Home | Sign Up | Log In | RSSThursday, 2018-12-13, 7:07 AM


Site menu
Section categories
Education [150]
Videos [1]
Music [0]
Chat Box
learn c language learn c language branching and loop learn c language functions learn c language Pointer Basics how web page works Oracle Solaris 10 Oracle Solaris 10 How Web Pages Work Images How Web Pages Work Adding images&gr How Web Pages Work Introduction to How to Install WAMP How to Host Your Own Website for Fr How to Install the Apache Web Serve Atomic Structure The text provides 000 tons of conversations Introduction to How Radio Works Rad often over millions of miles citizens band radio Introduction to How the Radio Spect Can information travel faster than How F-15s Work by Tom Harris Browse How F/A-18s Work by Robert Valdes B How F/A-18s Work by Robert Valdes B Flying Video Game: In the Cockpit I How F/A-22 Raptors Work by Gary Wol 10 Unidentified Sounds That Scienti 10 Things You Didn't Know About Ein Is glass really a liquid? by Laurie How Radio Works by Marshall Brain B How Radio Works by Marshall Brain B 10 Most Terrifying Vehicle Manufact How a Top Fuel Dragster Works by Ch but can Siri meet our nee How Siri Works by Bernadette Johnso How the Tesla Turbine Works by Will Lecture 1: Inflationary Cosmology: Lecture 2: Inflationary Cosmology: Lecture 4: The Kinematics of the Ho Lecture 5: Cosmological Redshift an Lecture 6: The Dynamics of Homogene Lecture 7: The Dynamics of Homogene Lecture 8: The Dynamics of Homogene Lecture 9: The Dynamics of Homogene Lecture 10: Introduction to Non-Euc IBPS Clerical Cadre Exam Pattern De Institute of Banking Personnel Sele Educational Qualifications: A Degre INTERVIEW Candidates who have been official trailer for Mission Imposs How Relativity Connects Electric an in 1738 Kinetic Theory of Gases: A Brief Re Frames of Reference and Newton’s La attempts to measure the UVa Physics 12/1/07 “Moving Clocks Run Slow” pl More Relativity: The Train and the 12/1/07 The Formula If I walk from Adding Velocities: A Walk on the Tr 3/1/2008 The Story So Far: A Brief Mass and Energy Michael Fowler Energy and Momentum in Lorentz Tran How Relativity Connects Electric an Analyzing Waves on a String Michael by f even earlier than the bra Fermat's Principle of Least Time 9/ Hamilton's Principle and Noether's meaning they have Mechanical Similarity and the Viria Hamilton's Equations 9/10/15 A Dyna A New Way to Write the Action Integ Maupertuis came up with a kind of p Maupertuis' Principle: Minimum Acti Canonical Transformations Point Tra Introduction to Liouville's Theorem Adiabatic Invariants and Action-Ang Hyperbolas Michael Fowler Prelimina Mathematics for Orbits: Ellipses Keplerian Orbits Michael Fowler Pre Newton's equations for particle mot Dynamics of Motion in a Central Pot A Vectorial Approach: Hamilton's Eq Elastic Scattering Michael Fowler B Driven Oscillator Michael Fowler (c Dynamics of a One-Dimensional Cryst I usefor the spring constant (is a a mass on a spring Motion in a Rapidly Oscillating Fie Anharmonic Oscillators Michael Fowl in which the distance betw Motion of a Rigid Body: the Inertia Moments of Inertia: Examples Michae Euler's Angles Michael Fowler Intro or more precisely one our analysis of rotational motion h Euler's Equations Michael Fowler In Motion in a Non-inertial Frame of R Ball Rolling on Tilted Turntable Mi live cricket score roll the ball ba

Total online: 1
Guests: 1
Users: 0
Home » 2017 » September » 1 » Small Oscillations
1:34 AM
Small Oscillations

Small Oscillations

Michael Fowler

Particle in a Well

We begin with the one-dimensional case of a particle oscillating about a local minimum of the potential energy  We'll assume that near the minimum, call it , the potential is well described by the leading second-order term, , so we're taking the zero of potential at  assuming that the second derivative , and (for now) neglecting higher order terms.

To simplify the equations, we'll also move theorigin to , so

replacing the second derivative with the standard "spring constant" expression.

This equation has solution

(This can, of course, also be derived from the Lagrangian, easily shown to be )

Two Coupled Pendulums

We'll take two equal pendulums, coupled by a light spring. We take the spring restoring force to be directly proportional to the angular difference between the pendulums. (This turns out to be a good approximation.)

For small angles of oscillation, we take the Lagrangian to be

Denoting the single pendulum frequency by , the equations of motion are (writing , so )

We look for a periodic solution, writing

(The final physical angle solutions will be the real part.)

The equations become (in matrix notation):

Denoting the matrix by 

This is an eigenvector equation, with the eigenvalue, found by the standard procedure:

Solving, that is,

The corresponding eigenvectors are and .

Normal Modes

The physical motion corresponding to the amplitudes eigenvector has two constants of integration (amplitude and phase), often written in terms of a single complex number, that is,

with real.

Clearly, this is the mode in which the two pendulums are in sync, oscillating at their natural frequency, with the spring playing no role.

In physics, this mathematical eigenstate of the matrix is called a normal mode of oscillation. In a normal mode, all parts of the system oscillate at a single frequency, given by the eigenvalue.

The other normal mode,

where we have written . Here the system is oscillating with the single frequency , the pendulums are now exactly out of phase, so when they part the spring pulls them back to the center, thereby increasing the system oscillation frequency.

The matrix structure can be clarified by separating out the spring contribution:

All vectors are eigenvectors of the identity, of course, so the first matrix just contributes to the eigenvalue. The second matrix is easily found to have eigenvalues are 0,2, and eigenstates and .

Principle of Superposition

The equations of motion are linear equations, meaning that if you multiply a solution by a constant, that's still a solution, and if you have two different solutions to the equation, the sum of the two is also a solution. This is called the principle of superposition.

The general motion of the system is therefore

where it is understood thatare complex numbers and the physical motion is the real part.

This is a four-parameter solution: the initial positions and velocities can be set arbitrarily, completely determining the motion.

Exercise: begin with one pendulum straight down, the other displaced, both momentarily at rest. Find values forand describe the subsequent motion.

Solution: At the pendulums are at Take This ensures zero initial velocities too, since the physical parameters are the real parts of the complex solution, and at the initial instant the derivatives are pure imaginary.

The solution for the motion of the first pendulum is


and for small ,

Here the pendulum is oscillating at approximately , but the second term sets the overall oscillation amplitude: it's slowly varying, going to zero periodically (at which point the other pendulum has maximum kinetic energy).

To think about: What happens if the two have different masses? Do we still get these beats -- can the larger pendulum transfer all its kinetic energy to the smaller?

Exercise: try pendulums of different lengths, hung so the bobs are at the same level, small oscillation amplitude, same spring as above.

Three Coupled Pendulums

Let's now move on to the case of three equal mass coupled pendulums, the middle one connected to the other two, but they're not connected to each other.

The Lagrangian is


The equations of motion are

Putting , the equations can be written in matrix form

The normal modes of oscillation are given by the eigenstates of that second matrix.

The one obvious normal mode is all the pendulums swinging together, at the original frequency , so the springs stay at the rest length and play no role. For this mode, evidently the second matrix has a zero eigenvalue, and eigenvector .

The full eigenvalue equation is

that is, so the eigenvalues are , with frequencies 

The normal mode eigenvectors satisfy

They are , normalizing them to unity.

The equations of motion are linear, so the general solution is a superposition of the normal modes:

Normal Coordinates

Landau writes . (Actually he brings in an intermediate variable , but we'll skip that.) These "normal coordinates" can have any amplitude and phase, but oscillate at a single frequency  The components of the above vector equation read:

It's worth going through the exercise of writing the Lagrangian in terms of the normal coordinates:


Putting in the above expressions for the , after some algebra,

We've achieved a separation of variables. The Lagrangian is manifestly a sum of three simple harmonic oscillators, which can have independent amplitudes and phases. Incidentally, this directly leads to the action angle variables -- recall that for a simple harmonic oscillator the action and the angle is that of rotation in phase space.

Three Equal Pendulums Equally Coupled

What if we had a third identical spring connecting the two end pendulums (we could have small rods extending down so the spring went below the middle pendulum?

What would the modes of oscillation look like in this case?

Obviously, all three swinging together is still an option, the eigenvector , corresponding to eigenvalue zero of the "interaction matrix" above. But actually we have to extend that matrix to include the new spring -- it's easy to check this gives the equation:

The equation for the eigenvalues is easily found to be  Putting  into the matrix yields the equation  This is telling us that any vector perpendicular to the all-swinging-together vector is an eigenvector. This is because the other two eigenvectors have the same eigenvalue, meaning that any linear combination of them also has that eigenvalue -- this is a degeneracy.

Think about the physical situation. If we set the first two pendulums swinging exactly out of phase, the third pendulum will feel no net force, so will stay at rest. But we could equally choose another pair. And, the eigenvector we found before is still an eigenvector: it's in this degenerate subspace, equal to .

Category: Education | Views: 273 | Added by: farrel | Tags: Small Oscillations Michael Fowler P | Rating: 0.0/0
Total comments: 0
Name *:
Email *:
Code *:
Log In
Entries archive

Copyright eduCampus.tk © 2018
Powered by uCoz