waves

[nibot]

One of the things I was taught in elementary school that has long bothered me is that "in [ocean] waves, the water isn't moving, it's just the effect that's moving." Watching the surfers outside my window (ha!), I can't help but remember this. Of course the spirit of the statement is correct, but the problem is that water is not a particularly compressable fluid, so conservation of mass says that water has to be moving. eh?

Comments

[nibot.livejournal.com]

http://www.mathcad.com/Library/LibraryContent/MathML/water_waves.htm

specifically refers to "[surface waves on] an incompressible fluid in a constant gravitational field"

[kaolinfire.livejournal.com]

well, the water's moving up and down, surely enough. just not traveling into shore so much... right?

[nibot.livejournal.com]

the point is that water can't stretch (it's incompressible), and there isn't going to be a vaccuum... so how can the whole column of water move up or down? I think "surface waves on an incompressible fluid in a gravitational field" are subtlely but importantly different than waves in an elastic material....

But think of a wave on a rope. Clearly the rope doesn't move left or right, but the wave propagates. Here the wave is transmitted via tension in the rope. Water doesn't really have 'tension'... ?

[kaolinfire.livejournal.com]

good point. I really wasn't making the incompressible connection.

[yonked.livejournal.com]

I think it's best to say that: in a wave through a rope the rope moves, and in a wave through the water the water moves. However the scale on which the rope-bits and water-bits move is << one wavelength.

Waves

[jqmold.livejournal.com]

I seem to recall that an individual particle of water in an ocean wave moves in a circular path - until the wave gets into shallow water. I'll look it up and recomment. R2

Re: Waves

[easwaran.livejournal.com]

That's what I recall as well. Of course, when the ocean gets really shallow (like at the beach), this interferes with some of the circular movement, so there's much more horizontal movement there, which surfers take advantage of.