Selecting a Projectile from the drop down list of projectiles will automatically fill out the Mass of Projectile and Projectile Diameter inputs. If the projectile you are using is not on the list, choose the custom option.
The baseball and golf ball options are special cases. They each have their own curve for calculating the drag coefficient. For all the other options, the simulation uses the smooth sphere curve for calculating the drag coefficient. For more information about calculating the drag coefficient see the Projectile Drag section.
Mass of Projectile is the mass of the projectile.
Projectile Diameter is the diameter of the projectile.
Projectile Diameter is used to calculate the drag of the projectile.
If you want to simulate your trebuchet without air resistance, use 0.0 for the Projectile Diameter.
Wind Speed is the wind speed in the direction the trebuchet is shooting, so if the trebuchet is shooting with the wind, the Wind Speed will be a positive number, and if the trebuchet is shooting into the wind, Wind Speed will be a negative number.