Re: [Phys-l] loud bag...physics?

[Brian Whatcott <betwys1@sbcglobal.net>]

Looks like a teachable moment....

Stiffness comes in two flavors: material stiffness, and structural 
stiffness.
Material stiffness can be visualized as resistance to bending.
Structural stiffness can be visualized as a structure which resists bending.
A warren girder made of balsa sticks is famously engaging for kids being 
introduced to structural design by way of load bearing structural 
competitions, given a limited set of starter materials.
Cotton thread, balsa sticks, glue.

Both stiff and floppy materials have a load limit, beyond which they 
tend to deform permanently.
Take a sheet of paper: crumple it noisily to a ball. Flatten it again 
and it has wrinkles.
Those are the tell-tales of permanent deformation. It doesn't take much 
force to deform a strip of paper to permanent deformation. The noise 
represents internal fibers breaking, so that it is easier to deform next 
time. This seems to be a material property.
The question arises: why should one wrap material be noisier than another?
It's reasonable to suppose that internal breakage, or internal slipping 
is responsible for the differential effect. The UTube videos seem to 
show well-creased bags of the noisy variety: plastic bags show much less 
creasing.

Brian W

Bernard Cleyet wrote:
> Again I "point out"  It's not blowing up the bag or stretching, but flexing it.  Here's a clip showing flexing and the resulting "speaking".
>   
> http://www.youtube.com/watch?v=sDRHWwiEg3E
>
> And here's a clue to it's construction:
>
> http://www.youtube.com/watch?v=czX8Ea6MvZE&NR=1
>
>
> At first I though it was Al backed, but then is it biodegradable because the Al film will oxidize readily?  One paper describes using C to make PLA conductive and this one found a high bulk resistivity.
>
> http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=20070650191856MT&q=&uid=789508895&setcookie=yes
>
> bc doesn't eat white potatoes
>
>
> the comments following the Sun Chips one belong in the efficiency thread:
>
> http://www.redditgadgetguide.com/r/AskReddit/comments/bkc4i/anyone_get_that_super_loud_sun_chips_bag_100/
>
>
>
> On 2010, May 29, , at 19:25, Brian Whatcott wrote:
>
>   
> > It is a sadness of composite materials, that when they reach their limit,
> > they fly apart - sometimes in an explosive hail of dust.
> > When structural steels and aluminum alloys reach their limit, they
> > yield, and if the excess load is  not too high,  return from the
> > exceedance with a permanent bend or stretch.
> >