Catapult experiment #1:
Use a ruler and an eraser or other soft projectile. Have a boy strike the short end of the ruler. How far did the projectile go? Now have him try it with half the ruler over the edge and hit it with the same force. Is there a difference in the distance? Why?
Catapult experiment #2
Build a catapult, like the one shown below, using a ruler, a plastic spoon, a block of wood and tape. You can use a cotton ball instead of a ball. Put different buckets on the floor, and measure the angle (with a protractor) it takes to get the ball into the bucket.
If you don't have a protractor, print these on card stock paper:
Check out these catapults: