Re: [Phys-l] surfing

["Bill Nettles" <bnettles@uu.edu>]

JD's points about the start-up transient are important.
 
Would it be incorrect to make an analogy between surfing and a particle being pushed along by a screw?
 
BNettles

> > > John Denker <jsd@av8n.com> 7/9/2009 4:21 pm >>>
On 07/09/2009 12:50 PM, Carl Mungan wrote:

> But I don't really get it - surfers don't seem to change altitude 
> much as they surf. Thus, the kinetic energy they acquire doesn't seem 
> to come from (direct) change in gravitational PE. 

During the steady state, i.e. excluding things like the start-up
transient, they don't need to change altitude at all, which is
consistent with the fact that they're not changing KE at all.

In the frame comoving with the surfer, which is also the frame
comoving with the wave, the KE remains zero during the steady 
state.

[snip]

Switching attention now to the start-up transient, you need to
be careful about what reference frame you are using.  
1-) Using the frame comoving with the surfer is possible ... but 
   it's tricky, because you're talking about an accelerated reference 
   frame.  
2-) Using the beach-bunny frame of reference is also tricky,
   because then you have a huge moving constraint, and the 
   aforementioned theorem does *not* apply to moving constraints.  
   (Hint: a bat changes the energy of the batted ball.)  
3+) Therefore I recommend the frame comoving with the wave.  In 
  this frame, the surfer starts out with some KE and /loses/ it as 
  he moves up the slope of the wavefront.  Nice and simple.  I 
  don't see a problem here.

As always, you can have all sorts of problems if you analyze the 
motion partly in one frame and partly in another.