Tuesday, January 12, 2010

Velocity Lab, Or Why We Don't Do "Real" Labs

One of the most interesting things I noticed in the PBS series Einstein's Big Idea was how the early investigators looking into physical phenomena were able to make such precise measurements using such primitive equipment and facilities. If the re-enactments are to be believed, scientists like Antoine-Laurent Lavoisier and Emilie du Ch√Ętelet were basically conducting their research in a spare room at home. It must have taken a tremendous amount of patience to gather the data that proved, for example, the principle of conservation of matter or that the energy contained in a falling object was proportional to the square of its velocity.

In this experiment, we recreated Mme. Ch√Ętelet's demonstration by dropping marbles into cups of flour from various heights and measuring how far down into the flour they sank. Although I don't usually bother recording data -- no one is particularly interested in keeping track of what we're doing besides me -- in this case I thought we'd do it "the real way," like the classroom instructions call for. But right away we had to make changes. First, we could only find yard sticks, not meter sticks. So we cobbled together a meter stick by taping a tongue depressor onto our yard stick and marking off the extra centimeters.

(Note marble in mid-flight)

Next, the lab called for regular glass marbles, but we found that our results were rather random -- the marbles fell different depths without regard to what height they were dropped from. And sometimes they barely broke the surface of the flour. So we used some steel marbles we found, which tunneled into the flour an amount that could be measured.

Finally, the regular drink-sized paper cups called for in the lab were too shallow, and the (metal) marbles hit bottom and bounced back up. So I dug out some super-sized plastic cups that were about twice as tall as the original cups.

But in the end, our data was gibberish. We were simply too careless, and we didn't do enough trials to get a reliable answer. The whole experience reminded me of my traumatic days in high school science, where the top students (the ones who went onto med school, as it happens) routinely made up data to fit the expected results, so they'd get a good grade in the class.

(Stick marked in cm for measuring depth of marble)
In the three years since I began really focusing on science, I've been very proud of myself for getting nearly all the "labs" we do to work -- by which I mean they provide an impressive demonstration of the principle I'm hoping to illustrate. But I just think it's too much to ask for these demonstrations to provide useful data as well. We're not scientists, but we like watching cool things happen. I'm sorry to admit it, but it's true.

So if you're using this blog to find ideas for your own home or classroom science activities, take our example with a grain of salt. We're dilitantes with a capital "d," and we cannot be held responsible if your results vary.