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xkcd #2904: Physics vs. Magic
(imgs.xkcd.com)
Alt text:
'At the stroke of midnight, your brother will be hurtling sideways at an altitude of 150 meters' is a regular physics prediction about your nonmagical trebuchet, whereas 'you are cursed to build a brother-launching trebuchet' falls out of the Lagrangian.
You should pick up some civil engineering books. I did 1 year before I realized I hated it, but they're full of lovely equation that are like [Size of sand grains] x [percentage of empty space filled with water] x ([speed of water flow] + 14) x [Bill Factor]
(We add 14 to prevent the formula from breaking down)
(The Bill Factor was created by Bill Johnson and is 3.11 when it's raining and 1.70 when it's not. It is based on practical observations and has no theoretical basis)
There is also the whole formula set that adequately describes the phenomenon. It is a three dimensional set of differential equations, where you can only ever know five out of six starting conditions, so you need to iteratively adjust the sixth one until your error term is small enough. The formula set was developed 80 years after Johnsons proposal, using the advancements in computing technology, but the results are not better than what we get with the Bill Factor
So we thank Bill Johnson every day we use his Factor.