Re: Speed of Waves

["John M. Clement" <clement@HAL-PC.ORG>]

Unfortunately this is a deeply rooted alternate conception that does not
respond well to either metaphor or telling them.  One of the best methods of
dispelling this is to have them do experiments where they actually see that
the speed is independent of the amplitude of the excitation.  There are also
some McDermott tutorials that touch on this idea, and there are some good
videos included with the Videopoint distribution that show this.  In
addition they will think that wider waves move a different speeds.  The
Videopoint distribution has wide and narrow pulses on it.  A problem with
the videos is that many students will think that wave pulses which start
earlier are faster, and that wider waves look faster.  They must do the
measurements to convince themselves that the speeds are equal.  These videos
might also be incorporated into an interactive lecture demonstration.

Unfortunately waves on springs tend to travel too fast to do reasonable
experiments, except with videos.  Torsional waves using Popsicle sticks and
2 parallel strings produce reasonable speed waves that can be used for
actual experiments.

For more information see the research done by Michael Wittman
http://www2.physics.umd.edu/~wittmann/research/

You will also find that they may not understand the frequency remains the
same when waves cross over boundaries, and this is also a difficult concept
for many of them.  You might be surprised if you ask them:  "There is a 1
mile freeway with only one entrance ramp, and only 1 exit onto a bigger
freeway.  Cars enter at 30mph and exit at 60mph.  All Tuesday 2 cars entered
every minute.  How many cars were exiting each minute on Tuesday onto the
big freeway.".  I tried this with 3 HS physics teachers one of whom teaches
AP, and they agreed that 4 cars exited every minute.  It took them quite a
while with Socratic dialog, and modeling of the motion of the cars to figure
out the correct answer.

John Clement
> -----Original Message-----
> From: phys-l@lists.nau.edu: Forum for Physics Educators
> [mailto:PHYS-L@lists.nau.edu]On Behalf Of David Abineri
> Sent: Sunday, January 28, 2001 9:32 AM
> To: PHYS-L@lists.nau.edu
> Subject: Speed of Waves
>
>
> We are discussing mechanical waves (sound) and some students feel
> (understandably at first) that the harder (faster) one 'hits' air
> molecules, the faster the disturbance will travel through the air. They
> are thinking that the air molecules are particles that simply move
> faster when hit harder thus making the disturbance move faster..
>
> What metaphor and explanation might help them to see that the medium
> dictates the velocity regardless of the 'hit'.
>
> Thanks for any help on this.       Dave Abineri
>
>
> --
>  David Abineri   dabineri@choice.net