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‘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.
Sadly a lot of math heavy textbooks love to present the equations and how to use them, but do a poor job explaining how those equations came to be.
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)