Re: The sign of g

[Jack Uretsky <jlu@HEP.ANL.GOV>]

I'll paraphrase and modify a slogan that I learned from Dick Hake, and
somehow I don't think that he will disagree:
        One kinesthetic experience is worth one teraword (=10^12 words).
                                        Regards,
                                                Jack


On Tue, 3 Sep 2002, Rick Tarara wrote:

> But one doesn't need to get too far away from the vernacular to talk about
> what is happening at the peak of the motion.  The key is to focus on the
> definition of acceleration--change in velocity/change in time.  We need SOME
> time interval over which to calculate the acceleration.  Ask what the ball
> is doing JUST before it gets to the top?  The class will answer--slowing
> down, and therefore from the previous discussion, accelerating downward.
> Then ask what is happening just after the ball has reached the top--speeding
> up, again accelerating downwards.  Ask how long the ball remains at the top.
> If you can get them to admit that it is for only an instant--one point in
> time--then you can get them back to having to look at _some_ time interval.
> Only when you look from a little before the top to a little after the top is
> the vector nature of the velocity critical.  Students might bulk at the fact
> that the change in time is getting very tiny, but then you must point out
> that so is the change in velocity.  However, a small number divided by a
> small number can still be reasonably large--about 10 m/s^2 in this case.
>
> Of course this is an excellent opportunity to develop the idea that as
> delta-t gets very small here, the ratio of delta-v to delta-t remains the
> same and you've got yourself a practical definition of the derivative dv/dt.
> I do this even with my gen-ed class.  We don't go beyond the basic
> definition of the derivative, but we do go there.
>
> Rick
>
> *************************************************
> Richard W. Tarara
> Professor of Physics
> Saint Mary's College
> Notre Dame, IN 46556
> 219-284-4664
> rtarara@saintmarys.edu
>
> FREE PHYSICS INSTRUCTIONAL SOFTWARE
> www.saintmarys.edu/~rtarara/software.html
> MacIntosh versions of the Laboratory Simulations
> are now available.
> **********************************************************
>
> ----- Original Message -----
> From: "John S. Denker" <jsd@MONMOUTH.COM>
> To: <PHYS-L@lists.nau.edu>
> Sent: Tuesday, September 03, 2002 7:06 PM
> Subject: Re: The sign of g
>
>
> > Tina Fanetti wrote:
> > > One thing I think conveyed to them that acceleration is still acting at
> the top of the balls flight is that I asked them why would gravity suddenly
> turn off for this ball at the top?
> > IMHO the issue here is not the meaning of gravity,
> > but the meaning of acceleration.
> >
> > The vernacular meaning of acceleration !!conflicts!! with
> > the technical physics meaning of acceleration.
> >
> > vernacular "acceleration" = speeding up
> > vernacular "deceleration" = slowing down
> > vernacular "acceleration" does cease at the top of the
> > trajectory.
> >
> > Physics acceleration has to do with velocity, not speed.
> >
> > Squarely facing this conflict is essential.
> >
> > I wrote up a discussion of this at
> >   http://www.monmouth.com/~jsd/physics/weight.htm

--
        "What did Barrow's lectures contain?  Bourbaki writes with some
scorn that in his book in a hundred pages of the text there are about 180
drawings.  (Concerning Bourbaki's books it can be said that in a thousand
pages there is not one drawing, and it is not at all clear which is
worse.)"
                                        V. I. Arnol'd in
                                Huygens & Barrow, Newton & Hooke