Re: [Phys-l] surfing

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

> -----Original Message-----
> From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
> bounces@carnot.physics.buffalo.edu] On Behalf Of Carl Mungan
> Sent: Friday, July 10, 2009 2:24 PM
> To: phys-l@carnot.physics.buffalo.edu
> Subject: Re: [Phys-l] surfing
>
> 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.
> Presumably you then adjust your position along the slope to balance
> air 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). There is an energy loss (but no change in kinetic energy
> after the initial acceleration up to the wave speed, my mistake
> indeed) equal to the air resistance times longitudinal distance moved
> (in the ground frame, in addition to water/air drag times lateral
> distance moved) which is being taken out of the wave (amplitude
> and/or speed, but imperceptibly small I suppose).

I would say that in steady state, there is a significant drag force
being exerted on the surfer (board plus person) by the water when the
surfer is going "straight toward the beach" .  The surfer is moving
forward (relative to the beach) at the same speed as the wave, but the
water on which the surfer is riding is not moving forward as fast as the
wave is.  The normal force is upward and forward, the drag force is
upward and backward for a net water force that is essentially straight
upward and equal in magnitude to the gravitational force on the surfer.
Air resistance is negligibly small compared to the drag force of the
water.

In the frame of the surfer there is water rushing backward along the
bottom surface of the board.  I base this on having felt it (and used it
for steering) when body surfing.  There is a brief period after the wave
collapses and you are moving forward (back in the reference frame of the
beach) in the swash at about the same speed as the water, that the drag
force of the water is negligible.

> Next question: How does plasma wakefield acceleration (which is also
> often described as "surfing") work? I think there are different...