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Author Topic: Do sea waves always move at the same speed?  (Read 14999 times)

Offline turnipsock

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Do sea waves always move at the same speed?
« on: 15/04/2008 11:42:39 »
Whilst I was bobbing around the Clyde Estuary last weekend, I noticed that the waves seem to always move at the same speed. Is this the case and, if so, what speed do they move at?



« Last Edit: 15/04/2008 22:57:15 by chris »


 

lyner

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Re: Do sea waves always move at the same speed?
« Reply #1 on: 15/04/2008 12:35:54 »
They don't move at the same speed, actually. The big (long waves) ones move much faster than the little ones. Just watch the tiny ripples on top of large waves; the large wave just rushes by, beneath them.
For very deep water the speed is proportional to the square root of the wavelength (i.e. four times the wavelength goes twice as fast).
The depth of the water affects the speed - shallower makes waves go slower. This accounts for the formation of 'breakers' where the seabed shelves; waves 'catch up' on themselves.
Surface waves are complicated because the particles of water actually go in circles - not just up and down. Even at depths of several metres, the water is still sloshing around when there are long wavelength waves on the surface.
see: http://hyperphysics.phy-astr.gsu.edu/hbase/watwav.html for the formulae.
btw, the tidal bulges are, effectively, waves and they go round the Earth in approximately a day! PDQ or wot, eh?
 

Offline DoctorBeaver

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Re: Do sea waves always move at the same speed?
« Reply #2 on: 15/04/2008 13:14:06 »
I didn't realise there was so much to it. I knew about waves trying to overtake themselves in shallower water, but the rest of it is fascinating. Why do the water particles go in circles?
 

lyner

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Re: Do sea waves always move at the same speed?
« Reply #3 on: 15/04/2008 22:23:42 »
Basically, there's nowhere else for them to go. They can't just move up and down or because something has to move 'out of the way'when the surface moves down and to fill up the gap when it moves up. The energy flows in one direction so the water on one side of the peak moves out of phase with the water on the other side. The result is that each particle on the surface has a combination of vertical and horizontal motion - giving a circle, when the wave is a progressive one. When you have a standing wave (like the one you can get in your bath) the water just moves up and down because the energy is not going anywhere.
Surface waves are a combination of transverse and longitudinal waves, 90o out of phase, basically.
 

Offline turnipsock

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Re: Do sea waves always move at the same speed?
« Reply #4 on: 15/04/2008 22:30:26 »
...so, in deep water the velocity is proportional to the wavelength.

In shallow water the wave speeds up, making You've Been Framed the program it is.
 

lyner

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Do sea waves always move at the same speed?
« Reply #5 on: 15/04/2008 23:14:15 »
...so, in deep water the velocity is proportional to the wavelength.

In shallow water the wave speeds up, making You've Been Framed the program it is.
Proportional to the square root of the wavelength.
 

Offline chris

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Do sea waves always move at the same speed?
« Reply #6 on: 17/04/2008 12:24:02 »
What is the basis for the association between the speed and the wavelength?
 

lyner

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Do sea waves always move at the same speed?
« Reply #7 on: 17/04/2008 21:59:12 »
It's some complicated maths (relatively) and it is in all the textbooks on waves. There isn't a two word answer to your question.
It might help if you remember that the frequency is the same as the frequency of the thing that started it off and can never change. The speed is equal to the wavelength times the frequency. If a wave comes upon a shallow region, the wavelength will become shorter - you could think of the constriction of the depth restricting the distance which a peak or trough can effect the water on either side of it. If the wavelength gets less then so will the speed.
« Last Edit: 17/04/2008 22:05:54 by sophiecentaur »
 

Offline turnipsock

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Do sea waves always move at the same speed?
« Reply #8 on: 17/04/2008 22:20:57 »
I thought the wavelength became longer is shallow regions.

Light has a constant speed doesn't it?

The reason for the association between speed and wavelength is to do with the formula in the link in the second post.
 

lyner

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Do sea waves always move at the same speed?
« Reply #9 on: 17/04/2008 22:54:08 »
The basic wave equation says speed equals freq times wavelength. That makes sense - if 10 long waves go past in a second they have to be going faster than if 10 short waves go past (in shallower water, of course). The waves in shallow water get shorter - that's why they  pile up and 'break' when they become too short for their height.
Light only has a constant speed 'in vacuo'. The wavelength is less in a dense medium so the speed is also less - see refractive index on google.
« Last Edit: 18/04/2008 10:31:44 by sophiecentaur »
 

Offline daveshorts

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Do sea waves always move at the same speed?
« Reply #10 on: 18/04/2008 12:35:46 »
I had a think about why low frequencies move faster recently. The hand wavey reason I have come up with goes as follows.

For the same amplitude the faster the particles in a wave are moving the faster a wave will travel, so if you are talking about a sound wave, if you use a less dense gas the particles will move faster so the wave will move faster.

In a water wave the horizontal force on a water molecule is going to just be proportional to the amplitude of the wave. So it will be the same for a  low and high frequencies.

This means that a low frequency will feel that force for longer in one direction, so the maximum (and average) speed of the water particles is goign to be higher. So the wave will move quicker.
 

Offline Madidus_Scientia

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Do sea waves always move at the same speed?
« Reply #11 on: 18/04/2008 13:45:28 »
I understand sophiecentaur's explanation:
Quote
The basic wave equation says speed equals freq times wavelength. That makes sense - if 10 long waves go past in a second they have to be going faster than if 10 short waves go past
, but i'm not sure i understand what you mean daveshorts, are you explaining the same thing in a different way? or do you mean that the less dense something is, the easier it is for the force creating the waves to move it faster? like how you can lift a 1 kg weight quicker than a 10 kg weight?
 

lyner

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Do sea waves always move at the same speed?
« Reply #12 on: 18/04/2008 21:11:44 »
Quote
In a water wave the horizontal force on a water molecule is going to just be proportional to the amplitude of the wave. So it will be the same for a  low and high frequencies.
But, for any mechanical wave, the restoring force is proportional to the displacement. It's just that, for a water wave, the force is due to the displacement of water and in sound, the force is due to distortion of the material.

Seriously, if you really want to see how waves work you have to look at the theory. It's all well established stuff.
The maths is very simple and elegant and gives you very different results for surface waves and 'bulk' waves. All verifiable by experiment, too.
 

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Do sea waves always move at the same speed?
« Reply #12 on: 18/04/2008 21:11:44 »

 

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