See my response to Mac's answer for other details 3. 5.both engines,once there is energy to move the arm, the energy moves into the arm, than into the sling, and into the projectile. Uh, yeah, sort of. They are both good at using energy if properly designed, so that comparison doesn't hold up. Trebs will deal with unused energy by re-lifing the counterweight.
Catapults slam into a stop.
I don't know when the Romans used the Onager and Ballista, but I think it was well before the Medieval period. Go to Russel's websight for more accurate dates, I could easily be off by a few hundred years. Catapults hurl lightweight projectiles very far and very fast. They pack a lot of power for thier size and weight. They are much more mobile than a Trebuchet and throw farther.