Re: [Phys-L] routine assumptions and plausible hypotheses ... or not

[brian whatcott <betwys1@sbcglobal.net>]

On 7/8/2012 12:38 PM, John Denker wrote:
/snip/
> O In the introductory physics class:
>
>   *) strings have negligible stretch but also negligible stiffness (u.o.s)
/snip/

This makes for an amusing paradox: on first acquaintance stiffness is 
reciprocal stretch.
But these terms describe two different properties: one goes with the 
material selection, the other goes with the choice of cross-section - or 
I suppose you could say, there is tensional stiffness, and stiffness in 
bending.

For bike fans, it can happen that an aluminum alloy pedal bike frame 
made of aluminum is much too stiff for comfort (whereas you will know 
that aluminum has just one third the "stiffness modulus" of steel.)

The tubes have been selected for a greater diameter in the alloy frame, 
of course.
Or consider a steel tape measure which easily rolls away into its case - 
yet a box section of the same material  and c/s area would be much less 
flexible.

Brian W