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Re: acceleration



>>>> The scalar acceleration has to do with speed. Speed is the
>>>> forward component of velocity.
>>>
>>> Is that in the instantaneous direction of the vector?
>>
>> Yes.
>>
>>> Will that not be differentially different in the next
>>> differentially different time?
>>
>> Yes, it will be different.

On 11/22/2003 08:14 AM, SSHS KPHOX wrote:
> Then I would call that acceleration and I thought you said the
> acceleration there was zero. Did I misread or misunderstand?
> This is the root of what confused me in this thread.

This is the perfect illustration of the point I've been
trying to make.

This isn't about physics ... it's about terminology.
Nobody on this list is confused about the physics.
Our students would be less confused about the physics
if we could disentangle the terminology.

Specifically, I said the _scalar_ acceleration was zero.
Ken evidently heard the word "acceleration" and didn't
hear the word "scalar". This is a fairly common pitfall
when talking with physicists and others who use the
vector acceleration day in and day out.

*) True fact: at the peak of the parabolic arc, the
scalar acceleration is zero.
*) True fact: at that event, the vector acceleration
is nonzero.
*) It is just plain ambiguous to say "the" acceleration
is zero, nonzero, or anything else ... except in the
case of D=1 forward motion (no stopping or turning)
when the two concepts merge into one.

This should give us some sympathy for the students,
because this is some percentage of what they are
going through. They face the mirror-image problem:
when we say _vector_ acceleration they hear the
word "acceleration" and don't hear the word "vector".

They have been using the word in the restricted
sense of scalar acceleration all their lives and
they cannot possibly unlearn that overnight, no
matter what we say.

I say again, the point I've been trying to make isn't
about what happens the ball in flight ... it's mostly
about what happens with the students. Unlearning is
hard. Students need to partially unlearn what they
know about scalar acceleration in order to make room
for vector acceleration.

> I guess I see no advantage in so doing but what you said later sure
> derived it as existing.
>
> I guess for my classes it is just simpler to stick with the constant
> vector acceleration associated with the downward directed field.

Several people have written to say that in the intro
course, acceleration always means vector acceleration.
My point is that even if that's your _objective_ it
cannot be not your _starting point_. Wishing doesn't
make it so. Students come with baggage, with
non-explicit but strong assumptions in their brains.
The terminological assumptions that a physicist makes
when talking with another physicist are not the
assumptions that a naive student makes.

Just to be 1000% explicit: I am not repeat !not!
repeat !!!NOT!!! recommending restating the laws of
physics in terms of speed instead of velocity. We
experts all know that notions of speed (and scalar
acceleration) that capture some of the physics in
simple situations are just a nightmare in more
general situations.

But I want to be able to say that to the students:
scalar acceleration captures some of the physics in
simple situations but it's just a nightmare in more
general situations. I need to say that to them
so they can say it to themselves. We need to keep
the old terminology in our vocabulary at least
long enough to say "this is where we started from,
and now we are leaving that behind and going to
a better place".