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.
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Oscillatory part of the underdamped trajectory
Given a set of initial conditions with initial position and initial velocity , and a system with angular frequency and decay rate , this function maps a time to the oscillatory component of the underdamped harmonic oscillator trajectory: This represents the part of the solution that is subsequently multiplied by the exponential factor to form the complete trajectory. The values are elements of the 1-dimensional Euclidean space .
Polynomial part of the critically damped trajectory
Given a set of initial conditions with initial position and initial velocity , and a system decay rate , this function maps a time to the polynomial component of the critically damped oscillator trajectory: This represents the part of the solution that is subsequently multiplied by the exponential factor to form the complete trajectory.
Hyperbolic part of the overdamped trajectory
Given initial conditions with initial position and initial velocity , a system decay rate , and an angular frequency , this function maps a time to the hyperbolic component of the overdamped oscillator trajectory: This represents the part of the solution that is subsequently multiplied by the exponential factor to form the complete trajectory in the overdamped regime.
Trajectory of a damped harmonic oscillator for given initial conditions
Given initial conditions (representing initial position and initial velocity at ), the function `trajectory` maps a time to the position of the damped harmonic oscillator in a 1-dimensional Euclidean space . The solution is defined piecewise according to the damping regime of the system: - If the system is **underdamped**, the trajectory is , where is the trigonometric base solution. - If the system is **critically damped**, the trajectory is , where is the polynomial base solution. - Otherwise (the **overdamped** case), the trajectory is , where is the hyperbolic base solution. Here, denotes the decay rate of the oscillator.
Underdamped trajectory equals
Consider a damped harmonic oscillator system with initial conditions (consisting of initial position and initial velocity ). If the system is underdamped (i.e., its discriminant ), then the trajectory is given by the product of an exponential decay factor and a trigonometric base solution: where is the decay rate of the system and is the trigonometric base solution .
Trajectory of a Critically Damped Oscillator equals
In a critically damped harmonic oscillator system , for any given initial conditions , the trajectory is given by the product of an exponential decay factor and a polynomial base solution: where is the decay rate of the system and is the polynomial base solution determined by the initial position and initial velocity .
Overdamped Trajectory is
For a damped harmonic oscillator in the overdamped regime (where the discriminant ), the trajectory for a given set of initial conditions is the product of an exponential decay factor and the hyperbolic base solution. Specifically, for any time , the trajectory is given by: where is the decay rate of the oscillator and is the hyperbolic base solution defined as for initial position and initial velocity .
The Trajectory of a Damped Harmonic Oscillator is
For any initial conditions of a damped harmonic oscillator system , the resulting trajectory is infinitely differentiable with respect to time, i.e., .
The trajectory at is the initial position
For a damped harmonic oscillator with given initial conditions , let denote the trajectory of the system as a function of time. The position of the oscillator at time is equal to the initial position specified in the initial conditions:
The trajectory of a critically damped harmonic oscillator satisfies the equation of motion
Consider a damped harmonic oscillator system with mass , damping coefficient , and spring constant . For any given initial conditions (specifying position and velocity at ), if the system is critically damped (i.e., its discriminant ), then the trajectory associated with satisfies the damped equation of motion: for all times .
The underdamped trajectory satisfies the equation of motion
Consider a damped harmonic oscillator system with mass , damping coefficient , and spring constant . If the system is underdamped (that is, the discriminant satisfies ), then for any initial conditions (initial position and initial velocity ), the resulting trajectory satisfies the equation of motion: where and are the first and second time derivatives of the position with respect to time .
The overdamped trajectory satisfies the equation of motion
For a damped harmonic oscillator with mass , damping coefficient , and spring constant , if the system is overdamped (i.e., ), then the trajectory corresponding to the initial conditions satisfies the equation of motion: where and are the first and second time derivatives of the position, respectively.
The trajectory of a damped harmonic oscillator satisfies the equation of motion
For a damped harmonic oscillator with mass , damping coefficient , and spring constant , the trajectory defined for any given initial conditions satisfies the equation of motion: where and denote the first and second time derivatives of the position , respectively. This holds regardless of whether the system is underdamped, critically damped, or overdamped.
Uniqueness of Solutions to the Damped Harmonic Oscillator Equation of Motion
Let be two infinitely differentiable () functions. If both and satisfy the equation of motion for a damped harmonic oscillator , and share the same initial position and initial velocity , then the trajectories are identical, .
Uniqueness of the Damped Harmonic Oscillator Trajectory for Given Initial Conditions
Consider a damped harmonic oscillator with initial conditions specifying an initial position and an initial velocity . Let be a smooth function (of class ) that satisfies the equation of motion . If satisfies the initial conditions and the time derivative at is , then is equal to the uniquely defined trajectory for those initial conditions.
