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.
July 20, 2010
June 19, 2010
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:
Back to clearing the desk.
- PhD student in Canada is studying the psychology of earworms. (What's an earworm?)
- Another Canadian psychologist has examined how Rembrandt draws viewers into his paintings.
- An engineering student in England has adapted a bike to pump water in Guatemala.
- NASA's Mars Reconnaissance Orbiter flew over the Phoenix lander and may have imaged the damage to the lander, explaining why it never woke up after the Martian winter.
- Infrared image of nebula from the WISE telescope looks like a pair of hearts situated next to each other in space.
- Want to go to Neptune and come back in 5 years? It will only cost you $4 trillion. (More links at bottom of that page.)
- Possibly the most adorably unfortunate misspelling of hadron ever. Let's hope this kid never loses his interest in science.
- Something hit Jupiter last year, and amateur astronomers saw it first.
- In May, The Big Picture featured a round-up of really great images of Saturn and its moons.
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.
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 28, 2010
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:
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.
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.
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:
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.
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
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
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:
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.)
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 alist roundup of iPad review roundups for you:
First is Macworld:
Then there is Buzzfeed:
The Wall Street Journal's Digits blog has a roundup:
Zatznotfunny has a roundup:
Looking across the pond, the Telegraph has a roundup:
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.
Happy roundup reading!
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
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.They then go on to quote 8 reviews. The infamous Stephen Fry gets a link, but no quote.
Here are the reviews we’ve found so far:
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 ofZatz 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.iPad coverage this week. However, we promise not to run a single April Fools Day post. Fair enough?
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.
Happy roundup reading!
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
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February 12, 2010
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:
 |
  |
 |
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:
 |
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.
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