Arm Lengths
The arm of the trebuchet is comprised of two sections: the long component of the arm which is the section that goes from the pivot point to the end where the sling attaches to the arm, and the short component which is the section that goes from the pivot to the end where the weight attaches.
The length of the long component of the arm is called Length of Long Arm.
The length of the short component of the arm is called Length of Short Arm.
The optimal ratio between the long part of the arm and the short part of the arm will depend mainly on the Mass of Weight. If the weight is very heavy, the long part of the arm can be many times the length of the short part of the arm, which would allow the projectile to attain high velocities.