First off, you will need a few things:
5 rulers with multiple holes in both ends
A large nail or bolt (must fit through holes in rulers)
Two very small nails
Some tape (duct tape works well)
A heavy keychain or something else which can work as a counterweight
A brick or block of wood.
Alright, lets get started!
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Step 1: Attach Rulers to Brick
Putting the brick or block of wood on its side (drawings do not show this), tape the four rulers to the brick as shown, so that the holes line up, in order to build the support structure. Figure out what angle the rulers should be calabrating them so that the holes on the rulers overlap, and you can line the third hole of your remaining ruler up with these holes while its bottom hole is slightly above the brick. Once its lined up right, tape the four rulers in place.
Step 2: Creating the Arm
Put a nail through the hole in the five rulers as shown. Like I said, the hole should be just above the brick.
Step 3: Create Release Mechanisms
Tape the two little nails to the brick as shown.
Step 4: Make Sling
Make sling out of tape and string as shown. Test it out by seeing if you can throw a marble with it.
Step 5: Tie Sling to Ruler
Tie the loose end of the sling to the ruler as shown.
Step 6: Create Loop
Create a loop of string going through the hole in the ruler. This loop should reach down to the nail and be able to hold the ruler in place.
Step 7: Add Counter Weight
Attach the counterweight to the hole on the arm. Should be as heavy as possibe, without messing up trebuchet.
Step 8: Cock and Fire!
Hook lose end of sling to nail on ruler. Stuff sling under trebuchet, and relese string to fire.