Speed of a wave and its particles

[Celluhh]

Which is why th speed of the wave remains constant right ? Oh yeah I meant their vertical speed so at the same phases thy travel at the same speed right ? Ok thank you !

 

[davenn]

Particles don't travel in the direction of the wave. Their speed is zero (so yes, they're all traveling at the same speed :) )...
Particles never travel along the wave.

Not true !
In a compressional wave like a sound wave and a seismic P wave the particle motion (oscillation) is back and forwards along the line of the wave motion

Their motion is NOT perpendicular to the wave direction of travel as with some other waves
eg a seismic S wave

Dave

 

[sophiecentaur]

For a very good reason, we use the term "displacement" to describe the temporary motion of the particles (or even the fields) which are involved in a Wave. This can correspond to movement in ANY DIRECTION and the actual direction is irrelevant in nearly every discussion of the wave itself and the direction of the energy flow. Half of the misunderstandings on this thread have been because people haven't used the right terms.

 

[xAxis]

... Oh yeah I meant their vertical speed so at the same phases thy travel at the same speed right ?

Yes, at the same phase they have equal speeds. Note that in this example, during the first period no two particles are at the same phase, and that only the first particle has finnished one full oscillation.

 

[BruceW]

Yes, at the same phase they have equal speeds. Note that in this example, during the first period no two particles are at the same phase, and that only the first particle has finnished one full oscillation.

Agreed :)

 

[Celluhh]

Omg
Ok thank you all I understand
Now !

 

[Celluhh]

I tried to help you visualise. So just imagine the picture I propose. there is a streep which is say one molecule thin, like a line of molecules.
Now imagine the moment when the first particle's just finished the first oscillation. Because the lag in transfering energy is so small, the second particle has almost finnished it's first oscillation. So if you can see that (together with "backwards time copy") the wave started traveling to the right almost the same instant that the first particle started first oscilation, it shouldn't be difficult to see that now, as it have finished, the wave has traveled down the line some distance. But remember the main point of wave? The motion of it is because the motion of the source (up down oscillation) is comunicated to the next particle. Now you should be able to see that that distance cannot be more than one wavelent because it would mean that some particles have already finnished one full osculation and started the second, but it cannot be cause their motion is just the copy of the motion of previous particle, and none of them had finnished full oscilation. Does this help?

Then again, why can't it be half a wavelength then?

 

[sophiecentaur]

Half a wavelength away the particles will be traveling in opposite directions. One going up and the other going down - they are in antiphase.

 

[Celluhh]

Ok wait um a wavelength is decided only after a stable and periodic wave motion had been set up right? It cannot be deduce after only one wave right?

 

[sophiecentaur]

That's right. To set up a wave that you can recognise as such requires some time to switch it on (and off). If you just try a, 'up/down' jerk of a string, the wave will tend to disperse into an odd shape because it will in fact consist of a host of different waves at different frequencies.