Now, the catapult at least holds itself together, but it is not the greatest structure ever created. The arm instantly falls down. What you want is the arm to stay up and then be able to be dragged backwards.
To achieve this behavior, you will use the Physics Spring Joint. The question however is what are you joining it to? Perform the following steps:
CCNodeas a child of the physics node. This node will have no size. Click on Enable physics for the node and set it to be Static.