## Strength of printed PLA parts?

The spring hook of a carriage broken apart in July 22 after 5 months operation.
The force of spring was about 2 kg of weight.

## DIY Body armor

Levels IIA, II, and IIIA are all soft armor (Kevlar, Dynemma, Spectra, etc.) They can stop pistol rounds and shotshells.

To stop rifle rounds you need Level III or IV rigid hard plates, which are typically constructed from polyethylene, hardened steel, or ceramics such as boron carbide.

## The Man Who Changed The World – by Martin Noakes

Lyrics

There was a man he was a scientist
Some say perhaps the greatest mind that ever lived
(The man who changed the world)

So far ahead of his time was he
We’re catching up in the 21st century!
(The man who changed the world)

He gave us microwaves, remote control and radio
Alternating current & fluorescent bulbs
X-ray, radar, laser beams, electric cars

Tesla, Tesla I believe
You’ve been erased from history
A man with such ability
Could set the whole world free
Tesla, Tesla now I see
How very different it could be
Would set the whole world free

Tesla was born in 1856
A multi-linguist, he could speak 8 languages
(The man who changed the world)

Alone and penniless the day he died
And even now most of his work is classified
(The man who changed the world)

Who would have thought that for a century
We haven’t needed hydra-carbon for energy?
Endless Power
Hidden from humanity

Tesla, Tesla I believe
You’ve been erased from history
A man with such ability
Could set the whole world free
Tesla, Tesla now I see
How very different it could be
Would set the whole world free

M8 – Danny Manning Rap

Tesla, Tesla I believe
You’ve been erased from history
A man with such ability
Could set the whole world free
Tesla, Tesla now I see
How very different it could be
Would set the whole world free

## Replacement a ballast

I soldered four wires, some heat shrink tubes, and used three zip ties to mount the ballast and organize the wires.

## Converting rotation matrices of left-handed coordinate system

In left-handed coordinate system, positive angle is clockwise direction. In right-handed coordinate system, positive angle is anti-clockwise direction.

I spent many hours to find out how to convert rotation matrices of left-handed coordinate system. The solution is to replace the angle $\theta$ with $-\theta$

The rotation matrices about three axis in left-handed coordinate system.

$Rx(\theta) = \begin{bmatrix} 1 & 0 & 0 \\ 0 & cos\theta & sin\theta \\ 0 & -sin\theta & cos\theta \end{bmatrix}$

$Ry(\theta) = \begin{bmatrix} cos\theta & 0 & -sin\theta \\ 0 & 1 & 0 \\ sin\theta & 0 & cos\theta \end{bmatrix}$

$Rz(\theta) = \begin{bmatrix} cos\theta & sin\theta & 0 \\ -sin\theta & cos\theta & 0 \\ 0 & 0 & 1 \end{bmatrix}$

The rotation matrix about arbitrary axis

${\bf R}({\bf \hat{n}}, \theta) = \begin{bmatrix} n_{x}^{2}(1 - cos\theta) + cos\theta & n_{x}n_{y}(1 - cos\theta) + n_{z}sin\theta & n_{x}n_{z}(1 - cos\theta) - n_{y}sin\theta \\ n_{x}n_{y}(1 - cos\theta) - n_{z}sin\theta & n_{y}^{2}(1 - cos\theta) + cos\theta & n_{y}n_{z}(1 - cos\theta) + n_{x}sin\theta \\ n_{x}n_{z}(1 - cos\theta) + n_{y}sin\theta & n_{y}n_{z}(1 - cos\theta) - n_{x}sin\theta & n_{z}^{2}(1 - cos\theta) + cos\theta \end{bmatrix}$

To get $Rx(\theta)$ in right-handed coordinate system, replace the angle $\theta$ with negative $\theta$. Then apply the following rules.

$cos(-\theta) = cos\theta$
$sin(-\theta) = -sin\theta$

$Rx(\theta) = \begin{bmatrix} 1 & 0 & 0 \\ 0 & cos(-\theta) & sin(-\theta) \\ 0 & -sin(-\theta) & cos(-\theta) \end{bmatrix}$

$Rx(\theta) = \begin{bmatrix} 1 & 0 & 0 \\ 0 & cos\theta & -sin\theta \\ 0 & sin\theta & cos\theta \end{bmatrix}$

$Ry(\theta) = \begin{bmatrix} cos\theta & 0 & sin\theta \\ 0 & 1 & 0 \\ -sin\theta & 0 & cos\theta \end{bmatrix}$

$Rz(\theta) = \begin{bmatrix} cos\theta & -sin\theta & 0 \\ sin\theta & cos\theta & 0 \\ 0 & 0 & 1 \end{bmatrix}$

${\bf R}({\bf \hat{n}}, \theta) = \begin{bmatrix} n_{x}^{2}(1 - cos\theta) + cos\theta & n_{x}n_{y}(1 - cos\theta) - n_{z}sin\theta & n_{x}n_{z}(1 - cos\theta) + n_{y}sin\theta \\ n_{x}n_{y}(1 - cos\theta) + n_{z}sin\theta & n_{y}^{2}(1 - cos\theta) + cos\theta & n_{y}n_{z}(1 - cos\theta) - n_{x}sin\theta \\ n_{x}n_{z}(1 - cos\theta) - n_{y}sin\theta & n_{y}n_{z}(1 - cos\theta) + n_{x}sin\theta & n_{z}^{2}(1 - cos\theta) + cos\theta \end{bmatrix}$

Reference

3D Math Primer for Graphics and Game Development 2ed, Fletcher Dunn, Ian Parberry
Mathematics for 3D Game Programming and Computer Graphics 3ed, Eric Lengyel

## How to stop firefox making automatic connections

dng <dng@lists.dyne.org>

there is a general tendency for certain end-user software to become
very ambitious and also build a lot of marketing, leading most often
to lesser quality of the software itself because of bloat.

to our great horror, there is one more thing Parazyd has found out in
Firefox: it connects to their servers *every time a new tab is
opened*, no matter if empty! I mean, it does all sorts of connections
to homebase to gather info of all kinds (that obviously doesn’t serves
to make good choices for the betterment of the software…) down to
the bare minimum action of opening a tab, that also is communicated to
Firefox cloud servers.

I think this is ridicolous. There was a time in which we called this
behaviour spyware and for reasons.

Here a demonstration (webm video) http://b.1339.cf/iliksmj.webm

the visualization behind is made with our router/hub http://Dowse.eu
(also based on Devuan eheheheh)