We're studying circular motion in regular physics, and I'm preparing my laboratory activity for the week after Thanksgiving break. (Not sooner -- we have our trimester exams next week.) I want to do the "swing a stopper on a string in a horizontal circle above your head" experiment, a classic developed in the PSSC era. I discovered my setups for this experiment to be a tangled mess, with numerous missing pieces, broken strings, and not enough hollow tubes for the string to go through in any case. Now you can tell why I haven't done this experiment with my class in about five years.
I looked on the PASCO site, hoping to find a reasonably priced set of hollow tubes with stoppers and the light, low friction thread that leads to quality data. And what to my eyes did appear: $39 big ones for a set of five stoppers, two tubes, ten zip ties, and some regular old string.
Okay, my department's budget is nearly unlimited. I can -- and do -- order any equipment I want or need for my class. Nevertheless, there's something to be said for intelligent use of resources. $39 for items available in the storeroom? Neither frugal nor intelligent. This is why a former Chief Reader for the AP exam defined "Pasco" as a Latin verb meaning "to rob."*
* Before the Pasco Police come my way, please note that I am a HUGE customer, and a huge supporter of the company in general. They sent me two loaner heat engines for my summer institutes -- no charge, no hassle, no problem. (Of course, I probably garnered them 5-10 orders for said heat engines, so they got their money's worth.) I tell anyone who will listen how reliable PASCO's products are, and how good their technical support is. But the downside: they're expensive. And in this case, obnoxiously expensive.
I had no trouble finding stoppers, thread, and zip ties. I'm going to use thread from Burrito Girl's* sewing kit rather than regular string; the chemistry department has stoppers of all sizes. The trick was finding the hollow tubes without a trip to the hardware store.
* Burrito Girl is my wife and sidekick.
My classroommate Alex Tisch looked at the picture, and offered up a suggestion that would make the editors of the Tightwad Gazette croon: what about a BIC pen with the ink part removed? Two decades ago I used to take apart these pens when I was bored in class... now I could use that experience to save my department some dough.
In the event, I used a papermate brand pen. The pen-tip is connected to a thin tube of ink, all of which can be removed from the pen casing easily; the cap on the other side took some wedging, but I got it out with a fingernail in less than one minute. Voila, a "hollow tube," at a cost of about a quarter.
(Oh, you want to know about the actual experiment? Attach the stopper to the string with the zip-tie, thread the string through the tube, and hang a mass from the bottom end of the string. Hold the tube and swing the stopper in a horizontal circle at constant speed such that the hanging mass hangs in equilibrium. The radius of circular motion can be measured with a ruler. The speed of the mass can be determined with a stopwatch, knowing that speed is circumference divided by the time for one revolution. A graph of speed squared on the vertical axis and radius on the horizontal axis yields a line whose slope is the centripetal acceleration of the stopper. This acceleration can be shown to be equal to g times the ratio of the hanging mass to the stopper mass.)
GCJ
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