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Interesting that his spool goes bigger at the end...?

I would think you want it decreasing to near zero to extract all the kinetic energy from the mass, leaving the mass stationary as it hits the ground.

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I think the idea is that at the end the sling starts to unwind too, increasing the effective arm-length (that non-linearity is one of the reasons trebuchets are so efficient to begin with). As a result it's possible that decreasing the spool to near zero is like putting the gears of your car too high. Reducing the spool size ensures there's enough torque left to put more power into the final swing.

(too late to edit my comment, but I obviously meant increasing the spool size near the end, not reducing)

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