# Week 3 (Fa14)

**Astronomy Lecture**

Students had worked on the Ecliptic tutorial over the weekend so we started Tuesday with some follow-up clicker questions and I asked for their responses on a few of the tutorial questions. Later that afternoon I found the Motions of the Sun simulation from Nebraska that helps illustrate the Sun’s motion along the ecliptic versus the Celestial Sphere’s daily rotation.

After finishing the ecliptic discussion, we moved into Moon phases. On Tuesday, students did a brief activity where you hold a styrofoam ball on the end of a pencil, turn off all the overhead lights except a single incandescent lamp in the front of the room, and students observe how much of the ball is actually lit and how much of the lit part can they see from their angle as they move the ball around the head. Students generally got the idea but it didn’t work as smooth as I would have liked because 1) I had to goad students into actually doing it. Students worked in groups and most groups seemed content to have a single person actually view the styrofoam Moon from different angles. and 2) For a class of 30 students I set up 2 incandescent bulbs, one on each side of the room, but this created a lot of ambient light so the phases were not as easy to see as I would have liked. We used the activity sheet from Hands on Science at Texas for this activity.

On Thursday we continued the Hands on Science activity that involves Moon collars with several Moon locations marked and a ping pong ball colored half black to represent the Moon. Students again worked in groups (you need more than two hands to hold everything) to view the ping pong ball at different positions and match each position to one of the Moon phases with names on a table I handed out. This went pretty smooth (again I had to push students to have everyone in the group wear the collar explaining that they will be much more likely to remember how this worked on an exam if they actually experience it rather than just watch). We continued using the Hands on Science activity packet which guides students through questions about where the Moon is in its orbit when the lit section visible from Earth is growing vs. shrinking, when the Moon is closer or farther from the Sun than the Earth, etc.

At the end of class I asked students where the full Moon phase would go on their collars (this phase is not one of the listed positions) and then asked if this position should be a full Moon or an eclipse. Students agreed that eclipses don’t happen every month so it should be a full Moon. I agreed but said that the way we did this activity, their position would correspond to a lunar eclipse. I explained that the Moon’s orbit is tilted relative to the Sun-Earth plane and illustrated how tilting the collar would put the Moon behind and above Earth. I also (for fun) briefly discussed the phrase “dark side of the moon”. Students are working on the Causes of the Moon Phases tutorial for homework.

**Astronomy Lab**

We started by finishing our Scale Model of the Solar System activity from last week. Last week, students cut out scale paper representations of the Sun and each planet. This week we started by taking students outside with each student holding a sign indicating what object (Sun, a planet, Pluto) they represented and measured them out to represent a scale model of the planet spacing. Using the scale they chose last week, this model spanned about 23 meters with the first few planets a little too close for students to squish together well.

Our lab this week was a variation on Eratosthenes’s experiment to measure the circumference of the Earth. The pre-lab starts by looking at shadows cast by vertical objects in different locations and explains Eratosthene’s original experiment and then leads students to think about the altitude of the Sun at different locations (how we do our experiment). In lab, students use a USNO site to look up the altitude of the Sun at different locations at the same time. I tell them to use Austin, TX and Fargo, ND to start which gives very good results and then ask them to repeat the experiment for two other pairs of cities that line along a primarily North-South line. This lab is based on a tidbit in Great Ideas for Teaching Astronomy (a really fantastic book of brief ideas suitable for a class discussion or that can be built into entire activities).

**Physics 2**

On Monday, I introduced different types of ideal gas processes and set students to work in groups on a question asking to find final temperature, change in internal energy, heat transfer, etc. for a constant pressure process. This took the entire period, but students did a good job in their groups of figuring out how to use our starting points (ideal gas law, integral definition of work, 1st law of thermodynamics, internal energy of an ideal gas) to build up solutions. I gave students a second problem to try at home, but hardly anyone worked on that between Monday and Wednesday.

On Wednesday, we did some clicker questions on adiabatic and isothermal processes that led to very engaged group discussions about whether heat transfer has to be zero for isothermal and whether temperature has to be constant for adiabatic. I demonstrated a fire syringe and asked students how we might describe what happens to the gas and why the cotton catches fire. That discussion went pretty well with me pushing a little bit for more explanation but contributions from different students building up a fairly complete description. We ended by introducing a cyclic process and thinking about how to determine Q, W, and &\Delta;U for each segment and for the process as a whole.

On Friday, I was going to talk more about degrees of freedom and how quantum mechanics leads to a breakdown of the equipartition theorem (or a freezing out of certain degrees of freedom) at certain temperatures but decided to write this into a guided homework activity instead. We’ll see how the homework went come Monday. In class, we worked on the ideal gas tutorial from Washington. Students were engaged and discussing in their groups, but they struggled more than I anticipated and we did not get as far as I had hoped. We don’t have a separate recitation section for this class and I’m trying to figure out where to fit in some of the tutorials. I can probably fit them into lecture, I just have to plan better and reorganize a little bit what I do in class and what I ask students to do on their own before coming to class.

**Physics 3**

I felt like Physics 3 went really well this week. Last semester, I wrote a tutorial to try to guide students through thinking about standing waves on a string. It starts by having students use the PhET Waves on a String simulation to figure out how to time pulses so that each pulse constructively interferes with the previous pulse resulting in a single larger pulse. They do this for one and two pulses on the string determining an expression relating the time between pulses to the length of the string and the wave speed. Students then look at the Walter-Fendt standing wave simulation to help us visualize incident and reflected periodic waves and how their superposition works. Finally, the tutorial introduces the idea of boundary conditions, how to use boundary conditions to sketch possible standing waves, and how to use the sketch to determine wavelength and thus frequency.

Last semester, lecture and lab were in the same room and every period students had access to computers which is necessary for students to complete the tutorial. This semester, our lecture room only has an instructor computer so I decided to change the tutorial to more of an interactive lecture demonstration. This worked very well. I would ask students to think about how to time the pulses and then try their suggestions. We discussed coming up the equations and the sketches together, and I was able to alleviate some confusions that arise due to poor wording in the tutorial. Students still worked through the tutorial, but we did it as a class and finished the tutorial (standing waves on a string with two fixed ends) in 50 minutes.

On Wednesday, I broke the class into three groups (it’s only 8 students this semester) and gave each group 25 minutes to accomplish one of three tasks: 1) calculate the expected standing wave frequencies for an experimental setup and use the setup to test their calculations, 2) use the PhET simulation to determine pulse timing for the case of one free end and to use this to try to determine what would be needed for two free ends, or 3) sketch the first two possible standing waves for the case of one free end and the case of two free ends. Students worked in their groups while I floated around offering minimal suggestions and then we spent the last 25 minutes of class with each group sharing their results with the rest of the class. Students didn’t take notes on other groups’ work so after class I wrote up a summary of everyone results to post on Canvas. Each group succeeded in their task and everyone seemed to be enjoy the class (some students even saying so explicitly). I think this worked well because everyone’s task was just different enough from what we did on Monday that students felt like they knew enough to succeed yet had to bring together enough ideas from previous classes that they felt that they were discovering things on their own. It seems like a less-than-ideal classroom setting actually helped me stumble onto some more effective ways of covering these topics.

In lab, we built Sound Sandwiches and used our new ideas about standing wave frequencies to think about why certain adjustments lead to changes in volume or frequency. Students look at the Fourier series for their Sound Sandwich and see multiple frequencies from which they are asked to determine the fundamental frequency. They are then asked to experimentally determine the fundamental frequency they would expect for their Sound Sandwich based on properties of the medium. My thought, and what students did, was to use a force spring to measure tension in the rubber band, measure the linear mass density of the rubber band, measure the length of vibrating rubber band, and use f = v/2L. This actually doesn’t work. It works well for predicting the lowest frequency of the rubber band when you stretch it between your fingers and pluck it, but it does not work well for predicting the lowest frequency on the Sound Sandwich. I do not know why. We talked about the possibility that maybe the Sound Sandwich only produces certain harmonics and so what we though were fundamental, 1st, and 2nd harmonics are actually more like 3rd, 6th, and 9th harmonics. A student brought up the possibility that the rubber band might not have uniform tension due to the way it is clamped and so maybe measuring tension in a free rubber band stretched to the same length doesn’t work well. This is an open question for us. The last part of the lab asks students to relate these ideas to guitars and explain why different strings produce different pitches, turning the tuning pegs changes the pitch, and why pressing down on a fret changes the pitch.

Forgot to mention that we started lab with a discussion of the Mythbusters Fun with Gases clip (introduced on Day 1). We listened to Adam’s explanation in the video and talked about what makes sense, what assumptions are involved in his explanation, and what questions we might still have. I used this to motivate doing the Washington tutorial on transmission and reflection as homework. The tutorial looks at wave pulses transmitting from one string to another so I assigned the tutorial with the overarching question of what variables (wave speed, wavelength, frequency) change when the sound passes from helium in Adam’s throat to air in the room.