Day 4 Physics II Brain Dump

I. Warm-up Mini Calculation, Clicker Question, and Problem: A quick warm-up to review.

Calculation: Students were given questions to find wavelengths in air of the frequencies at the extreme of human hearing range. They were first asked to state which should be larger and why, and also to give a ballpark estimate for what their guess this distances might be.

Clicker: Students were shown a story graph and a snapshot graph for a wave, both with quantitative information, and asked to determine the wave speed.

Problem: Students were given a problem about a boat on ocean waves. They were given information like wave amplitude, wavelength and wave speed, and they were asked to first construct a snapshot graph and story graph, and then to construct velocity vs. time and acceleration vs time graphs for the boat.

II. Direct Instruction: Showing a PhET Sound Demo, to introduce idea of waves traveling not just in a line but spreading out in 2D and 3D. The simulation makes it easy to link back to story graphs and snap shot graphs. Beyond that, I had two goals here. Point out that the amplitude of the waves seems to go down as it travels away from the source. Our goal today was to build up the tool’s that would help us explain why that happens. My second goal was to introduce the new representation of “wave front” diagrams, linking this idea to their reading about spherical waves –> plane waves.

III. Invention Task: Students were given the following task for an invention task

Students caught on to this pretty quick. We had some (rather limited) discussion, which led into direct instruction described below.

IV. Direct Instruction on Intensity, Power, and Energy, plus an Example Problem: I leveraged the invention task to introduce the concepts of Intensity. After briefly introducing the idea, I applied it to a problem about using a lens to focus light (How much energy passes through lens each second; what is the intensity of the light after focusing).

V. Student Problem-Solving: Students were given two problems–one about how much energy an ear drum absorbs at a rock concert; and another problem to estimate how big the largest solar array in america must be.

VI. Direct Instruction and then Clicker Questions to Extend the Intensity Idea:  Had some brief direct instruction and clicker questions to extend the idea of intensity to a sound spreading as a spherical wave (1/r^2). Some of the instruction and questions were meant to pose this as a way of explaining why sound loudness decreases with distance—and to distinguish dampening (energy transforms to thermal) vs energy spreading (same amount of energy becoming less concentrated). After class, a student had lots of questions about this and the 2nd Law of thermodynamics.

VII.  Direct Instruction and Clicker Questions on Decibel Scale: This was brief, mostly to give student an intuitive sense for the decibel scale, rather than a formula sense.

VIII. Problems we didn’t get to: Students were supposed to end the day with a problem applying the idea of intensity decreasing in a spherical wave to calculate how the sound intensity level in dB decreases, but we were tired and running out of time.

Notes: With no lab on the day, it was kind of a slog, but not as bad as some no lab days can be. There lots of opportunities for student to think, discuss, work problems, etc,… but ultimately we needed a little more space in the day for play in order for us not be exhausted. This should be my new design principle–is there enough play?