Physlib

Physlib.ClassicalMechanics.HarmonicOscillator.Solution

Solutions to the classical harmonic oscillator

i. Overview

In this module we define the solutions to the classical harmonic oscillator, prove that they satisfy the equation of motion, and prove some properties of the solutions.

ii. Key results

  • `InitialConditions` is a structure for the initial conditions for the harmonic oscillator.
  • `trajectories` is the trajectories to the harmonic oscillator for given initial conditions.
  • `trajectories_equationOfMotion` proves that the solution satisfies the equation of motion.

iii. Table of contents

- A. The initial conditions - A.1. Definition of the initial conditions - A.1.1. Extensionality lemma - A.2. Relation to other types of initial conditions - A.3. The zero initial conditions - A.3.1. Simple results for the zero initial conditions - B. Trajectories associated with the initial conditions - B.1. The trajectory associated with the initial conditions - B.1.1. Definitional equality for the trajectory - B.2. The trajectory for zero initial conditions - B.3. Smoothness of the trajectories - B.4. Velocity of the trajectories - B.5. Acceleration of the trajectories - B.6. The initial conditions of the trajectories - C. Trajectories and Equation of motion - C.1. Uniqueness of the solutions - D. The energy of the trajectories - E. The trajectories at zero velocity - E.1. The times at which the velocity is zero - E.2. A time when the velocity is zero - E.3. The position when the velocity is zero - F. Some open TODOs

iv. References

References for the classical harmonic oscillator include: - Landau & Lifshitz, Mechanics, page 58, section 21.

A. The initial conditions

We define the type of initial conditions for the harmonic oscillator. The initial conditions are currently defined as an initial position and an initial velocity, that is the values of the solution and its time derivative at time `0`.

A.1. Definition of the initial conditions

We start by defining the type of initial conditions for the harmonic oscillator.

#### A.1.1. Extensionality lemma

We prove an extensionality lemma for `InitialConditions`. That is, a lemma which states that two initial conditions are equal if their initial positions and initial velocities are equal.

A.2. Relation to other types of initial conditions

We relate the initial condition given by an initial position and an initial velocity to other specifications of initial conditions.

In this section, we implement alternative ways to specify initial conditions for the harmonic oscillator. The standard `InitialConditions` type specifies position and velocity at time `t=0`, but in practice it is often useful to specify initial conditions at other times or in other forms.

Currently implemented: - **Initial conditions at arbitrary time**: Specify position and velocity at any time `t₀`, not necessarily at `t=0`. This is useful for problems where the natural reference time is not zero.

Future work (to be added in separate PRs) : - Initial conditions from two positions at different times - Initial conditions from two velocities at different times - Amplitude-phase parametrization

All alternative forms can be converted to the standard `InitialConditions` type via conversion functions, and we prove that the converted initial conditions produce trajectories that satisfy the original specifications.

#### A.2.1. Initial conditions at arbitrary time

We define a type for initial conditions specified at an arbitrary time `t₀`, rather than at `t=0`. This is useful when the natural reference point for a problem is not at time zero.

##### A.2.1.1. Extensionality lemma

We prove an extensionality lemma for `InitialConditionsAtTime`.

##### A.2.1.2. Conversion to standard initial conditions

We now define the conversion from `InitialConditionsAtTime` to the standard `InitialConditions`.

The conversion works by "running the trajectory backward in time" from `t₀` to `0`. Given that we know `x(t₀)` and `v(t₀)`, we use the harmonic oscillator solution formula with time-reversal to determine what `x(0)` and `v(0)` must have been.

Mathematically, if `x(t) = cos(ωt)·x₀ + (sin(ωt)/ω)·v₀`, then setting `t = t₀`: `x(t₀) = cos(ωt₀)·x₀ + (sin(ωt₀)/ω)·v₀` `v(t₀) = -ω·sin(ωt₀)·x₀ + cos(ωt₀)·v₀`

Solving this linear system for `x₀` and `v₀` gives the formulas below.

The correctness proofs showing that the conversion produces the expected trajectory are given later in section D.1, after the trajectory machinery has been defined.

A.3. The zero initial conditions

The zero initial conditions are the initial conditions with zero initial position and zero initial velocity.

In the end, we will see that this corresponds to the solution which is identically zero, i.e. the particle remains at rest at the origin.

#### A.3.1. Simple results for the zero initial conditions

Some simple results about the zero initial conditions.

B. Trajectories associated with the initial conditions

To each initial condition we association a trajectory. We will prove some basic properties of these trajectories.

Eventually we will show that these trajectories satisfy the equation of motion, for now we can think of them as some choice of trajectory associated with the initial conditions.

B.1. The trajectory associated with the initial conditions

#### B.1.1. Definitional equality for the trajectory

We show a basic definitional equality for the trajectory.

B.2. The trajectory for zero initial conditions

The trajectory for zero initial conditions is the zero function.

B.3. Smoothness of the trajectories

The trajectories for any initial conditions are smooth functions of time.

B.4. Velocity of the trajectories

We give a simplification of the velocity of the trajectory.

B.5. Acceleration of the trajectories

We give a simplification of the acceleration of the trajectory.

B.6. The initial conditions of the trajectories

We show that, unsurprisingly, the trajectories have the initial conditions used to define them.

C. Trajectories and Equation of motion

The trajectories satisfy the equation of motion for the harmonic oscillator.

C.1. Uniqueness of the solutions

We show that the trajectories are the unique solutions to the equation of motion for the given initial conditions. This is currently a TODO.

D. The energy of the trajectories

For a given set of initial conditions, the energy of the trajectory is constant, due to the conservation of energy. Here we show it's value.

D.1. Correctness of InitialConditionsAtTime conversion

We now prove the correctness lemmas for the `InitialConditionsAtTime.toInitialConditions` conversion function. These show that the conversion produces a trajectory that passes through the specified position and velocity at the specified time.

E. The trajectories at zero velocity

We study the properties of the trajectories when the velocity is zero.

E.1. The times at which the velocity is zero

We show that if the velocity of the trajectory is zero, then the time satisfies the condition that ``` tan (S.ω * t) = IC.v₀ 0 / (S.ω * IC.x₀ 0) ```

E.2. A time when the velocity is zero

We show that as long as the initial position is non-zero, then at the time `arctan (IC.v₀ 0 / (S.ω * IC.x₀ 0)) / S.ω` the velocity is zero.

E.3. The position when the velocity is zero

We show that the position is equal to `√(‖IC.x₀‖^2 + (‖IC.v₀‖/S.ω)^2) ` when the velocity is zero.

F. Some open TODOs

We give some open TODOs for the classical harmonic oscillator.

42 declarations

theorem

Equality of Initial Conditions from Initial Position and Velocity

For any two sets of initial conditions IC1IC_1 and IC2IC_2 for a classical harmonic oscillator, if their initial positions are equal (IC1.x0=IC2.x0IC_1.x_0 = IC_2.x_0) and their initial velocities are equal (IC1.v0=IC2.v0IC_1.v_0 = IC_2.v_0), then the initial conditions themselves are equal (IC1=IC2IC_1 = IC_2).

theorem

Equality of Initial Conditions at Arbitrary Time from Time, Position, and Velocity

For any two sets of initial conditions at an arbitrary time, IC1IC_1 and IC2IC_2, if their reference times are equal (IC1.t0=IC2.t0IC_1.t_0 = IC_2.t_0), their positions at those times are equal (IC1.xt0=IC2.xt0IC_1.x_{t_0} = IC_2.x_{t_0}), and their velocities at those times are equal (IC1.vt0=IC2.vt0IC_1.v_{t_0} = IC_2.v_{t_0}), then the initial conditions themselves are equal (IC1=IC2IC_1 = IC_2).

definition

Harmonic oscillator initial conditions at t=0t=0 from state at t0t_0

For a harmonic oscillator SS with angular frequency ω\omega and a state specified at time t0t_0 by position xt0x_{t_0} and velocity vt0v_{t_0}, this function calculates the equivalent initial conditions at time t=0t=0. The resulting initial position x0x_0 and initial velocity v0v_0 are defined as: x0=cos(ωt0)xt0sin(ωt0)ωvt0x_0 = \cos(\omega t_0) x_{t_0} - \frac{\sin(\omega t_0)}{\omega} v_{t_0} v0=ωsin(ωt0)xt0+cos(ωt0)vt0v_0 = \omega \sin(\omega t_0) x_{t_0} + \cos(\omega t_0) v_{t_0} This conversion effectively runs the trajectory backward from t0t_0 to 00 to determine the starting state.

instance

Zero initial conditions for the harmonic oscillator

The zero initial condition for a harmonic oscillator is defined as the state where both the initial position x0x_0 and the initial velocity v0v_0 are equal to 00. This defines the zero element (additive identity) for the `InitialConditions` structure.

theorem

Initial position x0=0x_0 = 0 for zero initial conditions

For the zero initial conditions of a classical harmonic oscillator, the initial position x0x_0 is equal to 00. This confirms that the zero initial condition state corresponds to a particle starting at the origin.

theorem

Initial Velocity v0=0v_0 = 0 for Zero Initial Conditions

For the zero initial conditions of a classical harmonic oscillator, the initial velocity v0v_0 is equal to 00. This indicates that the zero initial condition corresponds to a state with zero starting velocity.

definition

Trajectory x(t)x(t) of the harmonic oscillator

Given a set of initial conditions ICIC with initial position x0x_0 and initial velocity v0v_0, the trajectory of the classical harmonic oscillator is the function that maps each time tt to its position in 1D Euclidean space, defined by: x(t)=cos(ωt)x0+sin(ωt)ωv0x(t) = \cos(\omega t) x_0 + \frac{\sin(\omega t)}{\omega} v_0 where ω\omega is the angular frequency of the oscillator system.

theorem

Trajectory Equation x(t)=cos(ωt)x0+sin(ωt)ωv0x(t) = \cos(\omega t) x_0 + \frac{\sin(\omega t)}{\omega} v_0

For a classical harmonic oscillator system SS with angular frequency ω\omega and initial conditions ICIC consisting of an initial position x0x_0 and an initial velocity v0v_0, the trajectory x(t)x(t) is given by: x(t)=cos(ωt)x0+sin(ωt)ωv0x(t) = \cos(\omega t) x_0 + \frac{\sin(\omega t)}{\omega} v_0

theorem

The trajectory x(t)x(t) is identically zero for zero initial conditions

For a classical harmonic oscillator system with zero initial conditions (where the initial position x0x_0 and the initial velocity v0v_0 are both zero), the resulting trajectory x(t)x(t) is identically zero for all times tt.

theorem

The trajectory x(t)x(t) is CC^\infty smooth

For a classical harmonic oscillator system SS and a given set of initial conditions ICIC, the resulting trajectory x(t)x(t) is nn-times continuously differentiable (CnC^n) for any nN{}n \in \mathbb{N} \cup \{\infty\}. That is, the trajectory is infinitely differentiable.

theorem

The Velocity of the Harmonic Oscillator Trajectory is ωsin(ωt)x0+cos(ωt)v0-\omega \sin(\omega t) x_0 + \cos(\omega t) v_0

For a classical harmonic oscillator with angular frequency ω\omega and initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0, let x(t)x(t) be the trajectory of the oscillator. The velocity at time tt, defined as the time derivative ddtx(t)\frac{d}{dt}x(t), is given by: ddtx(t)=ωsin(ωt)x0+cos(ωt)v0\frac{d}{dt}x(t) = -\omega \sin(\omega t) x_0 + \cos(\omega t) v_0

theorem

The Acceleration of the Harmonic Oscillator Trajectory is ω2cos(ωt)x0ωsin(ωt)v0-\omega^2 \cos(\omega t) x_0 - \omega \sin(\omega t) v_0

For a classical harmonic oscillator with angular frequency ω\omega and initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0, let x(t)x(t) be the trajectory of the oscillator. The acceleration at time tt, defined as the second time derivative d2dt2x(t)\frac{d^2}{dt^2}x(t), is given by: d2dt2x(t)=ω2cos(ωt)x0ωsin(ωt)v0\frac{d^2}{dt^2}x(t) = -\omega^2 \cos(\omega t) x_0 - \omega \sin(\omega t) v_0

theorem

The Position of a Harmonic Oscillator Trajectory at t=0t=0 is x0x_0

For a classical harmonic oscillator system SS with a given set of initial conditions ICIC, let x(t)x(t) denote the trajectory of the oscillator. If x0x_0 is the initial position specified in ICIC, then the position at time t=0t = 0 is x(0)=x0x(0) = x_0.

theorem

The Velocity of a Harmonic Oscillator Trajectory at t=0t=0 is v0v_0

For a classical harmonic oscillator system SS with initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0, let x(t)x(t) be the trajectory of the oscillator. The velocity at time t=0t = 0, defined as the time derivative ddtx(t)\frac{d}{dt}x(t) evaluated at t=0t=0, is equal to the initial velocity v0v_0. That is, ddtx(t)t=0=v0\frac{d}{dt}x(t) \Big|_{t=0} = v_0

theorem

The Harmonic Oscillator Trajectory Satisfies the Equation of Motion

For a classical harmonic oscillator system SS and a given set of initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0, let x(t)x(t) be the trajectory of the oscillator. The trajectory x(t)x(t) satisfies the equation of motion of the harmonic oscillator system SS.

theorem

Uniqueness of the Harmonic Oscillator Trajectory

For a classical harmonic oscillator system SS and a given set of initial conditions ICIC specifying an initial position x0x_0 and an initial velocity v0v_0 at time t=0t=0, let x:TimeRx: \text{Time} \to \mathbb{R} be an infinitely differentiable (CC^\infty) function. If xx satisfies the oscillator's equation of motion and the initial conditions x(0)=x0x(0) = x_0 and tx(0)=v0\partial_t x(0) = v_0, then xx is equal to the predefined trajectory of the system, given by: x(t)=cos(ωt)x0+sin(ωt)ωv0x(t) = \cos(\omega t) x_0 + \frac{\sin(\omega t)}{\omega} v_0 where ω\omega is the angular frequency of the oscillator.

theorem

The total energy of a harmonic oscillator trajectory is 12(mv02+kx02)\frac{1}{2} (m \|v_0\|^2 + k \|x_0\|^2)

For a classical harmonic oscillator system SS with mass mm and spring constant kk, let x(t)x(t) be the trajectory defined by the initial conditions ICIC, where x0x_0 is the initial position and v0v_0 is the initial velocity at time t=0t=0. The total energy E(t)E(t) of the trajectory is constant for all time tt and is equal to the initial energy: E(t)=12(mv02+kx02)E(t) = \frac{1}{2} (m \|v_0\|^2 + k \|x_0\|^2)

theorem

x(t0)=xt0x(t_0) = x_{t_0} for trajectory from converted initial conditions

For a harmonic oscillator SS and a set of conditions ICIC specifying a position xt0x_{t_0} and a velocity vt0v_{t_0} at an arbitrary time t0t_0, the trajectory x(t)x(t) defined by the equivalent initial conditions at t=0t=0 (obtained via the conversion function `toInitialConditions`) passes through the specified position at time t0t_0, such that x(t0)=xt0x(t_0) = x_{t_0}.

theorem

x˙(t0)=vt0\dot{x}(t_0) = v_{t_0} for trajectory from converted initial conditions

For a harmonic oscillator SS and a set of conditions ICIC specifying a position xt0x_{t_0} and a velocity vt0v_{t_0} at an arbitrary time t0t_0, let x(t)x(t) be the trajectory defined by the equivalent initial conditions at t=0t=0 (obtained via the conversion function `toInitialConditions`). The velocity of this trajectory at time t0t_0, defined as the time derivative ddtx(t)\frac{d}{dt}x(t) evaluated at t=t0t = t_0, is equal to the specified velocity: ddtx(t)t=t0=vt0\left. \frac{d}{dt}x(t) \right|_{t=t_0} = v_{t_0}

theorem

Energy at t0t_0 equals 12mvt02+12kxt02\frac{1}{2} m \|v_{t_0}\|^2 + \frac{1}{2} k \|x_{t_0}\|^2 for the Harmonic Oscillator

Consider a classical harmonic oscillator SS with mass mm and spring constant kk. Let ICIC be a set of initial conditions specifying a position xt0x_{t_0} and a velocity vt0v_{t_0} at an arbitrary time t0t_0. Let x(t)x(t) be the trajectory of the oscillator corresponding to these conditions. Then the total energy EE of the trajectory at time t0t_0 is given by: E(t0)=12(mvt02+kxt02)E(t_0) = \frac{1}{2} (m \|v_{t_0}\|^2 + k \|x_{t_0}\|^2) where \|\cdot\| denotes the norm in the oscillator's configuration space.

theorem

tan(ωt)=v0ωx0\tan(\omega t) = \frac{v_0}{\omega x_0} at times of zero velocity for the harmonic oscillator

Consider a classical harmonic oscillator with angular frequency ω\omega and initial conditions given by position x0x_0 and velocity v0v_0 at time t=0t=0. For any time tt, if the velocity of the trajectory is zero (i.e., ddtx(t)=0\frac{d}{dt}x(t) = 0) and the initial conditions are non-trivial (x00x_0 \neq 0 or v00v_0 \neq 0), then the time tt satisfies the relation tan(ωt)=v0ωx0\tan(\omega t) = \frac{v_0}{\omega x_0} where x0x_0 and v0v_0 are treated as the scalar values of the position and velocity in 1D space.

theorem

The velocity of the harmonic oscillator is zero at t=1ωarctan(v0ωx0)t = \frac{1}{\omega} \arctan \left( \frac{v_0}{\omega x_0} \right)

For a classical harmonic oscillator with angular frequency ω\omega and initial conditions specifying an initial position x0x_0 and initial velocity v0v_0 at time t=0t=0, if the component of the initial position x0(0)x_0(0) is non-zero, then the velocity v(t)=ddtx(t)v(t) = \frac{d}{dt}x(t) is zero at time t=1ωarctan(v0(0)ωx0(0))t = \frac{1}{\omega} \arctan \left( \frac{v_0(0)}{\omega x_0(0)} \right) where x0(0)x_0(0) and v0(0)v_0(0) are the scalar components of the initial position and velocity vectors in 1D Euclidean space.

theorem

The velocity of a harmonic oscillator is zero if and only if x(t)=x02+(v0/ω)2\|x(t)\| = \sqrt{\|x_0\|^2 + (\|v_0\|/\omega)^2}

For a classical harmonic oscillator system with angular frequency ω\omega and initial conditions given by position x0x_0 and velocity v0v_0 at time t=0t=0, let x(t)x(t) be the trajectory of the oscillator. The velocity of the trajectory at time tt, denoted by ddtx(t)\frac{d}{dt}x(t), is zero if and only if the norm of the position at that time satisfies: x(t)=x02+(v0ω)2\|x(t)\| = \sqrt{\|x_0\|^2 + \left(\frac{\|v_0\|}{\omega}\right)^2} where x02+(v0/ω)2\|x_0\|^2 + (\|v_0\|/\omega)^2 represents the square of the maximum displacement (amplitude) of the oscillation.

theorem

The velocity of the harmonic oscillator trajectory at t2t_2 equals vt2v_{t_2}

Consider a classical harmonic oscillator with angular frequency ω\omega. Let ICIC be the initial conditions determined by specifying velocities at two times t1t_1 and t2t_2. If sin(ω(t2t1))0\sin(\omega(t_2 - t_1)) \neq 0, then the velocity of the resulting trajectory x(t)x(t) at time t2t_2 is equal to the specified velocity vt2v_{t_2}: ddtx(t)t=t2=vt2\left. \frac{d}{dt}x(t) \right|_{t=t_2} = v_{t_2} where ddtx(t)\frac{d}{dt}x(t) denotes the time derivative of the trajectory.

definition

Conversion from Amplitude AA and Phase ϕ\phi to Initial Conditions (x0,v0)(x_0, v_0)

Given a classical harmonic oscillator SS with angular frequency ω\omega, and initial parameters specified by an amplitude AA and a phase ϕ\phi, this function defines the corresponding initial conditions (x0,v0)(x_0, v_0) at time t=0t = 0 as: x0=Acosϕx_0 = A \cos \phi v0=Aωsinϕv_0 = A \omega \sin \phi These initial conditions result in a trajectory of the form x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi).

theorem

Trajectory of the Harmonic Oscillator in Amplitude-Phase Form: x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi)

For a classical harmonic oscillator with angular frequency ω\omega, if the initial conditions are specified by an amplitude AA and a phase ϕ\phi, then the trajectory x(t)x(t) at any time tt is given by: x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi) Here, the trajectory represents the position of the oscillator in 1-dimensional Euclidean space.

theorem

Velocity of the Harmonic Oscillator in Amplitude-Phase Form: v(t)=Aωsin(ωtϕ)v(t) = -A \omega \sin(\omega t - \phi)

For a classical harmonic oscillator with angular frequency ω\omega, if the initial conditions are specified by an amplitude AA and a phase ϕ\phi, then the velocity v(t)v(t) (the time derivative of the trajectory x(t)x(t)) at any time tt is given by: v(t)=Aωsin(ωtϕ)v(t) = -A \omega \sin(\omega t - \phi) This velocity is expressed as a vector in 1-dimensional Euclidean space.

definition

Conversion of initial conditions (x0,v0)(x_0, v_0) to amplitude and phase (A,ϕ)(A, \phi)

Given a classical harmonic oscillator SS with angular frequency ω\omega and initial conditions ICIC specifying an initial position x0x_0 and initial velocity v0v_0, this function calculates the corresponding amplitude AA and phase ϕ\phi. The values are determined by representing the state as a complex number z=x0+iv0ωz = x_0 + i \frac{v_0}{\omega}, where: - The amplitude is the magnitude of zz, i.e., A=z=x02+(v0ω)2A = |z| = \sqrt{x_0^2 + \left(\frac{v_0}{\omega}\right)^2}. - The phase is the argument of zz, i.e., ϕ=arg(z)\phi = \arg(z). These parameters allow the trajectory to be expressed in the form x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi).

theorem

toInitialConditions(fromInitialConditions(IC))=IC\text{toInitialConditions}(\text{fromInitialConditions}(IC)) = IC

For a classical harmonic oscillator SS and any initial conditions ICIC (specifying initial position and velocity), converting ICIC into its amplitude-phase representation and then converting that representation back to initial conditions recovers the original ICIC.

theorem

Harmonic Oscillator Trajectory x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi)

For a classical harmonic oscillator SS with angular frequency ω\omega, let ICIC be the initial conditions specifying an initial position x0x_0 and velocity v0v_0. Let AA and ϕ\phi be the amplitude and phase calculated from these initial conditions. Then, for any time tt, the trajectory x(t)x(t) is given by: x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi) where x(t)x(t) is represented as a vector in 1D Euclidean space whose single component at index 0 is the shifted cosine.

theorem

Velocity of Harmonic Oscillator v(t)=Aωsin(ωtϕ)v(t) = -A \omega \sin(\omega t - \phi)

For a classical harmonic oscillator with angular frequency ω\omega, let AA and ϕ\phi be the amplitude and phase derived from the initial conditions (x0,v0)(x_0, v_0). Then, for any time tt, the velocity of the trajectory (the time derivative of the position) is given by the following expression in 1D Euclidean space: ddtx(t)=Aωsin(ωtϕ)\frac{d}{dt} x(t) = -A \omega \sin(\omega t - \phi) where x(t)x(t) is the trajectory associated with the given initial conditions.

theorem

Velocity x˙(t)=0    sin(ωtϕ)=0\dot{x}(t) = 0 \iff \sin(\omega t - \phi) = 0 for A0A \neq 0

In a classical harmonic oscillator with angular frequency ω\omega, let AA and ϕ\phi be the amplitude and phase determined by the initial conditions. If the amplitude AA is non-zero, then the velocity of the trajectory at time tt is zero if and only if the sine factor in the phase-shifted expression vanishes, i.e., ddtx(t)=0    sin(ωtϕ)=0\frac{d}{dt}x(t) = 0 \iff \sin(\omega t - \phi) = 0 where x(t)x(t) represents the trajectory of the oscillator.

theorem

x˙(t)=0    t=ϕ+nπω\dot{x}(t) = 0 \iff t = \frac{\phi + n\pi}{\omega} for non-zero amplitude

For a classical harmonic oscillator with angular frequency ω\omega, let AA and ϕ\phi be the amplitude and phase determined by the initial conditions. If the amplitude AA is non-zero, then for any time tt, the velocity of the trajectory x˙(t)\dot{x}(t) is zero if and only if there exists an integer nZn \in \mathbb{Z} such that t=ϕ+nπωt = \frac{\phi + n\pi}{\omega}

theorem

The velocity v(t)=0v(t) = 0 if and only if x(t)=A\|x(t)\| = A for a harmonic oscillator

For a classical harmonic oscillator with angular frequency ω\omega and initial conditions ICIC (initial position x0x_0 and initial velocity v0v_0), let x(t)x(t) be the resulting trajectory. Let A=x02+(v0/ω)2A = \sqrt{x_0^2 + (v_0/\omega)^2} be the amplitude of the oscillation. For any time tt, the velocity tx(t)\partial_t x(t) is zero if and only if the norm of the position x(t)\|x(t)\| is equal to the amplitude AA.

theorem

x(t)=0    cos(ωtϕ)=0x(t) = 0 \iff \cos(\omega t - \phi) = 0

For a classical harmonic oscillator with angular frequency ω\omega, let AA and ϕ\phi be the amplitude and phase determined by the initial conditions. If the amplitude AA is non-zero, then for any time tt, the trajectory x(t)x(t) satisfies x(t)=0x(t) = 0 if and only if cos(ωtϕ)=0\cos(\omega t - \phi) = 0.

theorem

x(t)=0    t=ϕ+(2n+1)π/2ωx(t) = 0 \iff t = \frac{\phi + (2n + 1)\pi/2}{\omega} for a Harmonic Oscillator

Consider a classical harmonic oscillator with angular frequency ω\omega. Let ICIC be the initial conditions of the system, with AA and ϕ\phi denoting the corresponding amplitude and phase respectively, such that the trajectory x(t)x(t) is given by x(t)=Acos(ωtϕ)x(t) = A \cos(\omega t - \phi). Provided the amplitude is non-zero (A0A \neq 0), the position of the oscillator is zero at time tt if and only if there exists an integer nZn \in \mathbb{Z} such that: t=ϕ+(2n+1)π2ωt = \frac{\phi + (2n + 1)\frac{\pi}{2}}{\omega}

definition

Period of a harmonic oscillator T=2π/ωT = 2\pi/\omega

For a classical harmonic oscillator SS, the period TT is defined as the ratio of 2π2\pi to the angular frequency ω\omega: T=2πωT = \frac{2\pi}{\omega}

definition

Notation for the period TT

The definition introduces the mathematical notation TT to represent the period of a harmonic oscillator.

theorem

T=2π/ωT = 2\pi/\omega

For a classical harmonic oscillator SS, the period TT and the angular frequency ω\omega satisfy the relation T=2πωT = \frac{2\pi}{\omega}

theorem

The period of a harmonic oscillator is positive (T>0T > 0)

For any classical harmonic oscillator SS, the period TT is strictly positive, satisfying 0<T0 < T.

theorem

The harmonic oscillator trajectory x(t)x(t) is periodic with period T=2π/ωT = 2\pi/\omega

For a classical harmonic oscillator SS with angular frequency ω\omega and period T=2πωT = \frac{2\pi}{\omega}, the trajectory x(t)x(t) resulting from any set of initial conditions ICIC is a periodic function of time with period TT. That is, for all time tt, x(t+T)=x(t)x(t + T) = x(t).

theorem

Return to initial state (x0,v0)(x_0, v_0) implies t=nTt = nT for the harmonic oscillator

For a classical harmonic oscillator with period TT, let x(t)x(t) be the trajectory with initial position x(0)=x0x(0) = x_0 and initial velocity x˙(0)=v0\dot{x}(0) = v_0. Assuming that the initial position and velocity are not both zero (i.e., (x0,v0)(0,0)(x_0, v_0) \neq (0, 0)), if at some time tt the oscillator returns to its initial state such that x(t)=x0x(t) = x_0 and x˙(t)=v0\dot{x}(t) = v_0, then tt is an integer multiple of the period TT. That is, there exists an integer nZn \in \mathbb{Z} such that t=nTt = nT.