Re: [Phys-l] Collision of irregular bodies

["R. McDermott" <rmcder@gmail.com>]

----- Original Message ----- 
From: "John Denker" <jsd@av8n.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Saturday, August 12, 2006 9:49 AM
Subject: Re: [Phys-l] Collision of irregular bodies


> R. McDermott wrote:
> > Regardless of what it "sounds like" to you, John, have you anything to 
> > add
> > that might clarify how it might be otherwise, or is a flip response to be
> > the sum total of your contribution?  You posted that this did not have to 
> > be
> > so.  I don't see how that could happen and asked you , nicely, for
> > clarification.
>
> As I previously stated, the OP specified "smooth", but that is not
> necessarily the same as frictionless.  If you want to leap from
> smooth to frictionless, that's up to you.

Understood, however not all of us here teach at the college level, and at 
the secondary level in most cases "smooth" implies "frictionless".  I can 
see that this might be something with which you may be unaware, but that is 
the reason for my assumption.

> If you think there is a trivial solution, fine ... but don't have a snit
> fit if somebody points out that your solution is trivial.  I usually 
> assume
> that when people ask a question, they have some nontrivial reason for
> asking.

John, I appreciate that your knowledge is more extensive than my own, and I 
mean that sincerely.  Your explanations in some cases go WAY beyond my 
experience in physics.  Despite that, I have a degree in physics and another 
in chemistry, so I am not dumb.  I don't mind being shown to be wrong, and I 
don't resent the fact that others (many) are more knowledgeable than I. 
What I DO resent is condescention, insulting remarks, and flip answers.  Of 
all the people with whom I've conversed on this list, you are the only one 
who consistently annoys me with your responses.  I think that says a good 
deal more about you than me.  Look at the other - helpful - responses I've 
gotten and then read your own.  Do you see a difference in the tone?  Well I 
do, and it does you no credit that a man with your obvious intelligence and 
training, cannot respond to questions without being abrasive.

Yes, I am asking about a trivial case, because I do not want to teach my 
students something that is fundamentally wrong.  My students are made aware 
that many of the problems we do are considerably simplified to make them 
accessible to them using the rudimentary tools they have at their disposal. 
Complexities, while they may be interesting to me, and may be alluded to in 
my class, are impractical to actually wrestle with at that level.  I've been 
teaching high school for almost thirty years now, and, for better or worse, 
I'm locked into a certain mindset as a default.

> For that matter, "smooth" is not necessarily the same as rigid.  So
> it requires another leap to arrive at the assumption that there is
> a _single_ point of contact.
>
> We don't even know whether it is an elastic collision or not.
>
> As I previously stated, I'm not sure the question, as stated, makes sense.
> So far, all we have is a word game disconnected from physical reality.

Absolutely legitimate concerns.  Yes, I guess my tendency, given the level 
of students I teach, is to look only at trivial cases, and that is clearly 
something I have to remember when conversing here.  Ok, so my question boils 
down to the following: IF the surfaces are frictionless, and there is a 
single point of contact, at any instant in time can the forces acting be 
other than normal to the surfaces (as per another response, deformation 
would likely result in the direction changing over time, but what about at 
any given instant)?  If so, how can that happen?

> I assume there is an underlying real physical situation and a nontrivial
> reason for asking the question.  Until we find out what that is, I have
> no idea how to answer the question.

Ok, I can see that expectation from your perspective.  If the poster is a 
secondary school teacher, as I am, his (or her, I can't remember) 
perspective is probably the same as my own.  You and I clearly bring 
different perspectives to the problems we read.
Again, MY first concern relates to the teaching of the fundamentals to high 
school students who mostly have NO calculus or previous physics backgrounds. 
Secondarily, I have a personal interest in understanding, at a deeper level, 
those things that are impractical to teach in my classes.  Sometimes, the 
latter impinges on the former, and I want to be sure I'm not setting my 
students up for problems down the line.