In the Laboratory edited by
Accident Anecdotes
Jay A. Young
The Label Didn’t Say Not to Do It
12916 Allerton Lane Silver Spring, MD 20904
Jay A. Young 12916 Allerton Lane, Silver Spring, MD 20904
In physics classes it is useful to be able to measure the velocity of a moving object, but this is not as simple as it might seem, especially without sophisticated equipment. With a little ingenuity and a willingness to accept a bit of uncertainty in the results, though, high school students could carry out a fairly good experimental investigation. One problem, of course, is to design a device that produces the same trajectory each time a measurement is attempted. Otherwise, the object might travel outside the range of the observing device. One way to have the same trajectory time after time is to expel the object whose velocity is being determined from a cylinder set at a known, fixed angle. A moving object to use could be a tennis ball, which will fit nicely inside an empty juice can, set at just the right angle: a juice-can cannon. To make a juice-can cannon you use two juice cans with the tops removed and with a big hole in the bottom of one can and a smaller hole in the bottom of the other. Use duct tape to tape the two cans together so that the can with the larger hole in its bottom is affixed to the open end of the can with the smaller hole in its bottom. Arrange the cans with the axis pointing at the desired angle, put the tennis ball in the top can, and pour a little methanol through the smaller hole into the bottom can. Then wait a little to give some of the alcohol time to vaporize and apply a lighted match at the smaller hole. On their first attempt, the student-experimentalists, with the teacher and the rest of the class watching, used a very small amount of methanol, which they poured directly from the gallon container of alcohol. Upon ignition the tennis ball
sort of popped out of the upper can, traveled about a foot through the air, landed ungloriously on the ground, and rolled listlessly toward the assembled witnesses, who laughed at the failure of the experimenters. Undaunted, on their next try the experimenters used more methanol, and the tennis ball zoomed several feet through the air in a magnificent and thoroughly satisfactory trajectorial manner. Unfortunately, in their haste to restore their image, the experimenters quite forgot to take any velocity measurements. A third attempt was necessary. The third time methanol was poured into the hole of the juice-can cannon, there was an immediate flash back. It was ignited by the all-but-invisible flame of the residual burning alcohol that remained in the cannon after the second attempt. The gallon container exploded, distributing flaming alcohol in all directions. The two experimenters were severely burned, one over 50% of his body. Fortunately, no flaming alcohol reached the other students or their teacher, who had checked the procedure beforehand and declared it to be safe, without hazard. “After all”, the teacher said, “the label on the gallon container of methanol said nothing about any hazard when using it in juice-can cannons. Besides, we have done this many times before and no one got hurt.” Editor’s Note Accident Anecdotes has been adapted from material that originally appeared in Chemical Health and Safety. It is reprinted here with permission.
JChemEd.chem.wisc.edu • Vol. 77 No. 12 December 2000 • Journal of Chemical Education
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