I got nuthun for this one, so have this little coffee in the meantime:
The final step of my master plan for adding a cannon-type trap was to make it rotatable. Luckily the last update already added the ability to handle different rotations on the arc line. But this time it required rotating the bow sprite, the child arrow sprite, and applying the same force at different angles.
The math for it was more trial-and-error than knowing a specific formula (the probably is a specific formula). It's like that saying about art and science.