Beast Academy had us doing a simple pendulum experiment today, as an enrichment activity in the chapter on measurement. We taped a pendulum made from dental floss and a metal weight to the top of the kitchen doorway. Alex swung the pendulum while I timed its period – the amount of time it took for a full back-and-forth. After a few repetitions, she cut and re-measured the string and we repeated the process.

There was some noise in the data, of course – it was easy for Alex to understand how I might be a quarter-second slow in my reaction time on the stopwatch, or how she might sometimes give the weight more of a push than at other times. The overall pattern was clear, though: the shorter the string, the faster the period.

The thing that amazed and delighted her most, though, was that it didn’t seem to matter how high she made the pendulum’s arc. If she pulled it back as far as she could, it had about the same period as it did if she barely lifted it. Fascinating! (And I learned a new word, thanks to the Wikipedia article on pendulums: isochronism.)

Why did that happen? I asked. She didn’t know. Experimentation didn’t help her decide. Finally, I had her sit down and watch while I swung it high, then low.

“OH! When you pull it back further, it’s faster!

“Yes! It is faster. Why?”

“Gravity pulls it down, and momentum pushes it back up,” she said quickly.

Forgive the mama brag here, but I think that’s a pretty good answer from a seven-year-old. I always feel like we’re not doing enough science, but clearly she’s picking things up and remembering them. (Erm… is she right? It sounds plausible to me. It’s good reasoning, anyway.)

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6 Responses to Isochronism.

  1. Bill G. says:

    “Gravity pulls it down, and momentum pushes it back up,”

    Galileo himself could not put it better. That’s pretty much exactly how he described it.

  2. tinderbox says:

    Bill, have you discussed this with her? Michael says he didn’t.

  3. Jinian says:

    Wow, go Alex.

  4. Bill G. says:

    No, I don’t think we’ve ever talked about how a pendulum works.

  5. hobbitbabe says:

    I didn’t talk to her about it either!

  6. tinderbox says:

    Well, goodness, Hobbitbabe, why not?

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