## December 19, 2010

### R wins 364-331

Renae won today. I got all the high value tiles, but she got the only bingo: refront.

### R wins 364-331

Renae won today. I got all the high value tiles, but she got the only bingo: refront.

## December 15, 2010

### Running track lengths

Yesterday, Rhett at Dot Physics had a comment about last week's puzzler on Car Talk.  He had an alternate solution to the question about how a pair of people could walk side by side for an hour and cover different distances.  His solution was that they were walking on a circular track. (Read his solution for a full explanation.)

At the end of the post he said:

It doesn’t even have to be a circle – it can be just curved at the ends and straight in the middle. Of course in this case, the husband would have to slow down on the straight parts (or the wife would have to speed up) in order for them to stay side by side. But it could be done.

There was something about this that didn't seem right.  I remember the track I used to jog on had one lane that was 8 laps for a mile.  Let's use that as the lane for the husband in our example.

Here's the layout of the track:
Let's use Rhett's values for the inner and outer radii: 4 m and 5 m, respectively. If the outer track is an 1/8th of a mile, then how long are the straight sections?

1/8th of a mile is about 200 m. The circumference of the curved part of the outer track is d = 2×π×5 m = 31.4 m. So the length of each straight section for the outer track is 84.9 m.

The circumference of the curved part for the inner track is d = 2×π×4 m = 25.1 m. But the length of the straight sections is the same.  So the total length of the inner track is 25.1 m + 2 × 84.9 m = 194.9 m.

The wife has to walk 33 laps to go 4 miles. If the husband and wife are walking side-by-side the whole way, he also walks 33 laps.  His lane was 1/8th of a mile, so he has gone 4 miles, plus an extra 1/8th.

I still like Dot Physics.

## December 14, 2010

### Results are in

Over the weekend, I was watching the Mythbusters episode which was testing the idiom "It's like taking candy from a baby."  The phrase is used to imply that something is as easy as, well, taking candy from a baby. But long ago, I realized that phrase makes no sense at all. Anyone who's every been around babies knows that if a baby wants something, then even if you can easily take it from them, what you're really going to have is whatever you took away plus an unhappy baby. If the something you are taking away is candy, then you're likely to get tears from the baby, too. So taking away candy from a baby is just mean.  I think that's a better representation of the idiom.  What said you on the poll?

Clearly, most people went with the traditional meaning.  The three "other" responses all included meanness or cruelty in some way.  Still, most people (3:1 from this non-scientific poll) seem to think it is easy and not that mean or cruel to take candy from a baby.

## December 13, 2010

### What can you do with this?

I just wrote this up over at the home.drewsday blog, but the real reason for taking the photos was to use this in class next quarter. We start with thermodynamics; one of the first topics is thermal expansion and contraction.

I regularly read Dan Meyer's blog. He was (is?) a math teacher, but I'm consistently inspired by his ideas. One of his regular features is something he calls "What can you do with this?" [WCYDWT]. The idea is that he finds an example of something in the world which illustrates a math concept and brings it into the classroom.

When I saw the contraction of the vinyl siding on my house, I knew I could bring it into the class next quarter.  The question is: What can you do with it?

I want to present this to class, so I have to think of the questions that would be appropriate.  Usually, when I start a problem in class I make a list of everything I know and everything I don't know (or want to know).

What I know

• temperature outside today was 12° F. ( T)
• nominal length of the siding was 12 feet. ( L)
• change of length (on one side) was 1/8 inch. ( ΔL )
What I don't know (would like to know)

• temperature when the siding was painted ( T)
• coefficient of linear expansion for the vinyl siding (α)
Relevant equation

• ΔL = αL0ΔT
The problem is that I have two unknowns (ΔT and α).

I have no idea what the temperature was when the house was painted. I didn't even know the house existed when it was painted.  I suppose I could come up with a reasonable estimate, but realistically, there is a pretty wide range of temperatures to work with. Conservatively, I would guess that the painting could have been done when the siding was anywhere between 60° F and 90° F. That is a pretty wide range, so I'm not sure I want to try to estimate that and use it to find α.  So let's make the initial temperature something we want to find.

This means that I need to figure out what the coefficient of linear expansion is for the siding. In class we discuss how the thermal expansion process is approximately linear over a certain range. I have no idea if the expansion of vinyl siding is linear or not.  I'm going to cross my fingers and assume it's linear-ish.

With a bit of googling, I learned that vinyl siding is made of unplasticized polyvinyl chloride (uPVC). I also found the coefficient of linear expansion is listed as 50 ×10-6 °C-1 for PVC. I couldn't find uPVC thermal coefficient, but every listed coefficient I could find was close to this value.

Using that thermal expansion coefficient, and solving for the initial temperature, I found that the initial temperature was 23.6° C, which is 74.5° F.  (Right in the middle of my estimated range, hmmm...)

I think that the value for the coefficient is reasonable.  I plan to work this example in class, then extend it by asking the class to calculate what the maximum expansion/contraction range would be for a 12' length of siding in Illinois weather.  I'll be looking for other ways to extend this.

Side notes:

1. The units specified online for the coefficient of thermal expansion are often given as [L]/[L][T] (e.g. m/mK). Of course, the length units cancel and all that remains is inverse temperature units, which is how our textbook lists the units.
2. Coefficients of linear expansion for common materials varied quite a bit (sorry, not scientifically quantified) from one table to another. Surprisingly, the PVC coefficient seemed to always be within 2% of 50 ×10-6 °C-1.

## December 03, 2010

### Loudness and sound pressure levels

This morning the blogs and twitter feeds were linking all over the place to a passive amplifier for your iPhone. (See it here:

The idea behind a passive amplifier is that it requires no electrical power to increase the amplitude of the sound wave which is presented to your ear. Just pop your iphone into the cradle and crank up the tunes. As long as the battery in the iphone is charged, you have amplified sound.

It sounded cool, so I checked it out. This is what caught my eye:
The Phonofone III is an elegantly designed passive amplifier crafted from ceramic and designed explicitly for iPhone. This clever device amplifies the volume emited from an iPhone internal speaker roughly 4x (approx. 60 decibels).
Wait. What? I get a 60 dB gain out of a device with no power?  Let's see if that passes the sanity check. I don't have an iphone, but I have heard the iphone playing audio from its internal speaker. Unamplified, at a moderate level, I would say the sound level observed from an ipod (at an average distance) is about the same as the sound level of a typical conversation.  Let's look at what the approximate sound level would be:

(I found this chart on OSHA's website. I assume the image is public domain, like most government images.)  From the chart you can see that conversation at 1 m is approximately 60 dB(A).  (The "A" means A-weighted, which is a weighting factor used to approximate the frequency response of the human ear.)   Let's be conservative and estimate the iphone unamplified level to be about 55 dB. That means that if I drop the iphone into the passive device the sound level should be 55 dB + 60 dB = 115 dB.  That's louder than the "Discotheque" rating in the chart (when/where was this chart generated?) which is also about 20 dB louder than a jackhammer at 15 meters.  Somehow I really doubt that the single horn + iphone is going to be able to compete with the speakers and amplifier of a dance club.

What about the other claim?  They say the sound will be about 4x louder.  Here's where the claim might hold some water. The basic idea of a horn is to provide better impedance matching  between the driver and the sound field and to control directivity of the sound radiation.  Assuming the horn is pointing at an observer, the sound level at the observers ears should be higher with the horn than without the horn.

What if their claim that using the horn causes a perceived 4x increase in loudness is true?  (Loudness is a perceptual quantity, where sound level - either sound pressure level or sound intensity level - is a measured quantity.)    Then, the only mistake that they made is equating a 4x increase in loudness with a 60 dB gain in sound level.  Here's a graph from a lab we do in my "Sound and Acoustics" class:

In this lab students hear a broadband tone which they assign an arbitrary loudness level rating. In this class, we all agreed to call the reference tone (relative sound level = 0 dB) a loudness level of 100.  Note, there are no units, since it's an arbitrary scale we made up for the lab. Then they hear several other tones where the level has been increased or decreased randomly and they are asked to rate the loudness of the tone with respect to the loudness of the reference tone.

What I've plotted above is the class average of loudness level vs the actual relative sound level (in dB) for all trials.  Each sample was presented twice (not in sequential order) so the scatter is a sort of approximation to the uncertainty in the measurement.  The solid line represents a model that is what we would expect to see for a larger sample of the population.  For such a small class, the trend is pretty close to the "expected" behavior.

Note that a 4x increase in loudness, from either 25 to 100 or 100 to 400, corresponds to a relative gain in sound level of 20 dB, not 60 dB.

Under ideal testing circumstances, I could believe that a passive amplifier like the horn amp would give a 20 dB gain right in front of the horn.  But someone should have caught the 4x = 60 dB nonsense.

## August 26, 2010

### A side project...

Moving into our first house that we own, I thought it would be fun to try to document all the home-improvement (and maintenance) projects that we undertake.

Obviously, that means I must blog.

I'm announcing today the home.drewsday blog.  This is where I am documenting everything that we do in the new house.  Hope you like it!

### Do what I do, not what I say

One of the things that I decided to do based on recent classes I've taught is to try to reinforce the importance of students reading the material before coming to class.  To that end, I decided to implement reading quizzes due every morning that class meets.  But I also want to have walked in my students shoes as much as possible, so I have decided to read every section that I've assigned to them and take notes like I want them to be doing.

It may seem obvious to do this for every class, and certainly when I've taught the upper level courses I read and re-read (and sometimes re-re-read) all the sections we cover.  But for introductory classes I find that to prepare I generally need only skim over the chapter to see what topics are being presented and in what order.  Then, I have enough to worry about with prepping demonstrations, clicker questions, homework problems, in-class tutorials and anything else I can think of to help cover the topic in an active-learning classroom to have a lot of time to actually read the whole chapter.  Starting early, though I hope to get far enough ahead that I can finish all the reading assignments before the end of the quarter.

The reading quizzes are going to have two questions which will be the same each time, and then one or two additional questions which will be unique for each quiz.  The questions which repeat are: "What are two important concepts that you learned from the reading?" and "What are two concepts from the reading which you struggled to understand?"

I plan to use these questions to guide my note-taking as I read the book.

## August 24, 2010

### School is starting soon

Yesterday, a bunch of colleges and schools started up for the year.  Our campus is on the quarter system, so I still have a few weeks left to prepare.  My browser has become cluttered up with education-related articles that I've been saving up to comment on, but I think I'll just dump them out here.

A conversation with Sean Carroll - in this NY Times interview from April, I was a little disappointed to read this response from Sean:
Whenever you say you’re a physicist, there’s a certain fraction of people who immediately go, “Oh, I hated physics in high school.” That’s because of the terrible influence of high school physics. Because of it, most people think physics is all about inclined planes and force-vector diagrams. One of the tragedies of our educational system is that we’ve taken this incredibly interesting subject — how the universe works — and made it boring.
I get what he's saying.  But I'm disappointed that he would paint all high school physics teachers with such a broad brush.  I believe that at some level, ALL physics teachers got into teaching physics because they found the subject to be terribly interesting.  What about giving them the tools and motivation to make their classes interesting?  What about structuring schools so that biology teachers are not teaching physics?  These are complex questions that the interview does not get into.

His response to the follow-up question which was about what he would do to improve physics education was a little better in spirit, but I think it's a little unrealistic to think that starting with the Big Bang and particle physics is something that will lead to better scientific literacy.

No grading, more learning - This was an article from "Inside Higher Ed" that I found via Metafilter. A professor at Duke dispensed completely with the traditional method of grading (where the professor assigns grades for work turned in) and chose to "crowdsource" the grading.   The class as a whole (I guess) assessed whether or not student work met the standards for the course.  The discussion on Metafilter rightly pointed out that many classes have cliques of students which professors may or may not be aware of.  I am impressed with how this professor was able to turn the focus of the class away from the grades and onto the learning.  I am, however, not deaf to the concerns raised by many of the commenters talking about this method.  I don't think this would work for my courses where almost everything we do is a "right-or-wrong" type of assignment.  Still, it was interesting and inspiring to see faculty get their students to focus on learning.

Does Professor Quality Matter? -  You should read this article (from the Journal of Political Economy) to get the whole story, but I'm just going to skip to the end and quote from the conclusion:
Our findings show that introductory calculus professors significantly affect student achievement in both the contemporaneous course being taught and the follow‐on related curriculum. However, these methodologies yield very different conclusions regarding which professors are measured as high quality, depending on the outcome of interest used. We find that less experienced and less qualified professors produce students who perform significantly better in the contemporaneous course being taught, whereas more experienced and highly qualified professors produce students who perform better in the follow‐on related curriculum.
I'm not sure how much to read into this as it relates to teaching physics, but it was a somewhat interesting article.

Cory Doctorow: What I do - Not education related, but if you're a nerd who follows Cory Doctorow, now you can do what he does.

You say up, I say yesterday - This is a great profile of a cognitive psychologist who has studied a fascinating topic of how language relates to thinking.  I was really blown away by these ideas. (Cog psy is ALMOST as cool as physics...)

Plan B: skip college - This NY Times piece doesn't say anything that hasn't already been said about whether or not college is the best economic choice for all people.  But, it is a good reminder that we (as a society) should constantly be questioning how ALL people can best be served by education at all levels.

What if College Tenure Dies? - A series of essays in the NY Times opinion section.  Really good, thought-provoking stuff here.

Tenure, RIP -  Closer to my heart (and life), though, was this article in the Chronicle of Higher Ed. about how the percent of tenure-track positions available across the country has shrunk dramatically in the past few decades.  I don't have the time to go fully into what I really think about this situation, but from a personal level, I would just be THRILLED if I could get offered some version of a permanent job.

This press release talking about a research project connecting a poem by Walt Whitman with a meteor shower caught my eye.  The research was cool, but I LOVED the interview with the student research assistant:
...get involved and be as active and connected with whatever department and professors they have. That's been the most beneficial thing to me. Once you start talking to your professors and meeting with your professors and they realize that you're a real human with a brain and you're interested and excited they'll do anything for you, and you'll do anything for them. With that, find someone -- it doesn't matter who it is or what department they're in -- find a professor and make a connection...Also, fear can get you down. That's why I didn't want to be a physics major [initially], because I was afraid I couldn't do it. Don't be afraid of boys, don't be afraid of professors -- once you get over that you can do anything.
The part about "Don't be afraid of boys" was because the question was about advice to give to other female physics majors.  But, I just found her overall attitude throughout the whole interview to be great. She has everything that every faculty member is looking for.

Hanny and the Mystery of the Voorweep - New graphic novel about the discovery of a weird astronomical object.

The Hot Young Teacher they Hired Instead - Along with the Tenure, RIP article, this sort of struck a personal nerve, simply by being about the hiring process in education and the pain that a teacher goes through.

Hot for the Wrong Teachers - This article from Slate is about 2 years old, but it was new to me.  This gets into some of the reforms that have been implemented at one school in NYC.  There is also a discussion of proposed ideas and what is and is not working currently.

## July 20, 2010

### The usefullness of solution sets

I'm teaching a intensive introductory physics course this month. We cover a quarter of intro physics (algebra based) over a 3 week span. I have been handing out problem sets every day, which students work on every other day in recitation-style sessions.  None of the problems are graded, although the students are strongly encouraged to complete all the problems as part of studying for quizzes and exams.

I was a little surprised to learn that the instructor for the previous course (I'm teaching the middle course) provided complete solution sets to the students. It's not something I had planned to do. So on the first day of class, I was asked if there would be solutions provided.  I agreed to try posting solutions to the course Blackboard page.  Of course, this meant writing out solutions in addition to preparing for the next day. Since I had only written down the answers to the first set and needed to prepare the second set, I posted the answers to the first set and the solutions to half of the second set. By today, I was ready to post a full solution set to third set of problems.

I thought I was being generous.

Generally, I would not provide a solution set to a class.  I believe that students develop a false sense of security by relying on having solutions for all their problems.  Because, eventually I will give them an exam. And there will be no solutions for them. It's a bit like someone learning to ride a bike using training wheels.  You don't go from always using training wheels one day to entering a bike race the next day.

But a few of my students gave some clear reasons why they wanted full solutions.  The reasons ranged from "sometimes I don't know how to start the problem" to "if I'm getting the wrong answer, I can use them to check my work" and everything in between.  So, I'm going to try to post as many solutions as possible.  We'll see how it goes.

Popcorn soup!

## June 15, 2010

### End of term clean-up.

Today I'm in my office to clean up and get ready for the summer term course I'll be teaching in July. I've got a stack of papers to go through, clear off my desk and dump in the shred bin. But, I've also got a collection of science news stories which have been cluttering up my browser. In the spirit of clean-up, here's a small link dump:
I think that is all the links I've been saving up. You can always see my shared google reader links, even if you aren't following me.

Back to clearing the desk.

## June 10, 2010

### A tourist in our own town...

These are some ideas for places to visit in and around Chicago this summer when we have visitors and/or free time to go exploring.

If you are coming to visit us, or if you have any opinions on these places, let me know what you think of them.

## May 20, 2010

### Why are there so few women in physics?

I cannot pretend to be able to address the answer to the question of why men overwhelmingly dominate (in terms of numbers) in physics.  It is a complex question with no simple answer.

The question does get raised enough, though, by people who have thought very deeply about it, that I often get to read and ponder what they are saying.  Usually I'm directed to an article via a tweet by Eric Mazur, such as this recent blog post from American Thinker which argues that science academics are at risk of becoming "PC" (politically correct) to the detriment of this country's science and technology pursuits.

You don't have to dig too far to figure out which way the editorial staff of American Thinker leans, but I was interested in the blog post they linked to (and quoted from) on the Washington Post's website written by Christina Hoff Sommers.  In her post, Sommers recapitulates the main thesis of a book she edited for the American Enterprise Institute, The Science of Women in Science, which says that institutional bias is not mainly to blame for the gender imbalance in science.

Sommers was also replying to a criticism of her book from an article in Nature, which she linked to. I don't agree with either "side" since I believe that the answer is too complex to be boiled down to a soundbite or quote in an article or blog post. (This blog included.)

The Nature article discussed one of the essays from Sommers book which cited psychological and behavioral studies of children and the toys that boys and girls seem to prefer.  I wonder what the authors would think of this comic which a friend of mine linked to recently:

It's funny, for sure, and it makes you think.  Anecdotally though, I know of at least one couple who are self described liberals who decided when they had their first child to make sure that their daughter was raised without any gender bias.  And they saw that by the time the daughter was old enough to start choosing her own toys, she went right to the pink section of the toy store.

Of course, as Mazur would say, the plural of anecdote is not data.  As more often than not is the case, I have to let experts who have done rigorous studies guide my thinking.  So while I might not agree with any of the opinions in the articles I linked to, at least there was a lot to read for me to think about.  I would hope that everyone would agree with one thing that Sommers said in her piece:
Scientific preeminence is one of America's greatest national resources. President Obama and NSF officials should be doing all they can to preserve it. That means finding creative and effective ways to encourage gifted students of both sexes to pursue careers in science and technology.
You can read the rest of the article to see if you agree with anything else she said.

Finally, it is not only this country which has an issue with gender imbalance in science.  In an effort to recruit 20 top-notch scientists and engineers for a \$200 million dollar program, Canada found 19 qualified applicants, all male.

## May 19, 2010

### Two wrongs certainly don't make a right...

Today in my astrobiology class, I was talking about Saturn's rings and ringlets and I showed this photo:

This is an image of Saturn's F ring as imaged by the Voyager 1 space probe in 1980. We were discussing the ring structure and I was showing that some parts of the rings were not perfectly circular. This is due to the gravitational effects of the small moons near the rings on the ring particles.

But in talking about the ringlet in this photo, I was also telling the class about the camera technology on the early space probes.  I said that the Voyager camera was actually a film camera. Space probes with film cameras have the capability to process the film on board and then used a scanner to digitize the image for transmission to Earth.  It's a great strategy that was used before digital cameras were regularly put on space probes.

Except, the Voyager missions didn't use film cameras.  Oops.  Guess I'll issue a retraction next class.

But on my train ride home I was reading my current book: That's the Way the Cookie Crumbles: 62 All-New Commentaries on the Fascinating Chemistry of Everyday Life by Joe Schwarcz when I realized I had made a mistake in my OTHER class that I'm teaching.

Yesterday in "Light and Atoms" we were talking about what holds water inside a capped bottle having a hole on the side of it. (Answer: the air pressure of the room holds it in until the cap is loosened.) But the topic of surface tension was raised by one student.  So I turned on the water faucet and splashed the water around in the sink with my hand.  I asked if the water molecules were being held together by chemical bonds and if so, was I producing a chemical reaction to break the bonds as I splashed the water around.  (My answer was no, which it still is, but it was still part of the mistake.)  I then went on to explain what surface tension means and how you can lower it with a tiny bit of soap.

Except that surface tension depends on the weak hydrogen bond between water molecules.  I didn't talk about that.  So, I may have left the class thinking that there is no chemical bonding between water molecules.  Clearly that's not true.  The mistake I made was mixing analogies.

I'm quickly trying to learn more about hydrogen bonding so I can figure out how to correct any confusions I may have introduced.

## May 17, 2010

### Students' study time analyzed

A recent post in the Freakonomics blog came up in conversation at lunch today. The question was whether or not the new schedule our university has adopted (MW and TTh classes only) was leading to students missing more classes on Mondays. (Short answer: we don't know for sure.)

But that question reminded me of the Freakonomics post, which was little more than a link to the original paper. Read the article, it's not hard to digest. If you don't have time to read it all, here's a graph which summarizes much of the results discussed in the paper:

The data couldn't be clearer: students on average are spending less time outside of class studying the material covered in their classes. This is probably one of my largest gripes with interacting with students: how do I convince them that they need to spend more time outside of class with the material so that we can optimize our time with them in class?  I don't know the answer to that question, but it is something I'm always looking for answers to.

What was a bit surprising to me was one of the ideas suggested in the very first comment:
The reduced time inputs between 1961 and 2003 are probably mostly a result of increased productivity through better technologies – i.e. internet research, graphing calculators, word processors etc.
Makes sense, right?  I mean, computers are faster, the internet now puts information on every student's computer almost instantly and no student really even needs to spend the time to walk over to the library for a research paper, right?

All of that is true, but I doubt seriously that students today are just more efficient studiers than students of the past.  No matter what mode I use to present a concept to a class, the students will not really understand it until each one of them has taken the time to engage with the material on their own such that it makes sense in their own head.

And, full disclosure: when I was a student, I probably didn't spend as much time studying outside of class as I should have.  But, the classes I did the most work on were the ones I got the most out of.

## May 08, 2010

### R. wins 329-266

Renae had the bingo today: goutier. Two in a row for her.

## May 06, 2010

### Reflections on reflection.

Today in "Light and Atoms" we looked at the ray model of light. The goal was to be able to understand how the rays from a light source (or object) behave in order to use ray diagrams for curved mirrors and eventually lenses.  We spent quite a bit of time on our discussion and had to rush through the activity at the end.

I handed out plane mirrors, corkboard and pushpins for a law of reflection activity.  Many of the students had to leave before they completely finished the activity, but those who stayed really liked it. That's definitely a keeper activity.

The lab we did today was a lab exploring the formation of images by converging mirrors. I had the students use the lab writeup in their textbook.  I got the sense that while the class completed the lab all right, I would be surprised if most of the students actually understand the reflection off concave mirrors at the level the lab was written at.  One of the TAs even commented that there are some labs which do not work well to facilitate learning and that this was one of them.

I don't have any answers as to how to better teach reflection off of curved mirrors.  I need to refer to some of my favorite sources for inspiration.

## May 03, 2010

### Friendly Metra parking lot.

Not-so-friendly commuter lot

## April 30, 2010

### R. wins 357-303

No bingos tonight, but Renae cleaned up with the Z, J, K, X, S (3 of them) and a blank.

I was at a gathering of physics teachers earlier this week and the teaching of modern physics at the high school level came up a few times during the meeting.  It was really cool to see that there are high school physics teachers who are able to cover topics like the photoelectric effect, Compton scattering, and of course, the meaning of E=mc2.

In conversation with a few teachers the topic of antimatter came up.  I've always been fascinated with the existence of antimatter (thank you, Star Trek) and the means of producing antimatter.

One of the ways of producing antimatter is through a process called pair production.  The pair production process is the transformation of a photon into a positron (the antimatter complement to the electron) and an electron.  The positron has the mass of an electron, but has a positive charge. The production of a positron-electron pair from a photon requires a minimum energy of 1.022 MeV be involved in the reaction.  The mass of the electron and positron are each 0.511 MeV/c2. So if all the of the energy carried by a photon is converted into the mass of the two particles, the required energy is just 2mc2, where m is the mass of the electron (remember the positron has the same mass).  If the energy involved in the pair production process is just at that threshold energy, then the electron and positron would be produced at rest.

Scientists (and especially physicists) love conservation relationships.  The pair production process conserves charge, since the photon is not charged and the electron-positron pair is oppositely charged. Energy is conserved as I was discussing above.  The real issue with pair production is the conservation of momentum.

I don't want to get into the math of the momentum conservation right now. But, if you think about our hypothetic case where the pair are produced at rest, there is a problem which should be obvious: before the production the photon had momentum, but if the pair is produced at rest, then they have no momentum.  This is BAD, since momentum conservation is a fundamental principle of physics.

The teachers and I were standing around talking about pair production and the conservation of momentum when one of the teachers said that he explained to his class that two photons were required to  produces an electron-positron pair.  The collision of two photons with opposing momenta satisfies the momentum conservation law.  Then another teach piped up and said that he had read a paper saying that in face FIVE photons were required.  I tried to explain that I was under the impression that the typical observation of pair production was done in the presence of a heavy nucleus which would account for the momentum conservation.

None of the teachers really seemed to believe my explanation and they probably didn't think that I was going to be swayed by their reasonings, either.  The next day I flipped through some textbooks to see if I could make sense of what we were talking about.   I only found the type of pair production that I was familiar with, and the question of photon-photon (or multi-photon) interactions was never brought up in any of the books I had with me.

So I went to google and did some digging.

The first thing I found was an astrophysics book which discussed photon-photon interaction leading to pair production. The relevant part of the text is on pages 127-8 of that book.  It turns out that photon-photon collisions leading to pair production provide a means of screening high energy gamma rays in some cosmic environments. (Gould & Schreder PRL 1966)

But I had to figure out if the five photon interaction claimed by the other teacher had any basis in reality. What I found was a paper by Burke et. al. in PRL from 1997. (If you aren't reading this from a place where you can see the full text of the article, I apologize.)  I haven't fully digested this article, but the relevant quote says:

"...the multiphoton Breit-Wheeler reaction

becomes accessible for n ≥ 4 laser photons of wavelength 527 nm colliding with a photon of energy 29 GeV."
Whoa!  That's cool!  The point of that article was to show that while in cosmic sources, two photon collisions can produce electron-positron pairs, it has not been observed in a lab.  But, using one high energy photon and four (or more) photons from a laser, the pair production can be done in the lab.

What I learned was that all three of us standing around were right in what we each understood, but that our individual understanding of the topic was incomplete.  I'm so glad that I went and had that discussion with the teachers!

(The prediction of the existence of antimatter was a surprising result of the unification of Einstein's relativity theory with the burgeoning field of quantum mechanics in the earth 20th century.  This prediction was made by Dirac, a brilliant and eccentric theoretical physicist.  There was a book recently published on Dirac which I have not had a chance to read, but I'm linking to below.)

## April 02, 2010

### Roundup of iPad review roundups

So there is a new device from Apple called an iPad.  If you want an opinion on what the iPad could do for you, you could either try to read Apple's marketing material on it, or you could look for reviews by people who have actually tried the device out.

Most of the people who have actually tried the device out and written about it are journalists (or really lucky geeks), but there are some journalists out there who have not had the opportunity to try out the iPad and write a review on it.

The next best thing to actually writing a review is to go out and review other people's reviews.  But, you don't actually critique the reviews.  You make a "roundup" which is a fancy way of saying a list of reviews.

But there are SO MANY roundups of iPad reviews!  Where to start?!  Never fear, I have a list roundup of iPad review roundups for you:

First is Macworld:
With the iPad a few days away, now begins one of the great rituals of the release of a new Apple product: the posting of the reviews from the lucky few journalists who were given a week to play with the product.

Here are the reviews we’ve found so far:
They then go on to quote 8 reviews. The infamous Stephen Fry gets a link, but no quote.

Then there is Buzzfeed:
Howdy, pardners, it's iPad review day! And all the tech cowboys out there are a hootin' and a hollerin'.
They got Fry's quote, but only a total of 5 reviews.  Photos from the reviews does not make up for the low numbers.  Slackers.

The Wall Street Journal's Digits blog has a roundup:
The reviews are in on Apple’s new iPad, and the consensus is no surprise:
I'm not going to ruin the surprise for you.  You can click the link and read their roundup.  It's a little excessive for the WSJ to have a roundup, since they were one of the news outlets to get a review unit.

Zatznotfunny has a roundup:
Yeah, there will be a disproportionate amount of iPad coverage this week. However, we promise not to run a single April Fools Day post. Fair enough?
Zatz not going to hide the fact that you only linked to five reviews as well.  Even a youtube clip at the top won't make up for your laziness.

Looking across the pond, the Telegraph has a roundup:
Here is what the critics had to say about the latest offering from Apple:
What a perfunctorily and typically British way to introduce a round up.  Where's the zazz!?!  No surprise, the Limeys link to Fry's review first.  Again, they could only dig up five reviews to include in their round up.  Lame.

So anyway, there's a bunch more roundups out there, but these are the ones that were at the top of my google search, so they are the only ones I actually skimmed.

## March 22, 2010

### How about some free music?

I was playing around on Amazon last night, and I found these free mp3 album downloads.  You can preview the tracks before deciding to download or not.  There were some other free mp3 albums that I came across which I did not download because I didn't really care for the tunes on preview.  Here's what I downloaded:

This morning I found some more recently released free mp3 albums that I have yet to download and listen to, but here they are:

Happy listening!

## March 19, 2010

### This blog has moved

This blog is now located at http://blog.drewsday.com/.
You will be automatically redirected in 30 seconds, or you may click here.

http://blog.drewsday.com/feeds/posts/default.

## February 09, 2010

### How powerful is a physics professor, anyway?

Over at Dot Physics, Rhett has been explaining the inaccurate calculation of power on a TV show that I have never heard of or had the chance to see. He posted a video of himself pulling the family minivan with the family inside. He even posted a video of his 6-year old pulling the vehicle!

I assume that in a future post, he'll be giving the right way to calculate the power, but I couldn't resist trying out video analysis in Logger Pro with his video.

I analyzed the first 3.5 seconds after the van starts to move by clicking on the same point on the van every few frames. I used the wheel base of the 2007 Mazda 5 as a guess for the scale calibration.

I was pretty surprised that on my first attempt, the position-time data had a nice parabolic shape. I could have probably spent a little more time making sure that I always clicked on the exact same spot on the van, but for quick and dirty analysis, I'm pretty happy. Logger Pro defaults to motion to the left as having negative velocity. I didn't really try to reset this, as I only care about the magnitude of the rate of velocity change (i.e. the acceleration).

The acceleration is the slope of the velocity-time graph, so let's look at that:

Okay, so Rhett either wasn't applying a constant force over the 3.5 seconds, or my measurement uncertainties are starting to show up. Either way, the average acceleration can be estimated by looking at the slope of the best fit line. Logger Pro says that the average acceleration was 0.15 m/s2.

So, using Rhett's free body diagram, we can apply Newton's second law:

The friction force in this case is rolling friction, so we need a coefficient of rolling resistance. The friction force is equal to the coefficient of rolling resistance times the normal force. The normal force (on the level ground) is just the mass of the vehicle multiplied by the acceleration due to gravity.

Rearrange to solve for the applied force:

I googled around a bit and found that a typical auto on concrete has a coefficient of rolling resistance between 0.010 and 0.015, so I ballparked it at 0.012. Rhett gave the mass of the car in pounds, so I converted to kilograms.

Finally:

I didn't carry out the video analysis the whole way through the entire movie. The van moves for about 9 seconds, and I know its acceleration was not constant, since the brakes were applied at the end. However, I'm sure that our physics professor could sustain the constant 300 N applied force if he wanted to all the way through the 5 yards (strange units they use in LA; we'll call it 4.57 meters). At the constant acceleration we measured, it should take him 7.8 seconds to pull the van that far.

So the work done by our professor is equal to the force applied (300 N) multiplied by the distance through which the force acted (4.57 m). His power output during the pull is the work done divided by the time:

300 N * 4.57 m / 7.8 s = 176 W

That's a mighty bright light bulb, but it's just under twice the power radiated (on average) by a human that is just standing around doing nothing.

I know he was pulling at an angle, so his applied force is larger by the cosine of the angle, which I guesstimated to by 15 degrees, so make the applied force 310 N and the power 182 W if you want to be nitpicky about it...

## January 28, 2010

### Two in a series of past travel exploits.

This is another blast from our past travels that I'm posting photos now only because of the Metafilter postcard exchange.

Sometime in 2005, my good friend from grad school invited (or I otherwise wrangled an invitation from him) Renae and me to come to his family's cabin in the French Alps for New Year's Eve that year. We planned an entire Paris/London/Alps trip around his invitation.

We flew from Chicago to Paris, spent about 2 days there, then took a train to the southern French Alps. Our friend picked us up and drove us to his family's cabin up in the mountains! Our first stop was at a small hut where everyone in his family was gathered to bake bread in a wood-fired oven. It was incredible!

The New Year's Eve celebration was outstanding! There was a multi-course meal, with oysters, foie gras, and many other delicious dishes. We started eating at 8:00 pm, and didn't stop until the champagne flowed at midnight. It was so, so, so cool!!!

Everyone in the family was so incredible hospitable and kind to us. We spoke not a lick of French, and they went out of their way to make us feel welcome. It truly was the best part of the trip.

I think we were planning to leave on the 3rd, but because of the holiday and because we had not booked a return train ticket to Paris, we had to leave on the 2nd. It was our fault for not booking a ticket, but we were used to traveling in 1996 on trains where you could always ride on trains as standing-passengers. I guess times had changed. So we stayed overnight at a hotel in Grenoble, then went back to Paris to catch the Eurostar to London.

Somewhere in Paris, Renae got sick (actually, hadn't been 100% well in the Alps) and probably got a bit of food poisoning from a weird fish pizza that she ordered at a cafe. She hadn't slept the night before we were supposed to catch the first train out in the morning, so we were bracing for a crummy ride from Paris to London.

We had bought our Eurostar tickets online a few months ahead of time. When I ordered the tickets, I bought coach seats (\$65 each, I think) for the return from London to Paris. But the Paris to London trip, all the coach seats were sold out. All the First Class tickets were around \$500 or more, which was way more than I wanted to pay. After poking around on their website, I had found some middle class ticket that was something like "Business Economy" or something similar. From all I could tell, it was going to be a coach seat for \$30 extra. Having no other options, though, we bought them.

When we finally got to the train platform and were directed to our seats, it turned out we were in First Class. In fact, the Eurostar only has First Class and coach. An in our car, there was only a family of four sharing the entire car with us. The seats were comfortable; there was a real table for us to use; and the breakfast was simple, but refreshing. We got toast with jam, which was the perfect meal for Renae's stomach.

London was great, but knowing that we'd have to cram a lot into a little bit of time, we chose to do only highlights. We spent a lot of time running around to some of the major sites. We paid for an all-city bus tour that allowed us to hop onto and off of any bus run by a popular bus tour company. We saw the Mousetrap. We shopped at Herrod's. We wandered through one of the residences of one of the Royals. We did not hang around for the changing of the guard.

After a few days in London, it was the Eurostar back to Paris. Coach was like riding in a plane on Southwest Airlines. It wasn't too cramped, but we weren't exactly comfortable. And no one brought us toast.

We had another night in Paris, then flew home.

Somewhere in this post, I should have pointed out that right about the time the plane took off from Chicago to Paris, I looked at Renae and realized that having been married for five and half years, it was the first actual vacation travel we had taken that did not involve a physics conference in any way.

## January 26, 2010

### One in a series of past travel exploits.

 July 15 to 19 New York City
As part of the Metafilter postcard exchange, I decided to send out a bunch of old postcards that I have collected from past travels, but never got the chance to send out.

I wanted to do something extra as a part of the exchange, so I thought I'd post some photos from my trips and include the links on the postcards.

This trip to New York City was in the Summer of 2008. This was the last year for old Yankee Stadium and for Shea Stadium. Sometime during the early Spring or late Winter of that year it was announced that Billy Joel would play a concert at Shea. The concert was billed as "The Last Play at Shea". I'm a huge Billy Joel fan, and had seen him in concert several times before 2008. But, my wife had never seen him in concert, so I asked if she wanted to go. She said she would only go if my brother would go with us. We called him and he said he was totally in. That Saturday, we called for tickets and I was able to get through before they were all sold out.

When we got to NYC we were supposed to meet my brother somewhere in midtown. It turned out that the spot we were supposed to meet at took us right to the Major League Baseball All-Star parade. The photo above is of Ryan Dempster, who was in the game that year. The All-Star game was playing that year at Yankee Stadium.

We did lots of things on that trip, including walking from Central Park to Battery Park. We took pictures of our panda in various spots, including on the Staten Island ferry and in the terminal.

One of the places we wanted to go was to a place called Peanut Butter and Co. They sell unique peanut butter, which my wife had given to me one year for our anniversary. But they also have a restaurant that you can go to and get different gourmet peanut butter sandwiches. At the restaurant, I picked up some postcards, and these are some that I am sending out to the Mefites.

(The link below the photo will take you to a small gallery of photos from this trip.)

## January 17, 2010

Since my last update on the 2-for-1 project in my room I have made some more progress. I've been able to clear off the table that does not have the computer on it. It was nice to be able to do a little grading on the table. I set up my roll of butcher paper on the top of the table and have been using that to take notes on as I'm working at the table.

Here's the latest stuff to leave the room:

• A handful of tools went to the garage

• 2 collector mugs (still in their boxes) went to the basement and will probably go out of the house this summer.

• 2 boxes of random projects and parts went to the basement where I'm putting all my current and future projects until I have time to work on them.

• A craft kit that I haven't had a chance to try out yet went to the basement.

• 2 stacks of papers/magazines/journals were properly filed, a lot of which went out with the weekly trash.

• My shop light for that I use for photography is going to live in the garage even though all my other photography gear lives in my room.

• 3 comic books that I discovered I had duplicates of were sent to a friend.

• Another paperbackswap book (a biography of William Randolph Hearst) was mailed out a week ago.

Also, an update to a few of the other things that I posted about previously: my firewire adapter that I put up for sale on ebay was sold but the data cable for an old phone did not sell on craigslist. I'll try relisting it either on ebay or craigslist again and if it doesn't sell, it will go to Goodwill.

Things that have come into the room:

• Stack of Christmas presents including a farkel game, a small poster, a toy robot and a CD. These have mostly been put away.
• A book I bought at a used bookstore.
• A power strip
• An Ikea tin for storing little things that tend to collect on the desk in the room.
Total items out for the year:

38

Total items in for the year:

13

## January 14, 2010

### Apple didn't like my homage to Nate Dogg

This is a screenshot of the podcast customer review page in the iTunes Store for one of the podcasts I listen to regularly. If you go read the reviews in iTunes right now, you won't see my review. It was removed, probably by Apple, most likely because of the critical comment I made about how someone at Apple did not like my full review of the podcast that I had tried to post before the one in the screenshot was posted.

When you write a podcast review on iTunes, you are presented with five "Tips for writing a great review":

• Keep them short and to the point. Average iTunes customer reviews are about 200 words.
Mine clocks in at 238 words. Granted, it is formatted a little differently and looks longer, but it's no significantly longer than what Apple calls average.
• Praise podcasts on their own terms, not at the expense of other podcasts or the audience of other podcasts or genres.
My review is nothing but praise, and says nothing about other podcasts or audiences of other podcasts.
• Take the time to copy edit your work so that you avoid embarrassing typos or grammatical errors.
• Do not use profanity or overtly sexual language.
No problem here!
• Do not use language that can be construed as hateful, especially in regards to lifestyle, religion, or race.
Nothing in my review has anything to do with religion or race, so that's a non-issue. With respect to lifestyle, I suppose you could make a case about the numerous references to drunks, but if you know anything about the podcast I'm reviewing, you know that the topic of alcohol and drunks comes up frequently in the podcast itself. In fact, it is often in the titles and descriptions of the podcasts as posted to iTunes. And, my review is not "hateful" with regard to any lifestyle, so I don't really see it as going against this tip, either.

At the very bottom after you write your review (not to exceed 6000 characters, mine clocks in at 1200 characters) there is one more line:

If a review is deemed inappropriate, it will not be posted to the iTunes Store.

I did some searching online to see if there were other people who had their reviews rejected from the iTunes Store. The only relevant information I could find came from a FAQ for Podcast Makers on Apple's site:

### Can I have a review removed?

iTunes does not remove reviews for editorial purposes. Reviews with profanity, hate speech, explicit/pornographic content, or commentary that is completely off topic are prohibited. To request removal of a review, click on Report a Concern for that review.

Again, if you know anything about the podcast I was reviewing, you would know that every line in my review was completely ON topic for their show.

My guess is that Apple doesn't want their Customer Reviews to become like the ones you see on Amazon. (e.g. The Story about Ping, Tuscan Whole Milk) Which is a shame. Because if fans of podcasts (or any content on iTunes) can't have fun to promote their favorite artists, everyone sort of loses an opportunity to discover something new.

Anyway, the full podcast review that I wanted to post is here. My apologies to Warren G and Nate Dogg...

[Andy:]
Podcasters
we podcast any speaking of his mind
But you can't be any geek off the street,
gotta be out of your parents basement, if you know what I mean, earn your keep!
PODCASTERRS!!! MOUNT UP!

[Broham:]
It was a clear black night, a clear white moon
Nick D was in the seat, trying to consume
some talk in the eve, chattin' with a drunk
I'm rollin in my ride, beers are in the trunk

[Andy:]
Just hit the Northside on the Kennedy
on a mission tryin' to find Mr. Nicky D.
Got a 'dog with fries and I need to chew
all you drunks know what's up with 312

So I hooks a left on Addison and Clark
The Cubs are out of town so I said "Let's park."
I jumped out the ride, and started to cough
Nick went on a rant and my pants fell off!

[Nick D:]
Since Andy's tweetin' me I'ma check my phone
Laughin' so hard that I start to moan
You can do it right now or at your leisure, bro!

[Andy:]
He's speaking
Nick fans,
Brew and view,
I dare ya.
Drunks
Are in the News

[Nick D:]
Wisconsin's where they're from and the "winner"'s gonna lose

Movies
Music
We brings
Culture
D-Funk
where Chicago is home
and home is Chicago

## January 03, 2010

### In with the new

Now that I've started taking things out of my room, I can start bringing things into my room. One of the projects I've been working on in the garage (for way too long) was the bookshelf in the photo here. This was the first major woodworking project I finished here at our place in Montgomery. Before we left for our Christmas road trip, I had applied a coat of polyurethane and had intended to do a light sanding and apply another coat. But, the temperature in the garage has dropped enough that I don't really think I should work on finishing any more for the winter. So, I brought it inside. It should help with organizing some of the stuff that stays in the room.

I took the cash that I got for the monitor I sold and spent it on a new USB headset for making skype calls, podcasts, and videos for class. I removed a microphone I got at American Science and Surplus to the basement.

Also coming into the room are 4 books I received for Christmas. Even though I have room in my 2-for-1 ratio to bring in the books without anything else going out, I thought it would be good to try to get rid of some books. Tomorrow I am mailing out a book that was requested of me via Paperback Swap. We really like this site for getting books that we'd like to read without paying cover price. You send a book you don't want to someone who does want the book, and in exchange you get a credit that can be spent to request a book you do want to read from another member. In essence, you end up getting books you want for the cost of mailing your books (about \$2.25) to other people. In addition to the book I'm sending out, I found 7 physics journals scattered around the room that are heading to the basement.

Other things removed today: a Rubbermaid tub of physics stuff that belongs in the basement, an old computer plus keyboard and KVM switch, a box of photos, some broken glass from a photo frame
empty cups that I use to keep spare change in, 2 kick-disk hovercrafts (physics demo) and my stereo unit from college. I figured since I am sort of using fuzzy accounting to count the physics journals as books, I will make up for it by counting the computer, keyboard, and switch as one thing. So in total for the year I have:

Total items out:

25

Total items in:

6

## January 02, 2010

### Out with the old...

I've made a bit of progress on the great 2-for-1 exchange program in my room. In the photo are some of the things that have left my room, most permanently. I'm keeping track of the stuff going out and coming in so that I can adhere to my rule. I've been getting rid of the stuff in different ways:

Put away

I decided to put my backpack that normally lives in my room in the hallway closet. The backpack only really gets used for biking, and I don't see any bike rides in the next few months.

My old laptop is essentially dead, but I need to remove the hard drive before I can get rid of it. So it is going to the basement for now.

I'm also putting away (in the basement) some boxes that have been taking up space in the room. One is empty (it had our digital photo frame in it) and the other is half-full of some of my stuff that I moved from Bloomington.

Throw away

This one sort of breaks my heart, a little. I threw away a tiny USB laptop mouse. I think R. got it for me as a gift and I really liked it. But, it eventually wore out and stopped working reliably. I kept it to try to repair it or figure out something to do with it. I wasn't able to repair it and never came up with a great idea for it. I don't like the thought of this ending up in a land fill, but I wouldn't feel right with someone else getting frustrated by it like I did.

Give away

One of the things to leave my room is a first generation DVD-recorder. It works, but it does not always read the DVDs properly when you first put one in the machine. I took it apart and put it back together hoping I had cleaned it up enough to make it work better but no go. I'm going to list this on Freecycle and hope someone can make use of it so that it doesn't end up in the trash. Otherwise, I'll have to find an electronics recycling event to take it to.

Sell away

The other three things that are leaving the room are things that I'm trying to sell. I found out that eBay has a promotion where you can list 5 things every 30 days with no insertion fee. So, I put up for auction a USB/Firewire adapter that I had tried using for a project. We ended up using a different adapter for the project, so I no longer need this one.

I listed on craigslist a monitor that I had purchased at a thrift store. I had fixed up the monitor and used it for about a year before getting my new computer last month and no longer needing the monitor. The monitor sold almost immediately after listing it.

The other item I listed on craigslist is a data cable for my old cellular phone. I used it a few times, but I have a new phone and no longer need it. It's still for sale, make me an offer! ;)

Total items out

8

Total items in

0