Me and my group are making a catapult, 4 wheels, wooden, rope and a bucket as the payload.

http://www.instructables.com/id/How-To-Build-A-Catapult/

Here is a link (AVAILABLE ONLY TO REGISTERED USERS) to the instructions PDF document.

We’re going to need

• 9 – 8′ two-by-four’s
• 1 – 12′ four-by-four
• 3 – 8′ four-by-four
• 50′ of surgical tubing
• Fiberglass windsurfing pole or a large section of steel pipe (Fiberglass pole preferred)
• 20′ thick nylon rope
• 1- 4 ft section of 4″ diameter ABS pipe
• 1 -4 ft section of 2.5″ diameter ABS pipe
• 2 – 4″-2.5″ step down ABS adapter
• T-adapter for ABS pipe
• ABS cement
• 2″ X 4′ aluminum pipe
• “S” hook
• “U” bolts and nuts
• Large Eye bolts
• 12 – 6″ lag bolts
• 3 – 5″ carriage bolts, nuts, and washers
• Assorted deck screws (at least 3″ long)

Tools we’re gonna need

• Chop Saw
• Reciprocating saw
• Drill Press
• Assorted Drill Bits (including large hole-saw or adjustable spade bit)
• Drill
• Wrenches
• Welder (if using steel pole)