Answer by
LeheckaG (1826)
Catapults evolved from crossbows into several-variations:
Ballistas, Onagers, Mangonel, & Trebuchets
Catapults throw projectiles using either a bow-like or swinging-arm/leg mechanism.
A crank/windlass tensions a bow or applies torsion-tension(turning) to a rope,
this stored-energy is dramatically/suddenly released by a quick-release mechanism,
thus hurling the projectile with great force/speed.
You can use:
rubber-band-bow-string, paper-clip-quick-release, thread/spool-windlass.