Waves Unit Intro

When I was teaching at a college prep high school last year, my advisor showed me how he always introduces waves via the pendulum. For the life of me I can not remember if that is how I was introduced to waves, but I think it’s fabulous! It was a great lesson today, so I thought I would share:

I have this tiny little pendulum (pictured above) one of my students made me last year, so I set it up and had my students brainstorm every type of physics they observed. We went through the whole list (velocity, centripetal motion, friction, gravity, momentum, energy, etc) and discussed each one for a short bit. Next, I described the importance of pendulums and we brainstormed a list of pendulums in “real life”. Some classes were more creative than others, but the point was clear that pendulums are everywhere.

At this point, I asked students what determines the period (time for one cycle) of the pendulum. The usual mass, gravity, amplitude, and string length came into play, so we attacked one at a time:

  • MASS – the pendulum is essentially in free fall and we know that mass in negligible in free fall, so no need to deal with it here either.
  • GRAVITY – will affect every pendulum equally, so it can not affect individual pendulums diferently.
  • AMPLITUDE – to test this we timed 10 oscillations at a low amplitude and 10 at a high amplitude. The times were almost exact, so the students understood that it does not affect the period.

Therefore, the length is the only variable to affect the period of the pendulum.

Vocab came to play here as we defined amplitude (height of pendulum), period (time it takes for one cycle to occur, measured in seconds), and frequency (number of cycles that occur in a given time period, measured in Hz). I wrote the definition of period and frequency on the board as equations as well as the inverse relationship between them. The students then did a few sample problems from the textbook (Hewitt Conceptual Physics – awesome resource!).

To finish up the period, we graphed the position-time graph of the pendulum and the light bulb went off and the kids realized the connection between the pendulum and waves. They drew a transverse wave and labeled the anatomy in order to conclude the lesson.

We’ll pick up with types of waves and the wave equation tomorrow.


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