Whats the difference between progressive wave and stationary waves ?

  1. Wats the difference between progressive wave and stationary waves ?
     
  2. jcsd
  3. HallsofIvy

    HallsofIvy 40,676
    Staff Emeritus
    Science Advisor

    Re: Waves

    Pretty much exactly what it says. Imagine a long bull whip, say, that you quickly "whip" up and then down exactly once. The ripple that moves down the whip is a "progressive" (I would say "traveling") wave.

    Now take the loose end of that whip and attach it to a wall. If you "whip" it up and down, that ripple will move down the whip, hit the wall, and come back, inverted. If you do that again and again at exactly the right frequency (the "natural" frequency which depends upon the density of the whip material and tension) the waves reflecting off the wall will add to and subtract from the incoming waves so that you have nodes (0 motion) at some points, highest and lowest points half way between them. That is the nodes and hightest and lowest are "stationary".
     
  4. Re: Waves

    What happens if you don't wave the whip at a natural frequency?

    When you pluck a stringed instrument you inject it with a continuum of frequencies. But you only hear the discrete natural frequencies. So what happens to the other frequencies?
     
  5. Re: Waves

    When you pluck a stringed instrument, you are making a "kink" in the string which contains the fundamental and its overtones (not a continuum!). The highest overtones dissipate much more quickly, and so the tone quickly settles to something more like the fundamental and a few overtones (which gives the instrument its characteristic "sound").
     
  6. Re: Waves

    O yes, you're quite right. When you pluck a string, you lift the string until it forms a triangle with respect to the equilibrium situation (i.e., the string is a straight line), and then let it go. That triangle (i.e., initial position of wave at time=0) can be written as the sum of waves with wave numbers corresponding to the natural frequencies. I thought for a moment that the triangle could not be written as such a sum, but that's not true - every possible configuration of the string can be written as such a sum, including a sine wave of wave number not equal to the natural wave numbers!

    Why do the higher overtones dissipate much quickly? Is it because of air resistance?
     
  7. Drakkith

    Staff: Mentor

    Re: Waves

    To take a guess I would say that every time the string reaches near maximum amplitude, it strains the string, which means that higher frequencies reach peak amplitude more frequently and lose energy each time.
     
  8. Re: Waves

    Basic difference between stationary and progressive wave is that progressive waves are transporting energy, but stationary waves trap energy on the antinodes (I mean in vaccum here, no damping). When the end of the node is removed, energy would be carried away.

    To this question, I think it is because of higher frequency has higher energy carried with it. Higher energy like kinetic energy, means it interacts more vigorously with, for instance, air molecules, then energy dissipate faster.
     
  9. Re: Waves

    There is a similar thread on the front page that talks about the harmonics on a guitar string that are activated just by lightly putting your finger on the string and plucking on it. The explanation given was that by placing your finger lightly on the string, you are forcing certain shapes, particularly the higher harmonics. However, if this is true, then shouldn't you hear nothing at all, since the higher harmonics die out because they vibrate faster? Yet you can hear the high-pitched sound.
     
  10. Re: Waves

    Actually, forget overtones. A stringed instrument usually has several strings whose fundamental are all different. Do you have to pluck the higher-pitched string to a greater amplitude in order to sustain the note the same length?
     
  11. Re: Waves

    Something like that, yes. It's a fun and easy experiment that I recommend trying! ;)
     
  12. Re: Waves

    does crest and trough cancel each other in stationary waves?and why are these waves called stationary waves ?
     
  13. Drakkith

    Staff: Mentor

    Re: Waves

    If there are two waves travelling in opposite directions and they are out of phase they will cancel each other out, yes.

    See here for more: http://en.wikipedia.org/wiki/Standing_wave
     
  14. Re: Waves

    does this apply to electromagnetic waves too?
     
  15. A.E

    A.E 2

    Re: Waves

    Electromagnetic waves are travelling waves.
     
  16. Drakkith

    Staff: Mentor

    Re: Waves

    Not quite in the same way. I can't explain it fully, perhaps someone else could.
     
  17. Re: Waves

    Sure you can have electromagnetic standing waves, in a suitable resonant cavity. They are used in eg. magnetrons in microwave ovens.
     
  18. Re: Waves

    I've always sensed that our description of light as a wave is misleading. I can imagine it ACTUALLY being a progressive wave moving up and down because wouldn't that contradict the very idea of special relativity? Some light would would travel faster than others because not only is there the forward motion at light speed, but also the up and down (or side to side if you prefer) of the wave. Higher frequencies would travel faster to achieve the same speed as slower waves that need not oscillate as frequently.

    So the question I ask is are we just calling it a traditional wave because it is simply convenient to do so or because light is, in fact, a wave jumbling back and forth while traveling at the ultimate speed limit across the universe?

    Feedback would be helpful. I haven't had the chance to study this more as of yet, so I may, and probably am, overlooking a critical element that would resolve this dilemma.
     
  19. sophiecentaur

    sophiecentaur 13,787
    Science Advisor
    Gold Member

    Re: Waves

    You get standing waves in signal lines and in some designs of RF filters. They are a real embarrassment in the lines between transmitters and antennae where then can introduce excessively high volts (High Voltage Standing Wave Ratio / VSWR).
     
  20. sophiecentaur

    sophiecentaur 13,787
    Science Advisor
    Gold Member

    Re: Waves

    Hi beantwin
    An electromagnetic wave is transverse but that doesn't mean anything is "jumbling" up and down. It's called transverse because the varying electric and magnetic fields are at right angles (transverse) to the direction that the wave is travelling.
    We call it a "traditional wave" because the same maths apply to em and all other, more tangible, waves.
     
  21. Re: Waves

    Thank you. That answers my question.
     
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