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Mandy would like this: popsicle stick bracelets: boil in water for 15 minutes then place in cup to dry~this is cool!
My teens (boys and girls) had a blast with this in my "Protect Your Peeps" program. Working in pairs, they first built the catapults. Then they drew castles on pieces of manila folder (keep it simple), which we taped to the edge of a table. Behind the castle walls, they set up banks of Peeps on different sizes of paper cups, positioning a king and queen (complete with crowns, of course) somewhere among the rows. We awarded points for each Peep hit, with extra points for hitting the royals. Ad...
Build the catapult in different sizes, or with different rubber bands, or pencils of different length - discuss the changes each variable makes to the resultant motion of the projectile. Demonstrate circular motion and rotational KE. Based on the relevant principles, have the kids design and build other working models of catapults. Here are some ideas.
Heidelberg houses Detroit Heidelberg project (look up on Internet--may have a local spin you can put on it)