I've been tweaking my catapult design a bit each year, and think I've finally found the best design to fire accurately(even in student hands). So if you tried the catapult project last year, but had trouble with student accuracy this post might help if you're willing to give it another shot. If you haven't tried the project yet but want to this year, I would highly suggest using the design and tips below. Here is the new design:
1) Mini clothespins give a more appropriate distance and height.
2) I now make the basket out of a thin strip of paper, complete the circle with a small piece of scotch tape and use a bit of glue to hold the basket on. Having a basket that is just barely bigger than an m&m laying flat helps accuracy. Make sure students know to load the m&m laying flat each time.
3) Cut off the rounded edge of the popsicle stick behind the basket. The flat edge helps fire more consistently.
4) This new firing mechanism allows student to essentially just pressing a button to fire. There is still room for some error here, so make sure they know to hit the button SLOWLY each time. The back clothespin is positioned so that it just barely overlaps the back of the catapult when it is all the way down. Here's a close up with the catapult loaded:
6) Okay, so there's no 6 on the picture, but thinking about it now it would make more sense to move the guide rails on the main catapult much closer to the back.
So there you have it. I hope this helps. Let me know if you have any other ideas for improvements in the comments.
Previous catapult posts:
BONUS: I let one of my classes build their own catapults last year, and after finishing the project one group decided to come in to my classroom during some free periods and make this ridiculously awesome trebuchet.