Re: g

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

The problem that Michael details is why I spend a lot of time throwing a
ball straight up and later catching it.  We analyze this motion 'to death'.
Related to the question at hand, the downward acceleration, which can be
assigned either the positive or negative direction, causes the ball to BOTH
slow down and to speed up.  Of course the first challenge is to get everyone
convinced that the acceleration is ALWAYS downwards, but after that, this
motion can be used to attack the preconceptions detailed below.

Rick


----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, September 10, 2001 4:43 PM
Subject: Re: g


> After my original post, and a reply from Tina about the problems her
> students seem to be having, I don't think the problem is actually a
problem
> of whether g is positive or negative.  Therefore, even though this thread
is
> interesting, I don't think it is solving the original problem.
>
> I think the original problem stems from the common feeling among students
> that positive acceleration means "speeding up" and negative acceleration
> means "slowing down."  This preconceived notion is strong and for some
> students I never succeed at getting them beyond it.  Another way to say
this
> is that if acceleration is in the same direction as the velocity, then the
> acceleration is perceived as positive.  If the acceleration is opposite
the
> velocity, the acceleration is perceived as negative.  Of course these
> preconceived notions about acceleration are incorrect... or at least these
> notions are contrary to how practicing physicists view it.
>
> I wrote an e-mail about this, but it never came back to me, so I wonder if
> anybody got it.  I'll repeat it now with apologize to any of you who might
> have already received it.
>
> * * * speeding up and slowing down * * *
>
> Some students have the gut feeling that acceleration is negative when the
> object is slowing down, and the acceleration is positive when the object
is
> speeding up.  This is not true.  Objects with negative acceleration can
just
> as likely be speeding up as slowing down.  Objects with positive
> acceleration can just as likely be slowing down as speeding up.  Students
> need to learn to disconnect the sign of acceleration from the sign of the
> velocity.  These signs are determined by the choice of coordinate system;
> they are not determined by each other.
> I find it useful to point out that the signs of acceleration/velocity can
be
> +/+ or +/- or -/+ or -/-.  Then I point out that when the signs are the
> same, the instantaneous speed (magnitude of velocity) is increasing.  When
> the signs are opposite then the speed is decreasing.  I also point out
that
> in the opposite-sign situation it can't stay that way for too long because
> the object will eventually come to rest then reverse direction and the
signs
> will become the same.
> If you can get students to understand the previous paragraph then they
will
> have "seen the light."  I certainly don't have 100% success with this, but
I
> keep trying.
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
419-358-3270
> Professor of Chemistry & Physics        FAX:
419-358-3323
> Chairman, Science Department            E-Mail
edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817