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Speed of a wave and its particles

  1. May 10, 2012 #1
    i have a few questions which need clarification, so right now i will state my assumptions.


    definition of the period of a wave : it is the timetaken for one point on the wave to complete one oscillation. and also the time taken to produce one complete wave.

    definition of wave speed: in a time of one period, a crest on a wave will have moved a distance of one wavelength.

    based on the definitions, i can deduce that the distance of one oscillation is the distance of one wavelength. which means that, if we freeze a wave at onepoint in time, and measure its wavelength, it will be of the same distance as that covered by a particle (thatmakes up the wave) after one oscillation. time taken to produce onecomplete wave is a period, tme taken for a crest on the wave to move a distance of one wavelength is also one period. since speed is distance over time, i can therfore deduce that fr the wave and the particles that make up the wave to cover the same distance over the same period of time, both their speeds must be the same. please correct me if i am wrong, however.

    ps: i still don't get how a period is the time taken for one point on the wave to complete a distance of one wavelength and yet also the time taken for one wave to be completed. logically, this means that all particles that make up the wave will hve to start moving at th same time, which is impossible, as the wave would not even exist. but how then, can the particles complete one oscillation, and therefore one wave(many particles movin up and down at different times create a wave) (particles cover the same distance of one wavelength in one oscillation) in the same amount of time at the same speed if they don't actually start moving at the same time? but yet they can't.
    no one has actually told me the answer i want yet, sadly.....):


    help will be greatly appreciated!;)
     
  2. jcsd
  3. May 10, 2012 #2

    phinds

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    I'll leave it to the experts to get this right in details, but I think your fundamental flaw is a misunderstanding of the wave/particle duality. Light ACTS like a wave if you measure it in a way that sees waves and it ACTS like a particle (a SINGLE particle) if you measure it in a way that sees particles. It's NOT like there was a series of particles along the wave.
     
  4. May 10, 2012 #3

    BruceW

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    Since this is in the general physics section, I'm going to assume Celluhh is not talking about light, but some classical wave such as waves on a string, or sound waves, or water waves, e.t.c.

    Now we need to specify if we are talking about transverse or longitudinal waves. Since Celluhh mentioned crests, I am going to assume that means he/she wants to talk about transverse waves. And this is good because transverse waves are easier to imagine than longitudinal waves.

    So after getting all that clear, I can say that this statement is wrong: "i can deduce that the distance of one oscillation is the distance of one wavelength" This is wrong because the oscillations are perpendicular to the direction of propagation (assuming transverse wave), so the distance of the oscillation has nothing to do with the wavelength. And if we talked about longitudinal waves, again the distance of oscillation is not related to the wavelength (but this is harder to imagine, since the oscillation and direction of propagation are the same).
     
  5. May 10, 2012 #4

    phinds

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    seems like a bad assumption, since those waves don't HAVE "particles", they have "points", but you may be right.
     
  6. May 10, 2012 #5
    This is untrue. The distance covered by the particle has nothing to do with how far the wave has moved. To illustrate this, consider two waves in the attachment and suppose they have the same time period. The first one has the particles of the wave moving equal distance as the other since their amplitudes are equal, but the distance travelled by the waves are different.


    Edit : BruceW already beat me to the post :biggrin:
     

    Attached Files:

  7. May 10, 2012 #6
    So y'all are saying that the distance of one oscillation of a particle that makes up the wave and the distance of one wavelength of the wave are two totally different things?

    But then how come my textbook states that the period is time taken for a crest on the wave to Move a distance of one wavelength is the same as that taken by the crest to complete one oscillation? It doesn't make sense....

    @phinds, brucew is correct. I am talking about transverse waves.

    Thank you all for helping!
     
  8. May 10, 2012 #7
    It sure makes sense as the two distances (wavelength and distance traveled during an oscillation) are "traveled" with different speeds.
    The speed of the oscillation is not even constant but it has a harmonic time dependence (in the simplest case) and the maximum speed depends on the amplitude and frequency of the wave. Higher amplitude results in higher speed so the time taken for one complete oscillation is the same.
    The speed of the propagation is independent of both amplitude and frequency (in linear, non-dispersive medium approximation).
     
  9. May 10, 2012 #8
    I think its a valid assumption based on the context. The particles are the matter of whatever medium the wave is propagating through, not the wave/particle duality of QM. In water, for example, the speed that these individual water particles move at plays a role in the speed of the propagating wave.
     
  10. May 10, 2012 #9

    jtbell

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    Think of a cork bobbing up and down on the surface of water, as ripples pass underneath it. The crests of the ripples move horizontally, with a distance between them that we call the wavelength. The cork oscillates up and down, and we call the time for one complete oscillation the period.

    Suppose we start a stopwatch when ripple (crest) #1 passes underneath the cork. One period later, ripple #1 has moved a distance of one wavelength away from the cork, ripple #2 is now underneath the cork, and the cork has completed one cycle of oscillation.

    The individual water molecules basically move the same way the cork does, that is, up and down. They don't move horizontally along with the ripples.
     
  11. May 10, 2012 #10

    BruceW

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    yeah, good explanation from jtbell :)

    The molecules go up and down (or equivalently, the small section of string, or the small parcel of fluid), yet the crest moves to the side. So I often like to think of the moving crest as simply the illusion of a moving object. Because certainly there is no sideways movement of fluid, or string, or whatever.

    It is natural to think that the fluid is moving along with the crest, but this is not true. So our intuition lets us down in this case (I remember I struggled with this when first learning it).
     
  12. May 10, 2012 #11

    jtbell

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    Another example is a "wave" produced by people raising their arms in a football stadium. The "wave" travels horizontally, but the hands go up and down and the people stay where they are. (I don't know whether this example is meaningful outside the USA, though.)

    However, this example is a different from the other waves because water waves, light waves etc. transport energy, but a "football-stadium wave" doesn't.
     
  13. May 10, 2012 #12
    I'm not so sure that a football stadium "wave" does not transport energy. If you placed a party sized beach ball at one end of the football stadium wave, would it not travel to the other end if the conditions were right?
     
  14. May 11, 2012 #13
    it's precisely because i understand perfectly how the particles making up a wave move up an down at different times to actually form a wave propagating to lets say, the mright, that i just cannot imagine how the particles all complete one oscillation in one period and at the same time one wavegets completed in that very same period. even if the distance of one oscillation and one wavelength is different,if the first particle starts its oscillation and transfers energy to the next particle to get it moving, and energy gets transferred from particle to particle until a wave is formed, the first particle will complete its oscillation way earlier than the other particles down thë wave, unless the last particle that gets the energy travels at about ten times faster a speed that the first particle and maybe five times faster a speed than the middle particles,will a wave be formed in one period. but logically, since the first particle starts off the wave, it also starts off the period, and hence only the firstparticle will be considered to have completed an oscillation in that period, whereas the other particles of the wave appear to have completed their oscillation in less than a period. do you get me?

    i can understand why the wavelength and distance travelled in an oscillation are different, but only if the particles that makeup the wave travel at different speeds. i can also imagine why the speed of a wave and speed of particles that make up the wave are different though.
     
  15. May 11, 2012 #14

    BruceW

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    hmm. Maybe start with imagining an arbitrary point on the wave. So this point will complete one oscillation in one period. And for a point next to that point it also completes one oscillation in one period. The only difference is that the phase is different.

    So for example, one point might be at the top of its peak, and the next point might be just below the top of its peak, still on its way up. So from there, both points are doing oscillations with the same period, but the second one will always be a little bit behind the first one (in terms of its phase).

    This is an explanation as if there were a discrete number of points. But it is simple to take the limit as the number of points goes to infinity, to get the behaviour shown by a continuous, ideal string.
     
  16. May 11, 2012 #15
    This description is OK and is difficult to see where your problem is.
    Maybe in the word "completed" use earlier, in reference to a wavelength.
    Considering a propagating wave, when the wave produced by particle A reaches a particles B situated one wavelength away, particle A has completed one period of oscillation whereas particle B is just starting to move. Particle B did not "complete" anything yet. But the wave traveled by one wavelength to reach B.
     
  17. May 11, 2012 #16
    @brucew and nasu, yes of course. But nasu, tha particles of the wave that travels one wavelength to to b start their oscillations all at different times too . Unless you are saying that the period we are talking about starts at different times for each particle but is always of the same length?but then again as I said one complete wave can't be considered to have been formed in one period then.
     
  18. May 11, 2012 #17

    davenn

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    if it was speeding up then the measurements of the speed of sound, for example, would be meaningless, as under your definition the speed would be contineously changing

    D
     
    Last edited: May 12, 2012
  19. May 11, 2012 #18

    HallsofIvy

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    The whole point is that there is no particle of the wave that "travels one wave length". You can have a wave (in a wire, for example) that has a wavelength of 100 meters and an amplitude of 1 cm. While the wave "moves" one hundred yards along the wire, any particle of the wire moves up and down a total distance of 4 cm.

    Are you really clear on what a wave is?
     
  20. May 11, 2012 #19

    davenn

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    ok some visualisations that maybe will help
    these are from my interests in geology They show the direction of propagation of a way V's the particle motion within the material that the wave is passing through.

    grrr ... discovered that they are too big kb size to post here
    did a quick www page to show them

    here's a link to my site that shows the wave animation graphics
    seismic waves

    Celluhh, take particular note of the particle motion

    Dave
     
  21. May 11, 2012 #20
    "One complete wave" is misleading. It may be used as a short for one wavelength.
    Other than that, it does not mean much. A wave (as a phenomenon) is not some sort of finite object so we can have fractions of it.
    The wave is not "formed" in a specific time and exists afterwards. There is a perturbation somewhere in the medium and this perturbation propagates to other parts of the medium. This propagation is what is called a wave. There is no moment at which the wave is more "complete" than at other moments.

    And yes, various points in the wave have different phases. This means that they all complete one oscillation in the same time but they are not doing the same thing at the same time. When one goes up another may go down and so on. Various particles will start oscillating at various times, the ones farther away from the source at later times. So you can say that they start their cycle at different times.
     
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