Mechanical Tennis Ball Cannon/Mortar Project Help

[noturn10]

For my Physic class, my teacher assigned us with a project of creating a mechnical cannon, meaning no human, electrical, chemical, pneumatic energy can be applied. I was wondering if you guys have any idea of the type of mechnical mechanism that I should use to fire tennis at both a 8 m and 12 m target. Two ideas that I have right now are either to use some spring loaded mechanism or rubber bands. The rules for the testing are that we will only have two minutes to fire as many shots as possible in the hope that it will land in a trash can at the prementioned distance. We get 3 pt if the ball gets in the can, 2pt if it hits the can but does not drop into it, 1 pt if it hits within a 1/2 meter of the can. As a result, a mortar would be preferred since it is over a relatively short distance where a arc would be greatly favorable. At the same time, the mechanism must not only be accurate but fast. In the two minutes, we must fire as many shots as possible for a max of 15 at each location. So five direct hit would be enough. Since time is of the essence, I was thinking that a spring loaded mechanism would be preferred over rubber bands. Please provide any suggestion on other possible mechanism that would produce better results. Also any help on the overall design of the cannon itself will be greatly appreciated. My idea is to have a tube with a spring attached at the bottom. The firer would first pull down the spring and then the loader would drop the tennis ball, upon which the firer would release the spring.

 

[Delphi51]

Welcome to PF!
Wow, what an exciting project! You guys will be up all night playing.
I think springs would be more reliable, but harder to find. Those desk lamps with long arms that you can adjust to any position have two really nice springs. I used them to make a marble cannon that shot horizontally. It was not very accurate, probably because the shooting tube had to be larger than the marbles and the marbles were bouncing from one side to the other while in the tube, emerging with unpredictable sideways speed. My aim was to hit a falling light bulb across a classroom and my record was about 2% success. A trash can is much bigger, though.
I always meant to replace the metal tube with a piece of wood with a channel narrower than the marble cut out of it on a table saw - possibly cutting down on that side to side bouncing. You may face different problems shooting at a high angle. A catapult approach might be better, holding the marble in the same trajectory every time.

 

[noturn10]

Thanks Delphi51 for that insight. But since I need to fire a tennis ball some 12 meters, I would need a much stronger spring than one that shots off a marble. Do you guys know where I could get a spring with a lot of potential energy. Also, if you guys have any other ideas as to how a different mechanism beside spring can better help me, please share. Thanks.