Re: acceleration

[Matt Jusinski <mjusinski@YAHOO.COM>]

--- "John S. Denker" <jsd@AV8N.COM> wrote:
> On 11/19/2003 05:52 PM, Matt Jusinski wrote:
>  >
>  > If the scalar acceleration is zero locally, and
> the
>  > speed is locally zero, Why does a projectile fall
> back
>  > down?
>
> This illustrates a useful general rule:  ask a
> different
> question, get a different answer.
>   -- The scalar acceleration has to do with speed.
>      Speed is the forward component of velocity.
>   -- Falling down has to do with the vertical
>      component of velocity.
>
> These are two different concepts.  At the peak of a
> parabolic arc, these two projections are
> perpendicular
> so in some sense have precisely nothing to do with
> each
> other.  The velocity vector is changing.  The
> downward
> component is changing.  The forward component is
> *not*
> changing.  No problem.
>
>  > It seems to me that scalar acceleration is not
> much
>  > help in explaining most common phenomenon.
>
> Once again: ask a different question, get a
> different
> answer.
>
>   -- If the question is what is the best way to
> analyze
> the details of the dynamics of a thrown object, the
> answer usually includes the vector acceleration.
>
>   -- If the question is what is going through the
> minds of intelligent non-physicists when they say
> the acceleration is zero at the top of a parabolic
> arc, the answer may well be scalar acceleration.
>
> There are many questions for which vector
> acceleration
> is the right answer.  There are also many questions
> for which scalar acceleration is the right answer.
>
> =====================
>
> I do a lot of writing, and I try to use words
> carefully.
> I try to make everything nitpick-resistant.  Once
> when
> writing a book I tried to impose the rule that "in
> this book, the word acceleration always refers to
> vector acceleration" ... but I found it was
> impossible.
> There are just too many situations where the scalar
> acceleration crops up, and no matter how hard I
> tried
> to find circumlocutions (e.g. "speeding up" instead
> of scalar acceleration, and "slowing down" for
> deceleration) there situations where I just gave up
> and used the natural, normal, centuries-old notion
> of acceleration ... meaning scalar acceleration.
>
> I know a big part of the agenda of a intro physics
> course is to teach people about vector acceleration.
> I know this is hard.  But pretending scalar
> acceleration
> doesn't exist would be a step in the wrong
> direction.
> Even if I thought scalar acceleration were the
> enemy,
> I would say: know thine enemy!
>
> But it's not the enemy.  If it were intrinsically
> bad
> it would have been killed off by now.  The problem
> is
> that it is sufficiently useful and sufficiently well
> established that it will never die out.  It is a
> cousin and a neighbor the vector acceleration.  So
> for pedagogical reasons as well as technical reasons
> my advice is: know thine neighbor!

All fair enough, and well put.  My task to to teach
high school students about the everyday world.
Acceleration is a very difficult concept for many of
them.  You can train them to get right answers in
certain instances, but a week later they are going to
be confused all over again when a projectile is at the
top of its path.  Although I may think that physics
know how is built up a few profound principles, the
students see it as abstract, complex, ornate,
whatever, but not straightforward.

Presenting one notion of acceleration, the vector
kind, for me, gives the biggest bang for the buck.




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