Update 1_lagrangian_mechanics.md

[dpgao]

First, thank you for documenting your work online! I've been going through SICM lately, too, and I'm sure these material will be very helpful. I do think, however, that this 'optimisation' to reduce the number of dimensions is a bit quick. A juggling pin spinning around its central axis will behave differently from one which is stationary, just like how a spinning bicycle wheel behaves differently from a stationary one.