How might a spider feel its prey sooner? (Phys101 PCOs, weeks 7-9)

(This is the third post in a semester long series about the pre-class overheads, or PCOs, Joss Ives and I use as discussion pieces in Physics 101, a large-scale first-year physics course. The intent of these posts is to create a record of the images we choose and of the thoughts behind our choices. See also the first and second posts in the series.)

As we continue to be inspired by student-generated content, our definition of ‘pre-class overheads’ is evolving also, to something more like ‘added activities’. In these weeks, we included content in the form of clicker questions (some generated by students), YouTube videos, and a combination of the two. The high density of videos in these weeks may be at least partially attributed to the content of the course: Waves are a time-dependent phenomenon and many characteristics are easier to visualize in real time than with a static image.

Through these three weeks of the semester, as mentioned, the focus was on waves: wave speed, intensity, reflection, interference, and standing waves were all discussed. (i.e. Almost everything you might want to know about waves!)

Week 7

Inspired by the recent visit by Derek Muller of Veritasium to the Department of Physics and Astronomy at UBC, in the first lecture of the week we opened with his excellent slinky drop video:

When the top of the slinky is released, the bottom of the slinky stays fixed until it ‘feels’ that the top has been released. In the slinky, this happens when the top falls down to the bottom, because the longitudinal wave speed in the slinky is quite slow compared to the time it takes for the top to fall to the bottom. This might be contrasted with dropping a solid metal rod. We know that if we drop a metal rod, it will appear as though the bottom of the rod begins to move as soon as we release the top. If we model the rod as a lattice of atoms connected by springs, these springs will be very stiff (so that the rod appears solid), and the longitudinal wave speed through the rod will be very quick. Therefore, the bottom of the rod will ‘feel’ that the top has been released very soon after it has been released, and it will appear as if the bottom begins to move immediately.

A possible way to facilitate this slinky video could be to use Veritasium’s series of slinky videos in conjunction with clicker questions for the students to predict the result of the demonstration.

For the second lecture in the week, we began with a clicker question adapted from a student’s Learning Object. In the Learning Object, the student sought a physical application of the equation for the speed of a wave on a string: a spider on a spider web. The interesting question the student posed was, how might the spider decrease the time it takes for it to notice a fly struggling in the web? From the wave speed equation, we see that increasing the tension of the web or reducing the linear mass density would increase the speed of the wave in the web, resulting in the spider becoming aware of the fly quicker. However, the student argued that decreasing the mass density of the web would make it weaker, which might have detrimental effects of the integrity of the web. Thus, she advocated that the spider should make a web with increased tension.

Week 8

In this week, the main idea we discussed was wave interference. To illustrate this phenomenon, we again turned to YouTube, using a video which demonstrates both constructive and destructive interference. With this video, we tried an unstructured peer discussion, by orally asking the students to turn to their neighbour and decide what was happening (why only some pegs fell down). During this time, I eavesdropped on a few discussions and participated in a few others. Many students quickly (and correctly) decided that the results shown in the video were due to ‘interference’. However, from my perspective, the depth of these discussions was unsatisfactory, and it wasn’t clear to me that the general student had a clear picture of what the video was demonstrating. (Some of this difficulty could have been due to the spring in the video being hard to see.)

Week 9

In the first lecture of this week, instead of having a pre-class overhead, Joss and I embedded the following video at some point mid-lecture:

In the video, a wave travels from one medium to another, with a partial reflection at the interface between the media. To facilitate this video and orient the students to notice interesting details, we asked three multiple choice response clicker questions (each with two choices) about what happens in the video. These questions (with choices and answers) were:

  1. In which medium is the wave speed the highest? (Silver spring, black spring.)
  2. Which property is responsible for this difference in wave speed? (Tension, linear mass density.)
  3. From the perspective of the incoming and reflected pulse on the silver spring, the boundary between the two springs behaves most like… (A fixed end, a loose end.)

These questions generated good discussion in the class. One piece of evidence that suggests that the students were indeed thinking about them is that students asked Joss to replay the video again, so they could watch with the questions in mind. They had not noticed some of these features during the first viewing of the video, but after being prompted by the questions, they were able to focus on them.

For the second lecture, we began the lecture with a clicker question taken from a Learning Object: “A 1.0m long string is fixed at both ends. What is the longest wavelength standing wave that can exist on this string?” This question directly connected to the then-current discussion of standing waves and provided a warm-up for the more difficult standing waves questions that were tackled later in the lecture time.

Notes about task structure and engagement

In these weeks, we used a few videos with different facilitation techniques. As described above and in the previous post, we found that the less structured task of ‘describe what happened’ (week 8 video) resulted in a less satisfactory response from the class than the more structured series of (simple) clicker questions (week 9 video) that scaffolded students’ noticing for the video. In the former case, we followed up the peer discussion with a class-wide discussion, which, like previous class-wide discussions, was not completely successful (students were not jumping over each other to offer their perspective). In contrast, by including clicker questions which were related to the video (week 9), we made space for every student to offer their thinking, which may be a more effective way to involve students in the class-wide discussion in such a large class.

Questions for the reader

  • What images or videos would you suggest, related to the images or topics described above?
  • Which content do you think should have generated the most interest?
  • Do you think its worthwhile to spend lecture time in this manner?
  • Where is the balance between enough structure (to focus students’ effort) and too much structure (such that the students aren’t pushed past being comfortable to the point where they are learning)?
  • (A practical question:) What features make a YouTube video suitable for bringing into class as content? Or, what features would make a YouTube video unsuitable for bringing into class?

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