Elite Engineer Posted February 24, 2016 Share Posted February 24, 2016 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 Link to comment Share on other sites More sharing options...
MigL Posted February 24, 2016 Share Posted February 24, 2016 Pressure is defined as force per unit area. Higher pressure makes penetration easier. ( sorry, that sounds 'dirty' ) Link to comment Share on other sites More sharing options...
studiot Posted February 24, 2016 Share Posted February 24, 2016 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 1 Link to comment Share on other sites More sharing options...
MigL Posted February 24, 2016 Share Posted February 24, 2016 Maybe for cutting, but not for pricking, Studiot. ( sorry, that sounds even 'dirtier' ) Link to comment Share on other sites More sharing options...
studiot Posted February 24, 2016 Share Posted February 24, 2016 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. 1 Link to comment Share on other sites More sharing options...
Endy0816 Posted February 24, 2016 Share Posted February 24, 2016 One can only hope you meant assess... 1 Link to comment Share on other sites More sharing options...
studiot Posted February 25, 2016 Share Posted February 25, 2016 Can I come to you for spelling lessons, endy? Thanks. +1 Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted February 25, 2016 Share Posted February 25, 2016 One can only hope you meant assess... Studiot was just trying to keep up with the "dirtyness" of the thread... Link to comment Share on other sites More sharing options...
Endy0816 Posted February 25, 2016 Share Posted February 25, 2016 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. Link to comment Share on other sites More sharing options...
Phi for All Posted February 25, 2016 Share Posted February 25, 2016 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. Link to comment Share on other sites More sharing options...
Elite Engineer Posted February 25, 2016 Author Share Posted February 25, 2016 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 Link to comment Share on other sites More sharing options...
studiot Posted February 25, 2016 Share Posted February 25, 2016 (edited) 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 Link to comment Share on other sites More sharing options...
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