What is the longest straw Superman could drink out of? (Phys101 PCOs, weeks 1-3)

(This is the first 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.)

In Physics 101 (Energy and Waves) this semester, we are continuing to use pre-class overheads, as described in my previous post. Since Joss and I tend to bang our heads together for a fair bit of time in order to decide what might be interesting and topical images for each week, here I will catalogue what we have come up with, in addition to some of our thoughts about what the interesting pieces and connections may be. Each week, we have two lectures, and so have at least two images that we display as peer discussion pieces.

Through the first three weeks of the semester, the topics we have covered include fluid density, buoyancy, and fluid dynamics (continuity and Bernoulli’s equation).

Week 1

On the first day of class, we used three images that we felt summarized some of the main points and connections of the course. The rough outline of the course is: 1. Fluids; 2. Simple harmonic motion, and; 3. Waves. The three images we used to address these are:

  1. A flask containing many layered fluids and objects floating in them.
  2. A guitar string vibrating (such that you can see the vibrations).
  3. A large bubble floating in the air, in which you can see a rainbow in the reflected light.

The flask with layers summarized the main point of the buoyancy portion of the course: density matters. We used this image for the very first class-wide discussion, because we felt that some of the ideas could be accessible even on the first day of class, before any content was covered. Joss asked the question, “Which of the layers or objects is the most surprising?” As expected at the start of the semester, the students were hesitant to respond. However, Joss did solicit a few suggestions and was able to have some sort of discussion. After the class, Joss and I decided that having a brief peer discussion about the question and image before attempting the class-wide discussion would be a good way to promote engagement with the image and to encourage more people to have and share ideas. This is a technique we now make sure to use each time we choose to centre a discussion around an image.

1024px-Dead_sea_newspaper

A man floats comfortably in the Dead Sea. (Copyright User:Pcb21 / Wikimedia Commons / CC-BY-SA-3.0)

For the second class of the week, we used an image of a man floating on his back in the Dead Sea while reading a book. Around this image we had a discussion about density, bringing into it facts about the density of fresh water, salt water, and humans.

Week 2

We began this week with an image chosen to tie in with both the then-current discussion about fluids at rest (including density and pressure) and the upcoming content on moving fluids: an airplane in flight. Several elements were brought up in the discussion, including the pressurization of the cabin, the pressure and density difference in air as you ascend in the atmosphere, and the lift generated by the pressure difference across the wings.

Later in the same lecture, we showed an image of a standing child who had fastened together several straws in order to drink from his soft drink that was sitting on the floor. Joss asked, “Is there a limit to how long of a straw you could have?” In the explanation of the situation, he restated the question as, “What is the longest straw that Superman could drink out of?” After a peer discussion, during the class discussion, Joss described how sucking through a straw works (as you decrease the pressure in the straw, atmospheric pressure on the top of the soda pushes the soda up the straw) and how you can derive the longest a straw can be. (Hint: For Superman, the pressure at the top of the straw would be zero. The answer I get, for a straw in a soda at sea level, is about 10 metres.)

Finally, to begin the second lecture of the week, we put up an image of a homemade science project: a salad spinner centrifuge. A centrifuge separates mixtures by density by spinning the mixture at high speeds. To connect this with the discussion in class about fluids of different densities floating on top of each other, I suggested the explanation that a centrifuge could be thought of as cranking up the gravitational force. We thought that this image may provide an interesting and novel angle on the density discussion we had been having.

Week 3

After starting on moving fluids, we chose an image that I hoped many in the audience could connect with. This was a picture of one of the fancy hose spray nozzles, which typically have multiple spray settings. Certainly, in my experience spraying things with the hose (watering the garden, having water fights with my brothers, etc.), I noticed that different settings on the spray nozzle resulted in very different results. Some settings sprayed water across the yard, while with others you could not hit something that was within spitting distance. Some settings provide a deluge of water; some a light mist. Although the complete explanation for some of these settings would be outside the scope of the course, some characteristics can be explained using the continuity equation and Bernoulli’s equation.

Karman_trefftz

Air flow around an airfoil. (Copyright User:Kraaiennest / Wikimedia Commons / CC-BY-SA-3.0)

For the second class of the week, we went back to a tried and true example for pressure and moving fluids: an airfoil. The image we used showed the flow of air around the airfoil. A common explanation for lift states that the travel time for the air, from the front of the wing to the back of the wing, is the same whether it goes above or below the airfoil. Of particular interest is that this animation shows that not to be true: in this simulation the travel time for the air below the wing is almost twice that of the air above. However, Bernoulli’s principle still applies, and the pressure of the slower moving air below the wing will be greater than that of the faster moving air above the wing.

Questions for the reader

  • What images would you suggest, related to the images or topics described above?
  • Which image do you think generated the most interest?
  • Do you think its worthwhile to add these to the course content?
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2 comments

  1. […] about Bernoulli’s equation: Unless the highest point of the siphon is too high (recall the maximum length of Superman’s straw), the siphon works just like a pipe going straight down from a higher area to a lower area, because […]

  2. […] 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 […]

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