Re: [Phys-l] Student Misconceptions

[Bernard Cleyet <bernardcleyet@redshift.com>]

On 2011, Sep 29, , at 03:09, Joseph Bellina wrote:

> I think we should be careful to distinguish misconceptions from misunderstandings.  Most of this list and I think BC's are more misunderstandings or unanswered questions.  I think perhaps preconception is a better term because it implies a mental model built on everyday experience prior to most instruction.  I think these are different from mistaken new ideas picked up in classrooms, or ideas not fully developed as a result of instruction.  Of course there is the possibility that a preconception is reinforced by classroom work done by an unwary teacher.


I agree some are (or may be) misunderstandings.  However, the one to which I referred, is, I've rather certain, a misconception.  

I elaborate:   

The conception is a higher Q increases the amplitude of a mechanical oscillator when driven at resonance with constant drive (energy), e.g. a clock.   This belief comes, I think, as a result of all those diagrams of Q(amplitude) in all those texts.  They show a narrowing and a greater amplitude with Q when the driver is frequency swept.   Well it ain't so.  In fact (I hate this expression.)  The opposite occurs when the Q is increased by a mass increase in the case of a pendulum (I suspect true for any mechanical oscillator.)   IF MY MATHS IS CORRECT.  To first order the masses cancel, but to second they don't.  More on that later.   

bc reminded of those diagrams of the exaggerated elliptical earth orbits and the resulting season misconception.

p.s. of course, if the Q is increased by a reduction of the dissipation, the amplitude will be increased.  But that's only one of the Q factors.