Video of me shooting the catapult:
(Sorry the video is sideways, I was having technical difficulties)
Step 1: Materials
Cardboard! - You will need A LOT of cardboard for this project unless you are lucky like me and find some quadruple-layer cardboard lying around.
Glue - I used Gorilla Glue and suggest that you use the same.
(Optional) Clamps - If you are using Gorilla Glue, you will most likely need clamps to keep your pieces together while the glue cures.
Pipes/Rods - You are going to need pipes for the axles and joints. I used a 3/4 inch acrylic rod for the axles and 3/4 inch PVC pipe for the axle to slide through.
Washers - You will use these to reinforce the joints that will be holding the counterweight and the the "arm" of the catapult. I used washers with a 7/8 inch hole.
Drill+Drill Bits - Depending on the size of your axle, you will need corresponding Drill Bits to make your holes. I used 13/16 inch and 5/8 inch.
Newspaper - Newspaper works really well when you are making your patterns for the cardboard
Rope/Cord - This will be for the "sling" that throws the projectile
Denim/Canvas Fabric - Also for the "sling"
3/16 Inch Steel Rod - This is also for the sling and only needs to be about 6 inches long
You will also need something to cut your cardboard into shape. I used a scroll saw but a knife or other saw might work.