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

[Bernard Cleyet <bernardcleyet@redshift.com>]

OK!  That's taught to all intro. engineering students.

Now my question is what makes the diff. in humidity for kraft paper?  I have two rather "fuzzy" ideas.

bc taught engineering materials at the community college level.

On 2010, May 31, , at 11:05, Brian Whatcott wrote:

> 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