🎓 Doc Freemo 🇳🇱 @freemo@qoto.org

#appleevent so
Nice
Fatter Mac (making products as thin as possible should not be at the expo of performance)
More ports.
MagSafe
Meh :
Notch

Not cool :
The price

Mistakes where made

Days since last timezone issue:

-1

6 most common types of Bias when working with Data

https://www.metabase.com/blog/6-most-common-type-of-data-bias-in-data-analysis/

Why the standard way to describe rotational inertia of a body is to specify its moment of inertia matrix?

Moment of inertia matrices are somewhat weird: not every symmetric semipositive-definite matrix is a valid moment of inertia matrix (note that there can be no body that has nonzero moment of inertia about exactly one of its principal axes).

Moment of inertia matrix is expected to satisfy I_{around e} = e^T*I*e [0]. At the same time I_{around e} = \sum m_i*r_{perp to e}^2 = \sum m_i*(r_{b1}^2+r_{b2}^2) where b1 and b2 are some orthogonal basis of the surface perpendicular to e.

This creates a natural idea: if we define J := \sum m_i*r_i^T*r_i, then I = C^T*J*C (see [1] for value of C), _and_ every semipositive definite J corresponds to an object that could possibly exist.

So, why don't we use this J instead of I? I think it is less confusing, and seems to be way better e.g. if we're numerically trying to find a moment of inertia that optimizes for something.

[0] So I = \sum m_i*||r_i||^2*Proj_{perp to r_i}^T*Proj_{perp to r_i}

[1] C = \sum_{i != j}e_i^T*e_j (btw. it's not immediately obvious to me that this definition is invariant wrt orthonormal base change, and if you have a succinct description of why it is so, I'd appreciate seeing it)

@Charlie @freemo
Because uncharted voyages into the endless void full of monsters hell-bent on eating you alive couldn't possibly have psychological repercussions.

Because not-killing an alien species on first contact after they leave their ship is an actually commendable thing to do.

1492 was a trippy time for everyone.

2092 will likely be the same thing in a different scene.

#xkcd: #UnsolvedMathProblems

https://xkcd.com/

Imagine a ball (of uniform density) rolling on a horizontal plane without slipping. Due to its symmetry its angular speed in the reference frame of the plane will remain constant over time if it's not disturbed[1]. Its angular speed has three components. What I found surprising (but is obvious if we present it this way) is that by only nudging the ball horizontally you can't get it to spin around the vertical axis: so, horizontal nudges allow one to explore only a 2d subspace of the 3d space of the angular speeds.

tl;dr I got surprised by a consequence of the fact that while rotations around different axes do not commute, infinitisemal rotations (and thus angular speeds) do commute.

[1] If its density was not symmetric, it could undergo precession and nutation.