Re: Conservation of ME and nonconservative forces

[Herbert H Gottlieb <herbgottlieb@JUNO.COM>]

Oren

You are on the RIGHT TRACT!

AMEN

I could not have said it better myself.

Herb


On Thu, 28 Jun 2001 14:27:21 -0500 "QUIST, OREN" <OREN_QUIST@SDSTATE.EDU>
writes:
> Come On People!
> The problem is for a freshman level physics class.
>
> "A ball rolls down an inclined plane without slipping.  Given the
> height
> from which it starts, find the speed of the ball at the bottom of
> the
> incline."
>
> In your attempts to be "absolutely perfect" you wish to include the
> following:
>
>         "internal friction" energy dissipation,
>         "ADHESION of the wheel with the table makes a difference".
>         "Also, if the rolling object and the flat surface are both
> insulators, then
> as the object rolls along, we'd see "electrification by contact."
> The
> rolling object would aquire a constantly increasing surface charge,
> and it
> would leave behind a trail of opposite surface charge.  Some of the
> work going into the "rolling friction" would be stored as
> e-fields."
>
> No one has mentioned "air resistance" yet, but that's about all
> that's left.
> Do wish to use a velocity squared resistance, or cubed, or some
> other
> power?????
>
> We wonder why students see physics as hard.  Yet, some insist on
> taking a
> very simple problem and  then making is so complicated that the
> poor
> freshman student throws up his hands and decides to major in
> business!
>
> A student with a stopwatch, ball, and inclined plane will never
> "see" these
> effects.  Even a computerized timer of some kind will not allow
> accurate
> enough measurements to be sure.  Don't take a nice simple experiment
> and
> complicate it to a degree that can only frustrate the student.
>
> Physics is a "science of approximations."  We can be as accurate and
> precise
> as we want to be.  But, we should not be more accurate than has
> meaning for
> a specific case.  Students need to see the simplicity and elegance
> of
> physics without getting totally bogged down in unneeded details.
> Leave
> these for the more advanced classes.
>
> I don't know if anyone is still reading, but I know I feel better
> getting
> this off my chest.
>
> Oren Quist, SDSU
>
>
>
> -----Original Message-----
> From: kowalskil [mailto:kowalskil@MAIL.MONTCLAIR.EDU]
> Sent: Thursday, June 28, 2001 11:58 AM
> To: PHYS-L@lists.nau.edu
> Subject: Re: Conservation of ME and nonconservative forces
>
>
> I remember seeing the following argument about rolling
> friction. Consider a wire which is bent and unbent many
> times. Its temperature goes up. A rolling object is not
> perfectly rigid; it is compressed and decompressed
> constantly along diameters as they become parallel to
> the normal reaction force. The so-called "internal
> friction" is a dissipation mechanism to be considered.
> Ludwik Kowalski
> **************************************************
>
> William Beaty wrote:
> > On Wed, 27 Jun 2001, Paul O. Johnson wrote:
> >
> > > There is indeed no slipping, Oren, so there is no KINETIC
> friction. But
> > > it seems to me that there must be STATIC friction to cause
> rolling. It
> > > also seems to me that the instantaneous point of contact is
> indeed
> > > moving down the plane, opposite to the static frictional force.
> > > Therefore, the frictional force does negative work, converting
> > > potential energy to thermal energy.
> >
> > I don't know if it would be significant for something like a
> train, but
> > for small tabletop objects the ADHESION of the wheel with the
> table makes
> > a difference.  Imagine trying to roll a steel ball across a glass
> > tabletop.  Now imagine that the glass has a very thin coating of
> > half-dried rubber cement...
> >
> > Also, if the rolling object and the flat surface are both
> insulators, then
> > as the object rolls along, we'd see "electrification by contact."
> The
> > rolling object would aquire a constantly increasing surface
> charge, and it
> > would leave behind a trail of opposite surface charge.  Some of
> the
> > work going into the "rolling friction" would be stored as
> e-fields.
> > ((((((((((((((((((((( ( (  (   (    (O)    )   )  ) )
> )))))))))))))))))))))
> > William J. Beaty                                  SCIENCE HOBBYIST
> website
> > billb@eskimo.com
> http://www.amasci.com
> > EE/programmer/sci-exhibits          science projects, tesla, weird
> science
> > Seattle, WA   206-789-0775          freenrg-L taoshum-L vortex-L
> webhead-L


Herb Gottlieb from New York City
(Where it's nice to live but I wouldn't
want to be a tourist here)
herbgottlieb@juno.com