Chapter 8

Mechanical waves

Transversal oscillations of a leaded rope

Take 15 minutes to prepare this exercise.

Then, if you lack ideas to begin, look at the given clue and start searching for the solution.

A detailed solution is then proposed to you.

If you have more questions, feel free to ask them on the forum.

An elastic cord of negligible mass is at equilibrium, tense with a force between two fixed points and of a distance from each other.

The rope supports, regularly spaced, three weights , and of the same mass .

Neglecting the weight of the leads, each string section of length is in the initial equilibrium state is characterized by the stiffness of the string and natural length .

Let .

Small transverse movements of the leads are studied ; ordinate of lead is at time ( ).

It is assumed that its abscissa is constantly equal to .

Transversal oscillations of a leaded rope


Establish a differential system of the second order relative to the studied movement.



The Newton's second law applied to each lead in vertical projection gives :

So :


We seek the solution of type (all weights vibrating in phase with the same frequency).

Determine the possible values of for such movements (proper modes of the system).



The linear system is obtained :

This system has nontrivial solutions if the determinant is zero :

We then obtain three angular frequencies of the oscillations :

SimulationAmazing pendulum wave effect

Do you know how to build this wave pendulum ?

Quand la physique des pendules vous présente des effets esthétiques
Waves in a pool
To test the understanding of the lesson