I'm a real beginner in physics with a really basic doubt about waves.

Suppose i have a string ( perfect elastic material ) whose left-end i can manipulate ( i can change its heigth ) and whose right-end can either be fixed, loose or actually be non-existant ( the string goes indefinitely towards the right direction ) .

I'm interested in the effect of manipulating the heigth of the left-end of the string.
Let's consider two cases.
i) The heigth starts at $c_0$ and i indefinitely ( non-stop ) make it grow .
ii) The heigth starts at $c_0$ and i lift it to $c_1 > c_0 $ , keeping it constant at $c_1 $afterwards.

What would be the effect of i) and ii) in the remainder of the string ?
I was thinking that we could consider a wave would be generated, that is, the action i) or ii) would be propagated sequentially to all points of the string, starting at the consecutive point to the right of the left-end, forming a kind of wave.

My motivation for thinking of it as a wave, is that we can perceive that effect as simply energy traveling through a medium ( the string ) withouth causing a permanent change in its constituents ( the points of the string ) ... and that is the common feature that all waves we know ( electromagnetic, surface waves, etc ) share.
P.S : Here we can consider no damping ratio at all ( the energy is transfered withouth loss ) .

But is it really a wave ? Can we talk about frequency,period, wave-length and velocity of wave in both cases i) and ii) ?

In the i) action, i was think of considering , only theoretically, the period of the generated-wave as infinite and the frequency as 0. What about the wave-length and the velocity of propagation of the wave ? On what parameters would it depend ? Would it depend solely on the properties of the string ( density, tension,etc ) , or would it maybe also depend on the velocity that the i) action is realized ?

In the ii) action, i was thinkg of considering the period of the generated-wave simply as the ammount of time that it takes for the heigth $c_0$ grow to $c_1$, and considering the frequency as the inverse of that . But again, What about the wave-length and the velocity of propagation of the wave ?

Thanks a lot , and sorry for elaborating the question in a simplistic way ( i don't have enough physic's knowledge to elaborate it better ) .

P.S : What motivated this question was exploring with the following web app :

  • 2
    $\begingroup$ You've actually come very close to recognizing the Heisenberg uncertainty principle. If a wave packet is very localized (i.e. near definite position) then properties like frequency and wavelength don't make much sense. In the reverse case, when the wave packet is very spread out over space, the wave will have a well-defined wavelength but undefined position. In quantum mechanics the momentum of a particle is inversely proportional to its wavelength, which is only defined if the packet is spread out over space. (I.e. the more you know about momentum, the less you know about position.) $\endgroup$ – Jold Nov 20 '14 at 16:13

From Wikipedia:

In physics, a wave is disturbance or oscillation (of a physical quantity), that travels through matter or space, accompanied by a transfer of energy. Wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass transport. They consist, instead, of oscillations or vibrations around almost fixed locations. Waves are described by a wave equation which sets out how the disturbance proceeds over time.

If we use that definition, in both cases you will have a wave, because there is a perturbation that moves and transfers energy without mass transport. But is not a sinusoidal wave, that is one that has a sine shape with endless peaks and valleys. However, Fourier's theorem shows that any shape can be decomposed into a collection of sinusoidal waves of different frequencies. What this means is that in your example the wave has multiple frequencies (an infinite number to be more precise) or wavelengths.

  • $\begingroup$ I had a bit of a hard time trying to understand that, although i'm a little bit acquainted with Fourier. Anyways, i'm a little confused about basic matters. Fourier is usually applied to a function, and returns a linear combination of an orthogonal basis vectors. What would the function be ? I thought that it this would be the time-function whose value is the heigth of the left-end point. So, the fourier transform would give us a linear combination of sinusoidal motions of the same left-end , such that combined, give exactly f. But what does this have to do with the wave ? $\endgroup$ – nerdy Dec 4 '14 at 0:21
  • $\begingroup$ The wave would be the propagation of f ( which has to do only with the left-end point ) to all other points of the string. Would the propagation in itself be a function again, say g ? Would the answer that you gave me be talking about the fourier transform not of f, but g ? These are really basic doubts about wave but i'm so interested in grasping this. Anyways, thanks a lot for your help ! $\endgroup$ – nerdy Dec 4 '14 at 0:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.