Sunday, March 10, 2013

The Physics of Ye Olde Tablecloth Pull

[post 328]

I don't know about you, but I still remember as a kid upsetting my parents by trying the old trick of yanking a tablecloth out from underneath some dishes and glasses with, er, mixed results. I've also written about this trick, including an advanced variation, in this blog post.  But now I can offer a more scientific explanation....

First a little background: there's an organization and web site,, that distributes free, interactive online classes from major universities. I decided to enroll in "How Things Work: The Physics of Everyday Objects" because, well, I'm interested in that kind of stuff. And here's the professor, Louis Bloomfield of the University of Virginia, beginning his very first lecture with — you guessed it — the tablecloth trick.


While Bloomfield makes a distinction between the scientist who shows how things work and the magician who hides things, he is in fact being a bit tricky here himself. For example, he says he's adding a degree of difficulty by pouring wine into the glass. Yes, the spilled wine would make a mess, but of course the added weight makes the glass less likely to tip over in the first place. And removing the wine bottle from the table hardly seems insignificant. Because of its shape and higher center of gravity, the wine bottle is far more likely to be displaced than the plate.

His very next demo is conisderably more interesting and uses a trick I'd never seen:

Drew Richardson is visiting and we were watching this together, leading us to brainstorm on how this could be turned into a clown bit. Hmm... It might be hard to find a reason to be sticking a pencil in a Coke bottle, but what if the pencil were a straw, a straw that somehow you couldn't insert in the usual manner? But wouldn't a straw, because it's so light, be more affected by air currents and be in danger of missing the opening? Maybe you could make your own "straw" out of heavier material. In fact, the object wouldn't have to be hollow so long as there were an opening visible on each end. (3D printer, anyone?) Now for the hoop. You can see why you'd need it rather than, say, a rectangular frame (too much friction), but what would the excuse be for having a hoop handy? A hat band? It would have to be perfectly circular. A spring-form pan?? Well, you get the idea. I'm sure this is how Grock worked.

Anyway, good stuff, and the course just started this week, so if you want to enroll, just go to Did I mention that it's free?

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