
Stack identical flat objects so they project over an edge as far as possible - dtnewman
https://www.quantamagazine.org/20161202-overhang-puzzle-solution/
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btilly
I've long enjoyed making impossible looking things out of Jenga blocks. For
example
[https://i.stack.imgur.com/xvEpe.jpg?s=328&g=1](https://i.stack.imgur.com/xvEpe.jpg?s=328&g=1)
has no glue, no nails, and is entirely balanced on a single Jenga block!

It looks impossible, doesn't it? But it is really quite easy.

Step 1: Find 5 blocks with 4 short and one long. You can test by putting
several upright on a table with one laid across with pressure. The ones that
slide are short, the ones that don't are long.

Step 2: Put the tall upright in the middle, the 4 short around it in a base.
Place 2 blocks whose middle crosses the tall one, standing on the others as a
base.

Step 3: Pile up your structure. Try to keep it from being off balance as you
go, but you have a surprising amount of leeway.

Step 4: Test the base, find which are loose and which are tight. Manipulate
the pile until all 4 at the base are loose.

Step 5: (Expect a 50% success rate.) Slide the base out. Everything is now
balanced on one!

Step 6: (For fun.) Put the remaining 4 on the very top.

Step 7: Take pictures, show people, etc. Your structure is unstable and can
collapse very easily. Which gives you opportunity to build another. :-)

A fun variation is to have 2 starting points, and build piles that extend both
ways. You can make a surprisingly long bridge!

~~~
kbart
Balancing symmetric objects isn't that hard, try something like this:
[https://www.youtube.com/watch?v=4Eo4gbLhYP8](https://www.youtube.com/watch?v=4Eo4gbLhYP8)

~~~
btilly
That's amazing!

And yeah, what I do is just a trick. The end result looks impossible but not
much skill is involved.

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Loic
If you have kids below 10, I heavily recommend the Kapla bricks[0]. You find
them in nearly all Kindergarten in Germany and kids love them. Our 6 year old
son built a 1000 piece bridge with only 4 bricks at the base.

[0]: [http://www.kaplaus.com/](http://www.kaplaus.com/)

~~~
sundvor
Vielen Dank, that looks brilliant. Just spent AU$188 on a Kapla 280 Box + Art
book, as well as architect book for my 6 year old. Kind of pricey, but it's
the right time of the year for it and quality looks great.

~~~
Loic
Yes, they are expensive, but they are very precisely cut and they do not
deform with the time. You will not be disappointed and your kid will have a
lot of fun.

~~~
dtnewman
I agree. The precision with which they are made is great. Also those blocks
will still be around for your grandchildren.

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ghusbands
The genie lamp configuration shown seems to have the top two bricks in a
position that would have them trivially topple. It leaves me wondering which
results in the article and mentioned paper are actually correct.

~~~
bendykstra
The article fails to mention that the genie lamp configuration relies on
additional "point weights," which are blocks of zero height and width and non-
zero weight. In the figure, those are represented by arrows.

[http://www.maa.org/sites/default/files/pdf/upload_library/22...](http://www.maa.org/sites/default/files/pdf/upload_library/22/Robbins/Patterson2.pdf)

~~~
thenewwazoo
I'd really like to buy some of those zero-height, zero-width, nonzero-mass
blocks!

~~~
SiVal
You'll have to wait for the Singularity when they go on sale.

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randallsquared
Given that the blocks are connected only by gravity, how could that bottom
block possibly help?

~~~
LeifCarrotson
It's only used to for the logical induction. It doesn't help the blocks above
balance any better.

The bottom block hanging over the table with a heavy weight on the top right
edge means the whole system is prone to tipping, and moving the bottom block
back so we're just worried about the next edge higher up means that the system
is, on average, less prone to tip over. It turns out to be easy to calculate
the difference in tippyness/torque between these two cases, so we can use this
to calculate how far the block can hang over.

One block obviously balances at 1/2, and has a center of mass at the edge. Two
blocks obviously still have a center of mass at the edge - else they would
tip, or could go closer to the edge without tipping. But how far out is this
second block hanging?

If we put the second block flush with the edge of the table, it has a center
of mass at -1/2 and a mass of 1, and the top block has a center of mass at 0
and a mass of 1. So the center of mass of the system is at -1/4 and we can
shift it this far to the right. With 3 blocks, we have a mass of 2 blocks with
the center at position 0, and another block below with a center of mass at
-1/2 and a mass of 1, for a system centered at -1/6th (and so on for more
blocks). We always shift it over in the end, but it makes the math of how far
to shift it easier to compute if we start with it backed up a bit.

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stcredzero
How to run an early 21st century science magazine like Quanta: Raid article
subjects from the back of late 20th century Scientific American articles.

[http://www.maa.org/sites/default/files/pdf/upload_library/22...](http://www.maa.org/sites/default/files/pdf/upload_library/22/Robbins/Patterson2.pdf)

~~~
sn9
This example is found as a problem in pretty much every introduction to series
in calculus and discrete math books.

I didn't realize popular science magazines get to call dibs on such problems.

~~~
stcredzero
Evidently, they don't.

~~~
sn9
Are you suggesting they should?

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stcredzero
That's ambiguous, isn't it? Maybe I'm just miffed I didn't think of it first.

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GrinningFool
The title alone brought immediately to mind a scene from childhood of insanely
stacked Nat Geo magazines, and the phrase "The Crazy Cantilever". Turns out it
the phrase (and I have no doubt the stacked Nat Geo magazines) came out of
this book:

[https://www.amazon.com/crazy-cantilever-other-science-
experi...](https://www.amazon.com/crazy-cantilever-other-science-
experiments/dp/B0007DO308)

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jrowley
What is the author referring to when he says it looks like a 'kingfisher'?

~~~
oh_sigh
The bird: [http://i.imgur.com/9A1wwRs.jpg](http://i.imgur.com/9A1wwRs.jpg)

~~~
jrowley
Thanks, yeah I am familiar with the bird but I guess I didn't see it.

~~~
Hondor
There's a photo of one made of glass blocks in the comments that made it click
for me.

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rukuu001
I've been doing exactly this with my 2yo nephew lately. I build it out, he
yells 'CRASH!!' and send the Jenga block flying. Keeps us both amused & out of
the way.

