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Re: When Physical Intuition Fails



The problem at hand is similar to the one where you have two parallel axles
with geared wheels on them (one or both spinning at some initial angular
velocity). If the gears interlock so that "suddenly" the two axles are both
spinning together, find the final angular velocity.

Energy is not conserved -- inelastic collision
Angular momentum is not conserved -- external torque

The problem is not difficult, but one must be careful with assumptions, and
begin with fundamental principles. At least, that's the only way I know how
to do it.

Oren Quist, SDSU


-----Original Message-----
From: Bob Sciamanda [mailto:trebor@VELOCITY.NET]
Sent: Wednesday, October 30, 2002 7:23 AM
To: PHYS-L@lists.nau.edu
Subject: Re: When Physical Intuition Fails

----- Original Message -----
From: "Brian Whatcott" <betwys@DIRECTVINTERNET.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, October 30, 2002 7:44 AM
Subject: Re: When Physical Intuition Fails


| Bob, having read your reasoned development, can I ask:
| does this treatment copied below lead to maximal translation
| velocity in the case where there is no initial slippage at the
| interface of the cylinder and substrate?

Newtonian mechanics requires that whenever the state v=Rw is achieved, the
obtained result for v (and w) is forced. It is not just the "maximal"
no-slip velocity, it is the only possible no-slip velocity (unless you
change the problem).

|
| I am taken with the suggestive comparison with the transfer of charge
| between capacitors that your result indicates: where if spark
| discharge (analogy:friction) is avoided, em radiation (analogy: ??)
| becomes significant.

Thank you for allowing ne to refer to an AJP article of mine (1996?)
"Mandated Energy Dissipation - E Pluribus Unum", where I found analogies
between the capacitor problem, the inelastic particle collision, and some
other mechanical/electrical phenomena. (At that time this rotational vs
translational interplay phenomenon was not recognized by me as yet another
facet of the story). To your point, whether the mechanism of dissipation
is frictional, radiative or ? is irrelevant to the salient point: In
order for the final state (equal "velocities") to be achieved, SOME
dissipative mechanism must be available and operating Note that even in
the particle collision and the spinning disk/rack collision, the
dissipative mechanisms may well include sparks, E/M radiation, sound
radiation, etc. The particular mechanisms which operate depend upon
circumstances - however, a definite energy dissipation is mandated
whatever these circumstances and mechanisms (unless you change the
problem).

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor



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This posting is the position of the writer, not that of SUNY-BSC, NAU or the
AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.