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	Catapult

See the Moving Chapter for more instructions, hints, tips, and ideas.

This is a great project for teaching children about engineering and teamwork. Bring a box of materials (pieces of wood, string, rubber bands, nails, screws, bottle caps) and tools. Let them design and build a catapult as much by themselves as possible. You may have to give them a little advice, but try to let them do as much of the work as possible. Do not show them a catapult prior to this activity. They have to use teamwork to design the catapult. Then they have to overcome design problems.

For the cap, I have found that the lids for bottled water are just the right size.

Anything can be used for ammunition. Dried beans, seeds, pebbles, pony beads, aluminum foil balls, or paper wads are good choices.

To make this project go faster in a meeting, cut the wood to the correct size, assemble the arm, and drill all of the holes in the base.

• Cut the base, Popsicle stick and dowel rod to size.
• Roughen up the top of the cap and glue it to the rounded end of the Popsicle stick using E-6000™.
• Glue the dowel rod just below the cap (and on the opposite side of the Popsicle stick) using white glue.
• Set the arm aside to dry.
• Drill a 1/16" hole in one end of the base for the nail.
• Drill a 3/16" hole in the other end of the base for the string.
• Drill a hole in the middle of the back of the base for the screw. The diameter of the hole depends on the size of screw you are using. You want the hole smaller than the screw that will be used to attach the PVC ring.
• Drill a hole in the middle of the "D" PVC ring. The diameter of the hole should be about the size of the screw. A little smaller or larger is fine.
• After the arm is dry, drill a 1/16" hole through the center of the cap and the Popsicle stick.

During the meeting do the following:

• Sand all wooden parts to remove jagged edges.
• Hammer the nail into the smaller hole in the wood base.
• Thread the string through the hole in the Popsicle stick and the hole in the cap and tie a big knot in that end of the string. This can be a simple overhand knot in which you make several passes through the loop instead of one.
• Thread the other end of the string through the larger hole in the base.
• Make the hinge for the arm. See the photo for positioning of the hinge. If the hinge is positioned as in the photograph, the force will pull along the strength of the hinge. If not, the hinge will tend to separate from the wood. Using white glue, glue half of the cloth to the front of the Popsicle stick (same side as the cap). Align the string in the arm so that it goes straight throught the cap, Popsicle stick, and base. Glue the other half of the cloth beneath the Popsicle stick (see photo above) and let dry.
• Adjust the length of the string so that the Popsicle stick forms an angle with the wood base of between 45 and 60 degrees. Bring the end of the string to the top of the base and tie it to the string at the point where it goes through the hole in the base.
• Attach the PVC ring with a screw.
• Wrap the middle of the rubber band around the nail and attach the ends to the ends of the dowel rod. The rubber band should be in the shape of a "V" with the point at the nail.