The Pendulum

DSCN1095Empiricism in science is vitally important to us, especially since even at the private school we discuss in How We Ended Up Home “Schooling” the first grade simply read about science rather than doing. So in the spirit of Michael Maestlin we’ve been observing science regularly with both boys, but up until this year it has been mostly empirical without attention to measurement.

This year, Primo is ready to start a physics curriculum with more hypothesis and measurement. And, what better way to start than with one of the most significant observations of the last 400+ years: the pendulum. Thank you Galileo!

We’re pulling several different sources as guidelines, including our own experience to design experiments based on our curiosity, much the way the ancients would have done it. The books we are using as starting points are Exploring the World Of Physics by John Hudson Tiner and Galileo for Kids: His Life and Ideas by Richard Panchyk, which provides a good history before and during Galileo’s life. However, home educators should be cautious as there are some subtle inaccuracies in Galileo that must be corrected, but do lead to some good discussion regarding the use of precise language in science.

Case in point: In the description of the pendulum experiment, the author describes acceleration as an increase in speed. At minimum it should have been described as a change in speed, or most correctly as a change in velocity (a vector concept for which only some students will be ready, but don’t underestimate what a child is capable of understanding; they will surprise you). The point is, the material should always be checked by the home educator before proceeding with the child.

The Experiment

The “experiment” is presented casually, probably catering to a younger crowd for which a simple observation is sufficient (e.g., Secondo, who is currently 8-1/2 years old). But for Primo (at nearly 12) we have to dig deeper and simply presented to him that he needed to keep track of the mass and length relationships in a table for analysis.

Since the experiment called for strings of different length (6-12-18 inches), the weightsDSCN1095 - Version 2 were constructed from simple nuts, wire, and paper-clip to have units of 1, 2, and 4 nuts. We let Primo tie the knots since his hands were smaller and he is the “knot expert” from his climbing experience.

The next step was to have Primo measure the length and mass of both the strings and weights. The strings were measured for length from loop to loop, and the weights were measured from nuts to the end of the paper clip. We’ll discuss more on why later, when we present options for improving the experiment.

DSCN1098The action of the experiment is very simple: 3 strings and 3 weights for 9 combinations. Hang the string/weight combination on a hook, pull back a modest angle and time (coarsely) ten swings. Be mindful of when and how the child counts in relation to when timing starts and stops.

Primo really enjoyed this experiment, especially when he figured out how to make the camera capture the full swing.DSCN1113

Obviously, if you’ve done this experiment in school yourself you know what the answer will be. But it was joyful to watch both Primo and Secondo discover what was happening. This actually brings up a really good point: you don’t always want to tell them what will happen (something some books will do); it spoils the joy of discovery. Let them guess, reminding them that a hypothesis is just a guess and it is okay to be wrong as long as we execute the experiment honestly and with sufficient accuracy.

Primo recorded his data and jumped at the chance to enter it into an OpenOffice spreadsheet so he could plot it:

Screen Shot 2013-09-07 at 11.16.27 PM

(click to enlarge)

So What Did We Learn?

Primo said he suspected what the results would be, but it was still fun to record and prove it. Secondo (channeling Aristotle) was surprised as he expected the heavy weights to swing faster. Primo got a chance to use the extended exposure setting on the camera and use a spreadsheet (although getting the plot just right did require quite a bit of help from dad).

From the data and instruments we had some small discussions about precision and accuracy, concepts often confused by the most seasoned scientists. This is a concept we will hit repeatedly over the next several years.

Where do we go from here?

An improved version of the experiment would better hit home the concept of center of mass relative to the fulcrum, and improved timing measurements. I.e., the implementation of the string and crude weights meant that each length varied slightly. Further, our timing methods were crude.

This gives us an opportunity to play with some tools and use Primo’s new-found love of electronics and programming.

So for now we will just summarize the plan to use pipes (PVC, copper, and steel) of “identical” length (short and long sets) as well as an Arduino micro-controller with some photo sensors to time the swings. The details of this we will post separately.

We look forward to hearing about the experiments you’ve tried with your children.


See Fusing Skills post for update on the sensing circuit; hopefully we will run the experiment in the next week or so.

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  1. Pingback: Fusing Skills: Writing, Math, and Science RocketRedNeck

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