| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next


Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

Data Mining Books; SimpleDateFormat should be synchronized; British Words; Chinese Govt Intercepts External Web To DDOS github; Numbering Permutations; Teenage Engineering - Low Price Synths; GCHQ Can Do Whatever It Wants; Dublinesque; A Cryptographic SAT Solver; Security Challenges; Word Lists for Crosswords; 3D Printing and Speaker Design; Searchable Snowden Archive; XCode Backdoored; Derived Apps Have Malware (CIA); Rowhammer - Hacking Software Via Hardware (DRAM) Bugs; Immutable SQL Database (Kinda); Tor GPS Tracker; That PyCon Dongle Mess...; ASCII Fluid Dynamics; Brandalism; Table of Shifter, Cassette and Derailleur Compatability; Lenovo Demonstrates How Bad HTTPS Is; Telegraph Owned by HSBC; Smaptop - Sunrise (Music); Equation Group (NSA); UK Torture in NI; And - A Natural Extension To Regexps; This Is The Future Of Religion; The Shazam (Music Matching) Algorithm; Tributes To Lesbian Community From AIDS Survivors; Nice Rust Summary; List of Good Fiction Books; Constructing JSON From Postgres (Part 2); Constructing JSON From Postgres (Part 1); Postgres in Docker; Why Poor Places Are More Diverse; Smart Writing on Graceland; Satire in France; Free Speech in France; MTB Cornering - Where Should We Point Our Thrusters?; Secure Secure Shell; Java Generics over Primitives; 2014 (Charlie Brooker); How I am 7; Neural Nets Applied to Go; Programming, Business, Social Contracts; Distributed Systems for Fun and Profit; XML and Scheme; Internet Radio Stations (Curated List); Solid Data About Placebos; Half of Americans Think Climate Change Is a Sign of the Apocalypse; Saturday Surf Sessions With Juvenile Delinquents; Ssh, tty, stdout and stderr; Feathers falling in a vacuum; Santiago 30m Bike Route; Mapa de Ciclovias en Santiago; How Unreliable is UDP?; SE Santiago 20m Bike Route; Cameron's Rap; Configuring libxml with Eclipse; Reducing Combinatorial Complexity With Occam - AI; Sentidos Comunes (Chilean Online Magazine); Hilary Mantel: The Assassination of Margaret Thatcher - August 6th 1983; NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions; LPIR - Steganography in Practice; How I Am 6; Clear Explanation of Verizon / Level 3 / Netflix; Teenage Girls; Formalising NSA Attacks; Switching Brakes (Tektro Hydraulic); Naim NAP 100 (Power Amp); AKG 550 First Impressions; Facebook manipulates emotions (no really); Map Reduce "No Longer Used" At Google; Removing RAID metadata; New Bike (Good Bike Shop, Santiago Chile); Removing APE Tags in Linux; Compiling Python 3.0 With GCC 4.8; Maven is Amazing; Generating Docs from a GitHub Wiki; Modular Shelves; Bash Best Practices; Good Emergency Gasfiter (Santiago, Chile); Readings in Recent Architecture; Roger Casement; Integrated Information Theory (Or Not)

© 2006-2013 Andrew Cooke (site) / post authors (content).

Paper Structures

From: "andrew cooke" <andrew@...>

Date: Mon, 9 Oct 2006 10:49:14 -0400 (CLT)

Although I no longer post on Mefi, I still read it from time to time. 
It's dperessing how ugly some of the behaviour is, and how
stupid/incorrect some of the AskMe answers are.

Today there's a bunch of people talking about paper structures -
http://ask.metafilter.com/mefi/48182 - and they are in a complete mess
about the basic principles.

If you are going to support weight (from below) then a structure must
handle compressive forces.  There's no avoiding it.  The weight of
whatever you are carrying is going to pass down through he structure,
compressing it.

For reasons I will explain below, it's much easier to make stable paper
objects that rely on tensile, rather than compressive forces (the replies
have this much right).  But given my previous statement on the logical
necessity of compressive forces, you are forced to one of the following:
1 - Suspend the load (from above) rather than support it (from below)
2 - Use some other material in compression to take the weight
3 - Find a way to handle the compressive forces with paper

Options 1 + 2 are not possible, so you have to go with 3.

And, despite what people are saying in that thread, paper can, and does,
handle compressive forces.  How else does a rolled tube work?  The idea
that "the compression force gets distributed and turned into tension" is
meaningless crap - and that was written by someone who teaches this!

The problem is not that paper does not withstand compression.  The problem
is that a flat sheet of paper bends easily - it folds.  More exactly,
paper does not handle torsion along an axis that lies within the paper.

Torsion is the problem.  When do we get torsion?  When the weight pressing
down and the support pushing up do not coincide.  And when does this
happen?  When the weight pressing down does not pass through the paper. 
And why does this make things so hard?  Because paper is thin, so there is
little room for error - the "line of force" that supports the weight must
remain inside the thin sheet of paper.

This is why a cylinder buckles when it is dented - because at the dent the
forces supporting the weight pass outside the sheet of paper.

So to build a paper structure that is strong under compression we must do
two things: make sure that the weight passes down through the paper;
ensure that this remains the case.

The second of these requirements makes the structure stable.  And the most
obvious way to achieve it is to make the structure rigid - a rigid
structure will not flex, and the weight will remain pressing down through
the paper.

Hence the need for corrugations and cylinders.


Paper in Compression

From: "andrew cooke" <andrew@...>

Date: Wed, 26 Nov 2008 13:04:20 -0300 (CLST)

I am not sure I explained very well above; one more attempt...

Weight "presses down" through a structure.  If you hold a heavy object
over your head, you can feel that weight pushing down through your arms,
body and legs to the floor.

But if you hold a weight out straight to one side you feel something else:
you feel the weight pulling your arm down.  The weight isn't pushing down
through your body, it's pulling your arm down.  Your shoulder starts to
ache - you want to let your arm fold down to your side.

Something very similar happens to paper.  When the weight passes exactly
down a vertical piece of paper there is no problem (believe me - it will
make more sense in a minute).

But when the weight doesn't press *exactly* down through the paper, the
paper bends or folds instead.  Just like your arm wants to fold down when
a weight is to one side.

You can see this by carefully rolling a sheet of paper into a tube (use
scotch tape so it doesn't unroll).  If you stand the tube on the floor, on
one end, you should be able to balance something on the other end.  The
weight passes down the walls of the tube.

But if you put a dent in the tube then it will collapse.


Because as soon as there is a dent the weight at that part isn't passing
exactly down through the paper.  Instead, the weight is making the paper
fold, so that the dent gets bigger.  And as the dent gets bigger the
weight makes it fold more.  And so it gets worse and worse (very quickly)
and collapses.

A dent is similar to holding your arm out - the weight is no longer
passing down in a straight line.


Comment on this post