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In association with Greg Winterrowd & Colin Leland Return to Research |
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Many years ago, people thought that a heavy object would accelerate faster than a light one. Galileo Galilei proved them wrong. He showed that at a given location on Earth and in the absence of air resistance, all objects fall with the same constant acceleration. Galileo derived a formula that illustrates that for an object falling from rest, the distance traveled will be proportional to the square of the time; d t2. What Galileo was proving in that acceleration due to gravity is equal to 9.8 m/s. If these laws apply to feathers, heavy stones, and sheets of paper, then they must apply to tennis balls, especially tennis balls at incredibly high velocities. Canons have been a very important military weapon for centuries. Just recently, however, canons have been introduced to the regular “Joe” for good, clean, and safe backyard fun. All you need is a little bit of PVC piping and glue and a little bit of courage and imagination. If we shoot a tennis ball out of a pressurized air cannon aimed straight up into the air, it will rise to a certain altitude, then fall back to earth with the same velocity it left the cannon with. We believe that a longer length of the barrel of our pressurized air cannon will result in longer hang time. Maybe the most important part of a pressurized air cannon is its design. There are two ways to make a pneumatic tennis ball canon, a piston or diaphragm model, or a valve model. We elected to use a valve model due to its ease to make, sturdy design, and uncomplicated design. Regrettably, the valve model is not as constant as a diaphragm model because the vale is an added variable of error, the valve is opened at a different speed every time. We used four inch piping for the chamber, two inch piping for the barrel (conveniently, a tennis ball fits snugly inside), a one inch valve to release pressure, and the appropriate fitting and couplings to connect them safely. As far as the experiment goes, our procedure was quite simple. First we had to find a nice open, yet secluded area to launch at(broken windows were a little out of our budget). Next we had to construct the cannon. We went to Home Depot, as well as every other hardware store in the greater Portland area, to find all of the pipe and fittings. Then we had to fight and bicker over what design would work best, and also not blow one of our legs off. After that, we had to gather up all of our prices to build a frame to support the cannon and hold in in position (we found out that it fractures under its own weight the hard way). We set up the cannon and positioned it so that it would shoot straight up, we would fire the cannon and simultaneously start a stopwatch and time how long the tennis ball was in the air. Every five shots, we would cut six inches off of the barrel to measure how barrel length effected the tennis balls hang time. We shot the tennis ball at 40 pounds per square inch (PSI) out of a barrel that ranged from nine feet six inches down to two feet six inches. Equipment List:
Our data supported our hypothesis for the most part, as the barrel length went down, so did the hang time. Our graph is all over the place which suggests that the errors to be mentioned later in the paper effected our experiment drastically. However, in the barrel lengths five (7’6”) through thirteen (2’6”) did show what we were hoping to see, a bell shape curve showing that there is a medium length that is most efficient. This seemed to be the six foot barrel. After this the data fluctuated, but continued on a downward slope, we are certain that if we continued to cut the barrel down even more, that cannon would perform excessively pathetically. (We also posted our data, go check it out
if you are oh so inclined.)
Errors that could have effected our experiment are numerous. As stated above, the valve is a variable of error, if opened slowly, the tennis ball launches out a lot more slowly than if it were opened quickly. A human has to open the valve and humans are not constant and can make mistakes. Humans also make mistakes on pressing the start and stop button on the stopwatch when the cannon goes off or ball hits the ground. Another variable is our pressure gauge. It could have not been incredibly accurate, it cost only thirty dollars, so it is not exactly what you would call “precision equipment”, we also could have simply read it incorrectly when we looked down at it. The ground that we set up on was rather spongy, so the base of the cannon could have settled off level and made our shots drift off in different directions. Our tennis ball also could have made our experiment inaccurate. As we shot the ball, it could have smoothed out or taken a different shape, making it perform differently. We believed that a longer barrel on our pressurized air cannon would result in longer hang time. In spite of the mind boggling number of possible errors, our data seems to support this hypothesis. Our launched tennis balls achieved less and less hang time as we cut the barrel down. There was one dip that we find ourselves unable to explain other than it had been two hours since our last doughnut and we were getting kind of tired. We have show, or at least like to think so, that longer is better to some extent. |
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