Re: [Phys-L] Energy & Bonds

[brian whatcott <betwys1@sbcglobal.net>]

On 11/15/2013 2:22 PM, Jeffrey Schnick wrote:
> No, I'm talking about the attractive particle-particle interaction with a force of magnitude
> k |q_1| |q_2| / r^2
> where the model is that the two particles are connected together by a spring that exerts less force the more you stretch it.  Here,
> http://www.phys-l.org/archives/2013/10_2013/msg00143.html
> John Clement actually used the expression "rubber bands" rather than springs.  Again, I find that, in the elastic region, the more you stretch a rubber band the harder it pulls (even though it gets thinner) and if you stretch it beyond the yield point, it won't return to its original length if you release it, and if you stretch it far enough, it breaks.  In the elastic region it acts like a spring.  In this thread, the model came up in this message:
> http://www.phys-l.org/archives/2013/11_2013/msg00061.html
> (and earlier in the message to which this message was a response).  That's the context in which my comment
> http://www.phys-l.org/archives/2013/11_2013/msg00049.html
> was written.
>
> The model has a definite appeal to me but because the force law for the springs/bands in the model is so different from the force law for ordinary springs and rubber bands, it seems like it could do more harm than good in terms of the beginner's conceptual understanding of the attractive particle-particle Coulomb interaction.  It seems that it could also set them up for misconceptions of harmonic oscillator models where (to first order) the net force acting a particle really is proportional to the displacement of the particle from its equilibrium position.  I'm interested in comments from people on this list who have used the spring or rubber band model of the attractive particle-particle Coulomb interaction in the classroom.  If you still use it, why?  If you don't, why not?  Is there research evidence indicating that it is a good model to use?  You can buy a constant force spring.  Can you buy one that exerts a force that decreases with stretch and yet still snaps back to its unstr
>   etched length upon release?
Jeffrey asks for list comments on the use of springs in conceptual 
models of electromagnetic interactions - he also asks about springs 
whose stiffness decreases with tension.
   In the development of his formulation of the known electromagnetic 
effects, Maxwell used a matrix of springs for his propagation model. He 
later stripped all reference to this mechanical model from his developed 
system of equations.

In the relatively recent design of the compound bow (for projecting 
arrows), a system of cams and cables tailors the draw string force to 
achieve an early peak to the force-deflection diagram, followed by a 
significant reduction of force (by 80 or 90% even 100%!! near the 
maximal deflection, which permits maximal energy storage with easiest 
pointing on the target. An aiming system using a peep sight imbedded in 
the string, along with one of more pins mounted at or in front of the 
bow riser is used to permit comparative ingenues to place a cluster of 
arrows in a one or two inch diameter circle at a range of 20 yards.   
Some bow sights include optics which allow a magnified target view (X4 
or more) for competition style results.

Brian Whatcott    Altus OK