Catapult Physics

hello

i am currently working on a game on iphone using cocos2d and box2d

the game is similar to the link below which involves using a catapult to throw the person
http://www.miniclip.com/games/catapult/en/

i have been able to make all the bodies using box2d and joints for the movement..
but i was a bit stuck at the how the string physics and the front barrier sliding backwards(the hydraulics part) was implemented.

i would like to have some suggestions on the same as to how to deal with these two things

Thanks