A normal mode of a closed system is an oscillation of the system in which all pa

Question

A normal mode of a closed system is an oscillation of the system in which all parts oscillate at a single frequency. In general there are an infinite number of such modes, each one with a distinctive frequency fi and associated pattern of oscillation.

Consider an example of a system with normal modes: a string of length L held fixed at both ends, located at x=0 and x=L. Assume that waves on this string propagate with speed v. The string extends in the x direction, and the waves are transverse with displacement along the y direction.

In this problem, you will investigate the shape of the normal modes and then their frequency.

The normal modes of this system are products of trigonometric functions. (For linear systems, the time dependance of a normal mode is always sinusoidal, but the spatial dependence need not be.) Specifically, for this system a normal mode is described by

yi(x,t)=Ai sin(2?*x/?i)sin(2?fi*t)

Find the three lowest normal mode frequencies f1, f2, and f3.
Express the frequencies in terms of L, v, and any constants. List them in increasing order, separated by commas.

f1, f2, f3 = ???

Explanation / Answer

if a string is hel d at both ends then

L = /2, , 3/2 ( is wavelength)

therefore the possible values of wavelength are

= 2L/3, L , 2L

hence frquencies will be

f1 = v/2L,

f2 = v/L,

f3 = 3v/2L

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