I had the most wonderful high school physics teacher, Mrs. Omland. I went to college wanting to study physics because of her class. One of my favorite aspects were the projects that she would have us do for each chapter. Two projects stick out in my mind; a hand powered electrical generator I made with some wire coils and giant magnets, and this Trebuchete:
We all also split up into teams and made potato canons and then had a competition to see whos would go the furthest. I pitted my trebuchet against the spud-guns and if I remember everyone was very impressed.
The design was based on good deal of research, and though there are more efficiant modern variations, I chose a more traditional model that included a hanging weight which provides greater power. The weight box was filled with scrap metal and sand.
A small stone (painted red to be easier to find) was easily thrown 100 yards quite reliably
Once set up, a simple pin is pulled to let-r-fly!