A fun project that's simple and easy to complete.

Step 1: Gather Materials

Materials that are needed to complete this project:

PVC Pipe 3/4 inch
Wooden Dowel Rod 1/2 inch
Wood Planks/Blocks (needed for the base and barrel)
Zip Ties
2 Extension Springs
Door Hinge
Screws
Nut (to place on one end of the dowel rod to launch the marble)
Drill
Saw (to cut wood and pvc pipe to the length you need)

Step 2: Build the Barrel

Using a block of wood, attach the PVC pipe to the wood using Zip Ties or hot glue (or both, if necessary). Make sure that there's some wood past the bottom end of the PVC pipe to be used for the dowel rod. Near the back end of the PVC pipe, use 2 screws and drill them in beside the PVC pipe (make sure they're lined up or the cannon won't launch the marble correctly). Attach an extension spring to each of the screws. Use 2 more screws and drill them into the dowel rod, one on the curved part and another on the flat end near the first screw. Don't drill the screws in all the way, as they are part of the firing mechanism. On the other end of the dowel rod, use hot glue to attach the nut to the flat end. Attach the other end of the springs to the screw on the curved part of the dowel rod.

Step 3: Build the Base

Use the wooden plank(s) to build the base. The length of the base is up to you, as long as it is able to support the cannon. If it's for a project, then the base must fit the requirements given beforehand.

Step 4: Attach the Barrel to the Base

Line up the door hinge in the spot you want to attach it to (preferably the middle). Drill 1 screw through the bottom hole of the hinge and the base, and another through the top hole of the hinge and the barrel. You can attach the hinge on the outside of the cannon (as shown in the picture) or on the inside of the base and the barrel (with the curved part on the outside of the cannon). Either method will work. This will allow you to change the angle from which you fire the cannon from.

Step 5: Finished and Ready for Testing

You have now completed the cannon and are ready to begin testing the cannon. We added a block between the barrel and the base that the barrel can sit on we can move and adjust the angle. This is optional, but adding this will help with the stability of the cannon as it won't require you to hold the barrel at the elevation you want.

Step 6: Our Work

Here's the math for three of our shots:

1st shot:

Angle: 30 degrees
Launch height: 0.6 m
Distance: 2.38 m
Time: 0.63 s
Initial velocity: 4.34 m/s

2nd shot:

Angle: 30 degrees
Launch height: 0.6 m
Distance: 3.12 m
Time: 0.7 s
Initial velocity: 5.15 m/s

3rd shot:

Angle: 30 degrees
Launch height: 0.6 m
Distance: 3.75 m
Time: 0.75 s
Initial velocity: 5.62 m/s