Backyard Ballistics  is one of the books my son got for Christmas. We have been waiting for some moderate temperatures so we could be outdoors to build a potato cannon.
We had great fun building it over 2 days. The second day was spent making some improvements after we saw that our original design had a flaw.
We all learned about Isaac Newton's 3 laws of motion and how French armies developed the petard, a type of cannon. My son took great delight in learning that the word "petard" comes from the French word "peter," meaning "to break wind." If you know any 10 year old boys, you'll know how often that new knowledge has found its way into every possible conversation.
I heartily recommend this book. It has wonderful step by step instructions and explanations on the physics involved in each project.