On 5/22/2012 12:12 AM, Existential Angst wrote:
Awl --

As is proly intuitively clear to most, you break a pencil by applying a
bending moment at the center, andsnap.
But it would be REALLY difficult to PULL a pencil apart, or crush it.

Iow, the forces req'd to snap a pencil are small, the forces req'd to pull
it apart would be huge.

I was wondering what the explanation for this is.
And I think it may be as simple as this:

Ito of INTERNAL stresses in the pencil, when you simply pull on a pencil,
you generate an internal psi in the pencil, which is just total Force
divided by the cross sectional area, which if less than the material
whatever whatever (section modulus or some ****??, the pencil stays intact.

However, when you BEND a pencil, now you are generating torques of r x F,
and given the sizable r and F of bending, and the very small r of the pencil
for resisting sed Torque, the net F generated internally in the pencil
becomes very large, with very high resulting internal psi's, which then
exceed the intrinsic strength of the pencil.
Think finger in a door jamb... at the HINGE!!!!ouch

Sound good? Bad???

I don't mind breaking a few pencils.

But I'm not sticking my finger in the door jamb.