Science Teaching & Technology by Multimedia Science
Tennis Ball Cannon Saga
Shoot First and Ask Questions Later
The Original Cannon
After teaching physics and chemistry for several years, I attended a physics demonstration seminar. One physics teacher mentioned that he had built a cannon out of tin cans that would shoot a tennis ball and could be used as a demonstration of momentum. Thus began a twenty year process of building and rebuilding the tennis ball cannon which became one of the most effective and popular demonstration in my physics classes.
Some research led me to the basic design which was to use several tin cans taped together attached to another can used as a firing chamber. A small hole was made in the firing chamber to both insert some propellant and to ignite it.
In order to make the cannon into a demonstration for momentum, it had to be able to recoil. So I constructed my prototype cannons on momentum carts. Finding the right tin cans was quite a challenge. Some cans had such a tight fit around the tennis ball that the ball would not come out. The balls were so tight, that I had to disassemble the cannon to remove the ball. Other cans were so loose that, after ignition, the ball would literally just roll out the end of the cannon.
After many tries, I found the perfect sized can, which turned out to be small cans of V8 vegetable juice. The cannon was set up on a laboratory bench, the tennis ball was inserted, and casually I lit the propellant. BOOM! The tennis ball shot out of the cannon, bounced off of the wall across the classroom, and hit me right in the middle of my glasses (no lie). The cannon, meanwhile, recoiled, hit my hand, turned over, and hit floor. For the next several years, the physics cannon had a large dent in its firing chamber. Obviously, I had become careless after so many unsuccessful attempts.
I devised a lab where the students were supposed to be government researchers investigating a new secret cannon. They had to use both momentum and projectile motion to calculate the velocity of the ball using two different methods. One student from each lab group were assigned to set up the firing range and the others worked on setting up the equations to calculate the two velocities.
After everything was set up, it was time to fire the cannon. Many of the students were not particularly impressed with the cannon. The propellant was added and the flame was set to the ignition hole. BOOM! The ball shot out of the cannon, hit the wall, and caromed back across the room. I saw that all of the student’s eyes were now wide open and they began clamoring to do it again. I usually stood with the student whose job it was to catch the cannon, since the recoil was unexpected enough that they often missed.
While there was some variation, the speed of the tennis ball often exceeded 100 miles per hour. All and all, it was the one demonstration / lab that students mentioned when they returned to visit after graduating.
The New “Improved” Cannon II
I continued to use the lone cannon for several years and soon other physics teachers began using it. Then one day I looked in the closet and the cannon was GONE! One of the teachers said that the cans had come apart so they had thrown it away. I was without my favorite toy that year and tried to recreate it without success. V8 juice was no longer sold in cans and the other cans I tried did not work.
Here is a YouTube video of an attempt to create a cannon using Pringles tubes which had a few problems.
The following year, I came into school one day to find a large white plastic object on my desk. The other physics teachers had built a new beautiful cannon out of PVC piping. I trotted the new cannon out for my classes and they were very impressed, since it was much more imposing looking that the original. Instead of lighting the propellant with a match, this cannon had an electric ignition. The cannon was set up and anticipation was high in the classroom. The cannon was fired and POOF! A large cloud of smoke appeared and the ball rolled slowly out of the mouth of the cannon onto the floor.
Over the next year, we all tried to make the new cannon work. We tried buying extra-large tennis balls and different propellants. Over the summer, I took the cannon home and worked on it with some other physics teachers. We increased the size of the hole to the firing chamber which did not work. We got better results by lining the barrel with waxed paper, but the ball came out followed by a flaming piece of wax paper.
The New “Old” Cannon III
At the beginning of the next teaching year, I arrived at my office to find a new cannon sitting on my desk. It looked just like the original cannon, just using different tin cans than in the original. The teacher who had thrown out the original spent a lot of time that summer trying out every tin can that he could find and hit upon a winner. It turns out that the size of the opening of the can is a very important variable. When it is just right, you get very high velocity.
That year several cannons were constructed so that every physics teacher had one and one as a spare. It was interesting that some of the cannons worked better than others, even though they were built using the same components by the same person. An some days a particular cannon would work better than it did on other days. It would make an interesting study to try and determine just what those variables were. I’m just glad to have my favorite toy back and I would think my students do also.
You can get a complete guide on how to build a tennis ball cannon and the associated lab report at Teachers Pay Teachers.