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# Re: Conservation of ME and nonconservative forces

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.

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)))))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://www.amasci.com
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