Publicly available lecture notes from universities mathematics classes

https://www.reddit.com/r/math/comments/9s5548/what_are_your_favorite_publicly_available_notes/

http://math.stanford.edu/~vakil/216blog/   

https://www.reddit.com/r/math/comments/4nbqag/lets_make_a_collection_of_lecture_notes/

https://dec41.user.srcf.net/notes/

https://www.reddit.com/r/learnmath/comments/98wwfv/undergraduate_lecture_notes_university_of/

https://www.reddit.com/r/math/comments/csgigv/collection_of_lecture_notes_on_mathematics_and/

https://www.maths.cam.ac.uk/undergrad/studentreps/tripos-specific-resources 

https://www.maths.cam.ac.uk/notes-web

Euler Disc and its mathematics

As described in wikipedia:

https://en.wikipedia.org/wiki/Euler%27s_Disk

Here are the videos where the Euler discs phenomenon is highlighted and explained:

https://youtu.be/f07KzjnL2eE?t=2351

https://youtu.be/eYBLAkJqMF0?t=2777

https://interestingengineering.com/math-behind-spinning-coin

https://www.youtube.com/watch?v=55gJgCoDld4  (interesting study of the Euler Disc motion from an engineering point of view).

https://www.real-world-physics-problems.com/eulers-disk.html   (here the mathematics is worked out in details - and quite easy to understand with undergraduate Physics as background)

https://www.techly.com.au/2015/09/28/eulers-disk-amazing-plastic-disk-will-melt-brain-hear/

https://www.youtube.com/watch?v=XXcWQWO-uHg

http://eulersdisk.com/pubs.html

https://research-repository.uwa.edu.au/en/publications/eulers-disk-examples-used-in-engineering-and-applied-mathematics-

https://www.nytimes.com/2000/04/25/science/simple-spinning-toy-poses-a-mathematical-challenge.html

https://aapt.scitation.org/doi/10.1119/1.1501117

https://pdfs.semanticscholar.org/cbd5/1d985ca03a177d9ca8c9bc4ccfb3d6b5bebe.pdf

http://www.maths.uwa.edu.au/~keady/papers.html

https://iopscience.iop.org/article/10.1088/1361-6404/aaaac9/meta   

https://physicslearning2.colorado.edu/QOTWSite/services/demos/demosd4/productinfo_d4-27.pdf 

https://arxiv.org/pdf/1706.10205.pdf