Wanna see something cool #2

nsaspook

Joined Aug 27, 2009
16,333

We used these old Gap-filler UHF birds in the 70's. I once managed the transponders for our fleet of ships. They made them to last back then. The old Gap-filler UHF birds had open transponders (raw bandwidth) that would take any signal from Morse to PSK low- complexity constellations for data.
 

WBahn

Joined Mar 31, 2012
32,908
While I liked the video and it provided a nice, pretty intuitive, explanation of why the rotation about the intermediate axes flipped, it failed to show why that exact same mechanism didn't result in the rotations about the other two axes also flipping.

This was one of the most interesting exercises when I took Intermediate Mechanics as a physics major back in 1989 or 1990. The instructor made a point of not saying anything about the intermediate axis theorem when he took the textbook and, with rubber bands wrapped around it to keep it closed, thus making it a decent approximation of a uniform rectangular plate, proceeded to flip it about its two of its principle axis, the one with the maximum and the one with the minimum moments of inertia. No big surprise. He then had us calculate the moment of inertia of a uniform rectangular plate about these two axes, as well as the third one (the one he hadn't demonstrated). He surveyed the class asking if we expected the same nice, stable behavior if the book were spun around that intermediate axis and we all said that we did. He then had use calculate the stability of the rotation about each of those axes and we discovered that the extremes were stable but that the intermediate wasn't. Thus, we had a mathematical prediction of something that was completely counter-intuitive to how we all expected things to work in the real world, at which point he demonstrated that, indeed, you couldn't get that book to spin stably about its intermediate axis.

That really drove home the power of mathematical modeling.
 
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