What's the "definition" for this law?

[Elite Engineer]

Suppose you have a watermelon. You gently push the round head of a pin against the watermelon, and nothing happens. Then you turn the pin around and gently push

 

the pointed end against the watermelon, and the pin penetrates the surface of the watermelon. Or how about the blade of a knife penetrating a watermelon relative to the

 

round side of a stick.

 

 

 

 

-How can this be defined relative to a physics standpoint? Objects with less surface area are able to move mass more effectively rather than an object with more surface

 

area, granted they have the same force applied. Essentially, the pin is "moving" mass when it penetrates the skin of the watermelon.

 

~EE

 

[MigL]

Pressure is defined as force per unit area.

Higher pressure makes penetration easier.

( sorry, that sounds 'dirty' )

[studiot]

The theory of using sharp edges or points to cut materials is dealt with in mechanical or industrial tooling engineering under the title

 

'metal cutting principles'

 

In general cutting action involves shear forces, which why you need to get the correct angles for cutting different materials.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&biw=&bih=&q=metal+cutting+principles&gbv=2&oq=metal+cutting+principles&gs_l=heirloom-hp.3..0l6.922.6016.0.6438.24.15.0.9.9.0.219.1767.1j11j1.13.0....0...1ac.1.34.heirloom-hp..2.22.2032.HQz2oHwvGKE

[MigL]

Maybe for cutting, but not for pricking, Studiot.

( sorry, that sounds even 'dirtier' )

[studiot]

Maybe for cutting, but not for pricking, Studiot.

( sorry, that sounds even 'dirtier' )

 

 

Actually it does.

 

The mechanical device similar to using the blunt end is called a (cone) penetrometer and is used in to asses the bearing capacity of soil.

 

https://www.google.co.uk/#q=cone+penetrometer

 

This measures the pressure applied and resisted.

 

Pressure is a direct stress, not a shear stress and does not lead to rending division of the material, as does shear stress.

 

The sharp end of a tool causes rending, not squashing.

[Endy0816]

One can only hope you meant assess...

[studiot]

Can I come to you for spelling lessons, endy?

 

Thanks.

 

+1

[J.C.MacSwell]

One can only hope you meant assess...

Studiot was just trying to keep up with the "dirtyness" of the thread...

[Endy0816]

Can I come to you for spelling lessons, endy?

 

Thanks.

 

+1

 

Only if it doesn't involve any rending.

 

Studiot was just trying to keep up with the "dirtyness" of the thread...

 

It did that.

[Phi for All]

So if I erect sheets of plastic, and prick holes in their stiff fronts, it's rendering rather than puncturing? Either way, it's hard on the plastic.

[Elite Engineer]

Pressure is defined as force per unit area.

Higher pressure makes penetration easier.

( sorry, that sounds 'dirty' )

So, then one could say:

With a force applied to an object, ↓ in area of the moving object = ↑ in the magnitude force of "penetration" the object has on a secondary object... ( not being 'dirty')... ~EE

[studiot]

I think MigL was referring back to your post 1 and the pin point v the pin head.

 

If you push the head against the melon with the same force as you push the point the pressure applied will be greater at the point.

 

But there are other factors in play such as surface hardness. These are exploited in industrial standard hardness testers (Vickers, Brinell etc)

 

https://www.google.co.uk/search?hl=en-GB&source=hp&biw=&bih=&q=hardness+testing&gbv=2&oq=hardness+testing&gs_l=heirloom-hp.1.0.0l10.1766.5906.0.7484.16.10.0.6.6.0.172.1203.0j8.8.0....0...1ac.1.34.heirloom-hp..2.14.1453.XJ_r7KaExYo

Edited February 25, 2016 by studiot