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bouncing ball or car acceleration



Thanks to John D., John M. and others for their lucid
posts. I apologize in advance to those who are weary
of this thread.

Consider a superball dx above the floor about to have
a perfectly elastic collision with the floor. Let the
system be the ball only. If one for the moment ignores
the "w" word (or the "p-w" word), it is absolutely
clear that total system energy does not change during
the ball/floor interaction. In addition it is clear
that there is a substantial delta p and, therefore, a
net external force (upward) in the direction of the
delta p vector. This net external force is the
floor-on-ball N3 reaction to the force of
ball-on-floor. Since the system energy has not changed
(no energy added or removed), using the "w" word
that's very clear evidence that there has been no net
work done on the system by the floor despite the fact
that there has been a net external force and an
acceleration. The key (I think?) as JD pointed out is
that a system with internal structure changes the
anlaysis (W-E theorem no longer applies). In this case
it seems preferable to me to focus on energy changes
(or the lack thereof) rather than resorting to
pseudowork.

But I'm still stuck on how the floor "knows" to push
on the ball if it doesn't deform. I've introduced
normal force (which is a very difficult concept for
many intro students) by resting a 1kg mass on a
meterstick suspended between two supports. The
students see the bend of the stick and are guided
(correctly?) to interpret that as a spring that has
been deformed from its rest position. It "knows" to
push up on the mass b/c of this elastic deformation.
Then you do the
laser-pointer-on-the-table-plus-mass-on-table demo to
show that an apparently rigid object deforms under a
load and "knows" to push up on the load b/c of this
deformation. Are these models not valid? If so, what
is (are) the misconception(s) being fostered? If the
models are OK, what is different about the mechanism
for the development of the N3 external floor-on-ball
force in the interaction above?

Fianlly, to return to JD's explanation the skater
pushing on the wall, breaking the skater into
arm+torso makes sense. But for the arm to push on the
torso, the wall must push on the hand/arm b/c the arm
can't accelerate the torso by pushing against the air.
And then how does the wall "know" to push on the arm
if the wall doesn't deform? I hope there's a better
explanation than "it just does". John Barrere





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