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10 November 2010

HOW MUCH? The heck you say.

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.)


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