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The tennis ball mortar is a great project for many reasons: all the materials are cheap and readily available, its construction is extremely simple, and it gives off a nice loud *pop* which the video doesn't do justice.
Quick and dirty explanation on how it works: The mortar is composed of three sections of Pringles cans duct taped together. The metal bottoms of the top two cans are cut to form baffles, which will focus the force of the expanding hot gas created by the burnt vaporized naptha, onto the center of the tennis ball. Thanks to the near exact match in diameter of the ball and can, the force of the compressed gases is trapped behind the tennis ball for the whole length of the top can.As you can probably guess this design has room for improvements. For example, if the length of the top can were extended, the ball would have a longer time to accelerate and thus achieve a faster speed, but at what point does the downward air pressure on the ball overpower the structural integrity of cardboard and duct tape? How does increasing the volume of the cans affect overall performance? What effect would adding a few more baffled cans have? I'm sure the answer is only a simple math problem or two away, but you look bored and like your enjoying a few too many fingers. (More after the video)Today (June 1st) I am beginning a contest (Shhh! Its not an official contest so don't tell my bosses Eric and Christy ) to see who can come up with the best tennis ball mortar design. No one characteristic defines "best", it is a combination of performance, creativity, adn 1's aBiLiTy 2 not tlak LIEK Dis!!!1!1 I am judge and jury of this contest, and my decision is final.How to enter: Join the group Tennis Ball Mortars (Its moderated) If you feel your design is truly innovative, and that you are making great advances in the field of tennis ball mortarology, write a new instructable and post it in the group. If you just have some pictures, or if the only difference between your mortar and someone else's is the color of the duct tape, post it in the forums, and then add it to the group.Now for the good part *THE PRIZE*. I'm sure your dieing to know what sacrificing several fingers, gaining numerous pounds, and clogging all your arteries will win? ::Drum roll:: The ULTIMATE SUPER GRAND PRIZE is .........a cookie, thats right you win a cookie (Specifically: (1) Entenmann's chocolate chip cookie mailed in plain white envelope to any address in the world ( Its internationally delicious!! )). I know what your thinking "Damn Tetranitrate you cheapskate, even Fred, the guy who sits in his van by the playground is more generous with sweets then you", to which all I reply is "After all those Pringles you don't need anymore junk food, and you should probably tell a trusted adult about Fred."**DISCLAIMER: By entering this contest, thinking about entering this contest, or even just reading this, you retain all liability should you happen to blow up or injure yourself in an attempt to build this.(edit: I ate the cookie)
Yes, and what i meant by height, is try launching the ball into a container 1 ft high and fairly small around, like a coffee can or something. As for the band length, make the band adjustable at the end so that depending on the distance and height, you can change how much of the band you want to pull back. So for example ( these values arent correct) let's say you know if you pull back 1 meter of the band ( all the way until it stops due to the self limitation of clothing elastic ) and release it, the tennis ball fires 6.3 meters and at a height of 1 ft. You plot the points, after trial and error and have something like this (once again incorrect values)Table for 1 ft HeightLength of Band Distance Launched2 ft 3 meters2.5 ft 3.76 meters3 ft 4 metersetc.. etc..then, on graphs, you can plot the points, using the length of band as the x-axis, and the distance launched as the y-axis. This way, if your teacher says 6.3 meters, with a height of 2 ft. you look on your 2ft height graph, and find the point where y is equal to 6.3 , then take the x-value, measure out that much of your band, tie it tight, and launch. Your almost guaranteed to make it. I have one other suggestion for you. Before each launch, no matter what elastic you use, stretch it ALOT and i mean ALOT like 50 or 60 full stretches before your tests and launches...If you do this, your band is a consistant temperature, and is less likely to vary. ( If you launch the elastic cold, your ball won't go as far as if it was warmed up, and so if you launch warm on your trials, and then launch cold on your test, you won't do so well.) Just something to keep in mind. If you would like more suggestions, or are still confused, let me know, and I'd be glad to help, I'm also in the process of designing a trebuchet for a competition in the spring.Good luck with your projectP.S. By The Way for the best results on your project, I suggest doing multiple launches with every band length, and multiple height's. When I made my physics catapult, I did 20 launches per height, and did 15 heights (giving me 15 graphs worth of data). I only missed one shot ( out of 10 random height and lengths) and it was due to a miss fire. If you want other ideas on catapults I'd be glad to give you a few of my best