions. If both xx and yy satisfy the equation of motion for a damped harmonic oscillator mx¨+γx˙+kx=0m \ddot{x} + \gamma \dot{x} + kx = 0, and share the same initial position x(0)=y(0)x(0) = y(0) and initial velocity x˙(0)=y˙(0)\dot{x}(0) = \dot{y}(0), then the trajectories are identical, x=yx = y.

theorem

Uniqueness of the Damped Harmonic Oscillator Trajectory for Given Initial Conditions

Consider a damped harmonic oscillator with initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0. Let x:TimeRx: \text{Time} \to \mathbb{R} be a smooth function (of class CC^\infty) that satisfies the equation of motion mx¨(t)+γx˙(t)+kx(t)=0m \ddot{x}(t) + \gamma \dot{x}(t) + k x(t) = 0. If xx satisfies the initial conditions x(0)=x0x(0) = x_0 and the time derivative at t=0t=0 is x˙(0)=v0\dot{x}(0) = v_0, then xx is equal to the uniquely defined trajectory for those initial conditions.