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comment Maximum reading after dropping a mass onto a scale
I think you are right that approximating scale just as a spring is unrealistic. This is because in fact it is heavily damped! When you step on the scale, it takes it only a few oscillations to stabilize. That means the first spike might be significantly lower compared to the undamped case. This however introduces new unknown quantity - damping constant, but I guess we would be able to estimate it if we measure the time it takes the scale to stabilize. By measuring frequency of the oscillations we can also improve our estimate of spring constant.
comment Accelerating a glass of a water across a table?
This is almost true, just that in the reference frame of the glass, the acceleration of the glass presents itself as inertial force in the opposite direction (the g is also equivalent to glass accelerating upwards, not downwards). So the picture is incorrect. Water level will be higher on the back side of the glass.
comment Rigid body dynamics joints
Sorry, I should've included the article name. It's "Advanced character physics" by Thomas Jakobsen. It's pretty well googlable.
comment Conservation of Energy and the Poynting Theorem
As I can't comment on the question above, I have to do it here: Dear ganzewoort, perhaps you should have commented here, rather than modyfying your question, thus disrupting the chronology of posts. As of the subject matter, you could have clearly read in my reply that I basically agree with you. Poyinting vector is rendundant in your sense. However, in the same sense the energy conservation law is also redundant, as it is fully derived from Maxwell equations, therefore states nothing new.