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-----Original Message-----
From: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu] On Behalf Of cliff parker
Sent: Friday, November 21, 2003 6:32 PM
To: PHYS-L@lists.nau.edu
Subject: more acceleration
A test question on yesterday's exam involved periodic motion.
I offered the example of an object bobbing up and down on a
spring suspended from above as
an example of periodic motion. Several questions were asked about
velocities, accelerations, and forces when the object was in
various positions. One question asked what the acceleration
of the object was when it had returned to the equilibrium
position on the way up. I was looking to hear answers and
reasoning such as -- At this point the forces are balanced,
there is no net force, therefore no acceleration. A
particularly bright student responded as follows.
"This is a tough one. Because the forces are balanced, there
should be constant velocity (no acceleration). However
velocity is never constant in this situation, so the best way
to describe acceleration is that it is transferring from
positive acceleration to negative acceleration."
I had not thought about the problem as thoroughly as he did.
I like it when my students stretch my thinking! I know we
have had discussion of situations similar to this over the
past few days but I was wondering what others thought the
best answer to this question would be. Is acceleration
simply undefined at this point?