Experiment

Building the Cannon

The two PVC tubes are anchored to a piece of plywood (½” works fine) approximately 9” wide and 60” long. I used a very large hose clamp to go around both tubes near the near the top with a wood block in between to hold the PVC pipe apart.

Around the 3” long piece of iron pipe used to connect the two PVC tubes, I used a 1” conduit strap to anchor the base of the cannon to the plywood. To add support for the mounting of the cannon and plywood to the base, add a piece of 1X4” wood across the end of the plywood (This also is needed as a spacer between the iron pipe and plywood.)

I also used several deck screws to anchor the 1X4 to the plywood. Then bolt the conduit strap and 1X4 all to the plywood. Under the bottom end of the plywood and directly beneath the 1X4, I anchored 2 additional conduit straps for a wood dowel. The wood dowel (1 inch diameter) is used to allow the cannon to hinge enabling you to adjust the launch angle plus easily disassemble the launcher from the base.

Two additional conduit straps, one on each side of the base, are mounted to the 2×4’s used to build the base. The base is framed out of 2X4’s and has a dimension of approximately 42” by 12” with a plywood deck. The base is not completely covered with the plywood to allow the hinging of the cannon (4” to 6” of the 2X4’s are sticking out from under the base’s deck to allow for the hinging of the launcher).

Two vertical 2X4’s are placed near the front of the deck to allow the elevation of the cannon to be adjusted. The vertical 2X4’s are bolted into place to allow easy disassembly. Using a simple trigonometry formula for your right triangle being created, you can calculate the height to drill holes in each upright for a second 1” dowel. (A good website to do the math for you is… http://www.easycalculation.com/area/triangle-angles.php )

It is on this 2nd dowel that the launcher rests and allows you to set your launch angle. (When figuring the height of the hole placement, take into consideration the thickness of the plywood upon which the cannon is mounted and the top of the dowel. You will also need to know the length of your adjacent side, adjacent to the known angle, to calculate the placement of the holes.) I used 15, 30, 45, and 60 degrees as my known angles.

By building the launcher with adjustable launch angles, students are able to hypothesize which launch angle will give the greatest distance, which angle gives the greatest speed, and allows you to calculate the height of the potato (trajectory or path of the potato). To calculate the height I use a simple trig formula and let students see how ratios are used in a practical sense in higher levels of math.

Using the Cannon

To load the cannon with a potato, you are cutting a 2” hole into your potato by pressing and turning the potato over the open end of the cannon. Use a smaller diameter PVC tube (1” works fine) as a ram rod. Be sure you do not push the potato too far into the tube. You can go down near the end of the PVC, just not beyond because you do not have an easy way of clearing the tube besides shooting out air. When finished using the cannon, always run a wet cloth down inside the cannon. This keeps it from becoming a sticky mess. I usually just push it down like a potato and blow it out with air pressure. (Do whatever works best for you.)

Additional Info

Epoxy was used to cement the air valve into the end cap. Pipe thread compound needs to be used with each threaded joint. PVC Primer and PVC Cement are used to join all other PVC connections. Because you are using air pressure, you need to have tight fitting joints. I use a Dewalt air compressor and generally used 90 to 120 pounds of air pressure (psi). (Of course, this depends on how long of a range you have.) I am able to get upwards to 400 to 500 feet distance with this pressure and the potato travels around 130+ mph.

Observations

I also have most students timing the potato with stopwatches, several groups using 100 foot tape measures measuring the distance, and a recorder for each group of students. The potato cannon is able to shoot a fairly straight line so students set a distance of about 200 or 300 feet and pull the tape the last part of the distance. Screwdrivers stuck in the ground are used to anchor the tape and give you a good pivot point when measuring distance. All times and distances are collected and work as points of discussion, even if they do not fit the range of majority numbers. This allows discussion on scientific process. We write the data on the board. Students are to copy the raw data, do the math to determine the average time and distance, then calculate the speed (showing all work), and finally I ask them to write out the process in words explaining each step they took starting with the raw data to the determination of the speed of the potato.