These are some ideas to get you thinking on how to construct your catapult! Remember to make sure you read the directions and that it completes all the requirements

http://spaghettiboxkids.com/blog/catapult-designs-for-kids

http://littlebinsforlittlehands.com/popsicle-stick-catapult-kids-stem-activity/

These are some great resources that give you even more information about project based learning. Some of these resources have webinars, google hangouts and even actual projects you can do with your class.

https://www.bie.org/resources

https://www.edutopia.org/project-based-learning?gclid=CLeBjLixvdMCFUm5wAodupMDKw

https://www.iste.org/explore/articleDetail?articleid=386

https://www.iste.org/explore/articleDetail?articleid=337&category=ISTE-Connects-blog&article=
http://www.ascd.org/publications/educational_leadership/sept10/vol68/num01/Seven_Essentials_for_Project-Based_Learning.aspx