I must say I've been enjoying the responses to my query about whether
mechanical waves carry momentum. I hope people will continue to post,
because as far as I can tell at present, some folks think mechanical waves
clearly do carry momentum, and others (including me) are not so sure. I'm
with Barlow: confused!
I should clarify that I am thinking of a net time-averaged momentum. Of
course I grant that Newton's balls swing. But then they swing back! James
Wheeler computes a momentum flux of -sqrt(mu*T)*(dy/dt)*(dx/dt). But the
dy/dt averages to zero, I would think.
Ludwik asks what is a wave? Maybe that's a good question to start with -
anyone care to take a crack? In my mind, I was counting pulses as waves -
after all, you could Fourier compose many sinusoids to get a pulse. I am
however ruling out standing waves - there does have to be a net energy
transport, hence a traveling wave.
Brian's Arabian story is interesting. But I'm not sure what conclusion to
draw from it. I'm not teasing, really, so do give me the moral of it.
I actually heard someone else recently say that light wouldn't carry
momentum if the ether actually existed. I don't know what that means, but
it does sound like James McLean's comment, "I'm tempted to suggest that a
wave carries momentum only when there is no medium." Please elaborate -
sounds intriguing! I appreciated your example of the Maxwell tape ad. But
what about Eric's example of striking the end of a metal bar and seeing
what happens at the other end? (Presumably, the bar is clamped loosely at
its midpoint - is that analogous to managing to surf because of a sea
bottom effect?)
Bob's point that purely vertical motion of a string wave is over-simplified
is definitely related to my question. Again, please elaborate and I'll go
look up the AJP article next time I'm at the library. Carl
Dr. Carl E. Mungan, Assistant Professor http://www.uwf.edu/~cmungan/
Dept. of Physics, University of West Florida, Pensacola, FL 32514-5751
office: 850-474-2645 (secretary -2267, FAX -3323) email: cmungan@uwf.edu