What are the properties of transverse waves?

[Celluhh]

Ok I get the gist of it, but isn't this more like telling us how a transverse wave is formed??ok pardon
Me for my ignorance I may have inferred wrongly

 

[lntz]

do you understand about standing waves on a string, and where abouts the nodes and antinodes must be? this is a first step to understanding waves on a string

 

[Celluhh]

Huh really? No I don't... My teacher didn't mention anything about them.

 

[Celluhh]

Hold on I think I was taught just that I wasn't taught the term

 

[lntz]

the general idea is that you can only fit certain wavelengths on strings of certain lengths.

at either ends of the string the waves amplitude will be a minimum (node), and at certain intervals along the string, the wave will be at a maximum (antinodes)

to have a wave on a string you must have atleast 2 nodes and 1 antinode, from there you can create shorter and shorter wavelengths by 'squeezing in' more nodes and antinodes.

2 nodes and an antinode would be half a wavelength.

 

[Celluhh]

Ok, it seems I haven't learned this yet. But how is this related to the period?

 

[lntz]

this relates to wavelength.

i don't know if anyone has made the point yet that time period is = 1/frequency.

that is what the time period is. so your questions about how can 1 wave be completed in the same time period as all the other parts of the string, seems strange. unless your frequency is changing, the time period is constant.

 

[sophiecentaur]

You can get traveling waves on strings too as long as the far end can move and dissipate (absorb) the energy reaching it. Waffling a long rope on the ground can produce a convincing traveling wave.
But, as has been said, most waves that we see on strings are standing waves in which the energy travels to the ends and is reflected many times, building up into a standing wave pattern.
Standing waves are usually the next step on from being taught about traveling (progressive) waves.

 

[Celluhh]

this relates to wavelength.

i don't know if anyone has made the point yet that time period is = 1/frequency.

that is what the time period is. so your questions about how can 1 wave be completed in the same time period as all the other parts of the string, seems strange. unless your frequency is changing, the time period is constant.

It is not that I do not know the definition of period, it is because I know it that I am asking about it. I am tryin to visualise the definition, .

 

[sophiecentaur]

Does the definition need to go further than saying that the period is the time for one cycle to be completed at any fixed point in space?
Likewise, the wavelength is the space between identical points on the wave at a given time.

You can't get much more succinct than that.

 

[Celluhh]

Does the definition need to go further than saying that the period is the time for one cycle to be completed at any fixed point in space?
Likewise, the wavelength is the space between identical points on the wave at a given time.

You can't get much more succinct than that.

Yeah, why any fixed point, they don't even receive energy and start moving at the same time, do they have the same speed?

 

[sophiecentaur]

Yeah, why any fixed point, they don't even receive energy and start moving at the same time, do they have the same speed?

I don't exactly understand your question but the wave takes time to travel so the displacement at different points on the wave will be different. This is obvious when you look at a wave in action. A ball, bobbing on the sea moves away from its highest position and returns, at regular intervals (1/frequency). Balls in other positions are not at the same part of their cycles. If what you seem to suggest were true then the whole of the sea would be going up and down at the same time.

I say again that, to get a handle on this, you can draw a wiggle on a piece of paper and move it past your eye. You can see the wavelength (measured on the paper) and you can follow the variations of 'height' as it goes past. No fancy computer simulation - just a piece of paper, a pen and a brain needed.

 

[Celluhh]

Uh I don't think you understand my question. Precisely because I understand how displacement of particles at different points of the wave is different, which is why the wave is formed in the first place, that I am wondering why, when the wave travels, or more specifically, energy is transferred from one particle to the next and the particles start moving consecutively, that they actually manage to all complete one
Oscillation in the same period to form one complete wave in that one period. Unless of course they are all moving at different speeds, or they all start moving at the same time, which is impossible.

 

[sophiecentaur]

Ah, I think I've got it now. Well - all the 'particles' involved are in identical situations to the others (assuming a homogeneous medium) They can all be modeled as a chain of identical masses, coupled together by identical 'springs' (all non-EM waves boil down to this basic model). Energy is passed along the chain of particles at a rate that depends upon the 'masses' and 'spring stiffnesses'. They will all respond the same and the forces acting on them will be the same - just a bit later as you look along the wave.
The time taken for one particle to get energy from its earlier neighbour and deliver it to the next one is what defines the speed of energy transfer. Each one is being dragged to follow one neighbouring particle and is dragging the next one in the chain. The more stiff the spring is, the closer the movement of each particlal follows its neighbour (higher wave speed), for a more sloppy spring, the delay will be greater and the wave speed will be lower. The frequency of the oscillations will all be the same and determined by the frequency of the source of the wave.

I could suggest that you try to show that they would behave in a different way - in order to justify your reservations. If you can't show that the standard model is not true then that would be a reason to assume that it could be true.

BTW, what actual type of wave are you using 'in your head' when you are visualising all this?

 

[Celluhh]

A transverse wave.

 

[Bob S]

Uh I don't think you understand my question. Precisely because I understand how displacement of particles at different points of the wave is different, which is why the wave is formed in the first place, that I am wondering why, when the wave travels, or more specifically, energy is transferred from one particle to the next and the particles start moving consecutively, that they actually manage to all complete one
Oscillation in the same period to form one complete wave in that one period. Unless of course they are all moving at different speeds, or they all start moving at the same time, which is impossible.

Here is a youtube video of a traveling wave on a long rope:



Every segment of the rope is oscillating up and down at the same frequency (and the same period), but each adjoining segment has a slightly different phase, so the wave appears to be traveling.

 

[sophiecentaur]

@ celluh Which particular 'transverse wave'? The problem is that we come across very few progressive transverse waves.
Whether transverse of longitudinal, the same arguments all apply because you always have a displacement of some sort and a restoring force.

 

[sophiecentaur]

Here is a youtube video of a traveling wave on a long rope:



Every segment of the rope is oscillating up and down at the same frequency (and the same period), but each adjoining segment has a slightly different phase, so the wave appears to be traveling.

That movie is quite good but the wave suffers from severe attenuation because it is dragging on the ground. But at least it is a truly progressive wave as there is no reflection so standing wave is generated.

The best progressive waves to study are on water - the only snag being that they are not transverse waves but a combination of both transverse and longitudinal. If one accepts that then all the basics are there and easy to watch and to visualise.

 

[Celluhh]

i consulted my teacher today and one sentence she said really woke me up.She said, the definition of period is the time taken for a particle to complete one oscillation, and it is also the time taken for one wave to be completed. However, the important thing to note is that although every particle completes an oscillation in the same time period, they do not complete their period at the exact same time, the particles are all in different phases of their oscillation with different speeds when the wave is completed. the definitions of period are just telling me that the time taken to complete one wave is one period and the time for one particle to complete one oscillation is also one period.
perhaps its the last sentence that made me accept the fact that i was complicating things too much.

So, my next question is, how do the particles "know" how to form a complete wave in one period, and make the wave periodic? ok, here i go again.

 

[sophiecentaur]

Particles need to "know" nothing. It is entirely the other way round. The result of how each (identical) particle behaves as it is pushed and pulled by its neighbours is what forms the wave and the mass / stiffness involved, determines the speed of the energy passing through ( the wave speed).
In a medium with almost zero mass particles and with very high stiffness there is a very short lag in passing the effect of a disturbance. The wave would not be 'visible' because there would be no apparent difference in their position. Wave speed would be ' infinite' (very high).

 

[Celluhh]

So it just happens that the time taken for one wave to be completed and one particle to
Complete its oscillation is the same ?

 

[sophiecentaur]

So it just happens that the time taken for one wave to be completed and one particle to
Complete its oscillation is the same ?

"Just happens"? The one follows from the other. Try to imagine what would happen if they weren't and what you would observe at one point in space.

 

[Celluhh]

I understand the movement had to be consecutive for it to be a wave but why are they nth exactly one period? Cant one be more or less?

 

[sophiecentaur]

How could a particle be moving at a different frequency from the frequency of the wave? Where would it be getting the force to make it move at that (different) rate?

I think you need to stop and take a breath on this one. You seem to hanging onto something that just has to be nonsense. Why?

 

[Jadaav]

Simplest I can think is this :

1. Wavelength is the distance between 2 crests or troughs. 1 crest/trough is a wavefront. The distance between 2 wavefronts is a wavelength. Wavelength is one complete wave motion indeed. Because between 2 crests you have one wavelength.

You can only denote wavelength when it is one particular instant of the wave. Time is not considered here; like in a picture.

2. If you're seeing 2 graphs of displacement against distance and displacement against time, you'll notice that the representation of wavelength and time are both the same. Time period is the time taken for a wave ( considering all the particles synced ) to do 1 complete wave motion. So you now understand the relationship between wavelength and time period.

 

[Celluhh]

Ok so since the frequency f the particle is dependent in the frequency of the source so for example in a ripple tank when the dipper goes down particles start To move and then a wave is formed then when the dipper goes down another time the first particle starts off another round of oscillations and another wave is formed. Is this what you are trying to say?

Ps:I realized I forgot about the source

 

[Celluhh]

@jadaav thanks! Very clear explanation on your part. I understand what you are trying to say.

 

[Jadaav]

Yeah I know. I had some problems with this chapter also. But finally got to understand it.
I'm sure you're learning the same that I was some months back ;)

Also the frequency of a wave cannot change when it is in motion ( atleast up to I know; I'm still learning myself lol ). So what was I saying ?

Yeah, the frequency of a wave depends on the frequency of vibration of the source. So once the wave is produced, the frequency cannot change.

V=f\lambda

You'll notice that in the ripple tank, when the wave travels from deeper into shallower :

1. Its frequency does not change ( as mentioned above )
2. Its speed decreases.
3. Its wavelength decreases.

There are many things more. Well that's up for you to learn.