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Re: [Phys-l] surfing



On 07/10/2009 11:24 AM, Carl Mungan wrote:
Thanks for the discussion and references. I think I've got a clearer
picture now: You want to be going a bit slower than the wave (so it
catches up to you) but not too much slower (so it doesn't just pass
under you quickly). Then as you start riding up the sloped front of
the wave, the "normal force" of the water surface pushes you forward.

OK so far.

Presumably you then adjust your position along the slope to balance
air resistance

Plus lots of water resistance. Depending on what the wind is
doing, the drag could be 100% or even more than 100% water
resistance.

and maintain equilibrium (in the wave's frame of
reference, so there is no water drag, at least until you start moving
laterally along the wavefront and no longer straight toward the
beach).

I wouldn't have said that. The motion of the wave is not the
motion of the medium! To demonstrate this, set up a rope,
clothesline style. Tie a ribbon onto it somewhere. Whack
the rope to set up a transverse wave. The wave moves along
the rope, but the ribbon moves (mostly) perpendicular to the
rope.

Or better yet, set up ye olde ripple tank. Or even better,
watch the motion of a waterlogged stick (density near 1) in
the ocean as a wave comes along. Mostly the stick just stays
there as the wave passes under it.

Ocean waves are transverse. The motion of the parcels of water
is not /exactly/ perpendicular to the surface; actually it's
more like one turn around a small circle ... but approximating
it as purely vertical is much better than approximating it as
moving along with the wavefront.


Next question: How does plasma wakefield acceleration (which is also
often described as "surfing") work?
http://en.wikipedia.org/wiki/Plasma_acceleration

If that isn't what you wanted to know, please ask a more specific
question.