Hawk-Eye Ball Tracking

Being a tennis nut, I was obviously glued to the TV whenever I can during major tennis tournament, such as Wimbledon that has been going on the past 2 weeks. The most significant progress in tennis during the past couple of years has been the more rapid introduction of the Hawk-Eye system into various tournaments, including Wimbledon. This is the ball-tracking system that can "replay" where the tennis ball lands on the court in case there is a disputed call.

Now, what is interesting is that it doesn't involve the actual picture of a ball landing on the court. Rather, it is more complicated than that. It takes a series of video frames of the trajectory of the ball and then calculates where it will land on the court. So it isn't a direct picture of where there ball lands, but rather, using mechanics, calculate where it would have landed. Of course, the factors involved in producing such calculation is a bit more complicated since a number of parameters must be considered.

Tests have always been conducted outdoors, encompassing situations that take the following factors into consideration:

* Wind (and therefore camera wobble);
* Bright sunlight at different times of the day;
* Shadows covering part or the majority of the court;
* Dark or overcast conditions;
* Artifical floodlights.

Still, considering how winds can be swirling in different directions, and how many of these tennis balls are hit with a variety of spins, one would tend to wonder if some of the very, very close calls (ball landing 1/4 inch on the line, etc.) are valid. The problem with this is that there isn't any kind of documentation of the degree of accuracy of its call. Where are the "error bars"?

Still, I suppose it is better than the human judgment call, especially on the very close one. At the very least, it stopped the players from arguing with the umpire. Would have been interesting to see how this would have changed how John McEnroe behaved on the courts if it had been around back then. :)

Zz.

Making Sense of the Legendre Transform

I don't mind admitting that, while I was a graduate student, doing the Legendre transform in statistical mechanics was more of a "automatic" response rather than anything I actually understood. All I knew was that it got me from one place to the other, and that's that. Luckily, I don't quite use that piece of information, and that skill, in my everyday work. Unluckily, it means that, while I can still tackle such problems, I don't think I have that good of a grasp of it as I do with other aspects of physics.

That's why I was rather interested in reading this preprint on an effort to clearly introduce the Legendre transform. The authors used the standard, useful examples from classical mechanics and statistical mechanics, so physics majors should be well-familiar with the coverage. I've only looked through it rather quickly, with the intention of reading it more carefully later, but I think this could potentially be highly useful to many, especially if you are still in school learning this subject.

If you have read through this more carefully, and have some comments, please post them here. I'd like to hear them.

Zz.

Physics In Action at Theme Parks

Summer is almost here (at least here as in the northern hemisphere). With that, many theme parks are gearing for their busiest time of the year. Taking advantage of this are several schools and programs that try to marry the fun of theme park rides with physics lessons.

There are two recent examples to this. The first one is the Physics Day for area students at the Six Flags theme park in Largo.

Barnabas Adekanye, Irving Delco, Frailen Ramirez, Ludwin Romero and Johnny Wilks, all sophomores at Northwestern High School in Hyattsville who study engineering, were somewhere in the middle. They had brought a 28-page workbook of problems to solve. It was complicated stuff with a lot of formulas.

"Compare the change in potential energy to the gain in kinetic energy," went one question about the Mind Eraser. "Within experimental error, was energy conserved? Explain your answer."

Johnny, 15, had an easier explanation for what they were learning: "Like how the gravity and force relates with the loops and stuff."

The second example is a trip to Ceder Point by students from Windsor, Canada. I think it is a valid concern to look at it closely and see if the students are learning something, rather than simply using it as an excuse for a trip to a theme park. There has to be a conscious program to clearly demonstrate the physics principle to the students, and that the students actually got something. I think most of these programs try do that.

Zz.

The Physics of Baseball

OK, I've posted many entries related to physics on this blog. Someone might have the impression that I'm a big baseball fan - I'M NOT! :)

Still, this is a rather fun and exciting news report on the physics of baseball. It was a talk and a lot of demonstration given by Paul Doherty, senior scientist at the San Francisco's Exploratorium. It sounds like it was a lively and somewhat "interactive" presentation, even if one could get hit on the head with a foam ball. :)

Zz.

Snowmobiles Faster Than A Speeding Bullet, Maybe, But Not Light

I'm glad this writer spotted something fishy about the accident report.

Mr. ... and his brother ....were traveling across (the) lake around 10:30 p.m. at an apparent high rate of speed when (they) came upon the shore and hit a maple tree ... 'He was overdriving his headlights. The shore came up too fast and he hit a maple tree

The writer certainly questioned, and rightly so, the phrase "overdriving his headlights", because it implied that the snowmobile was moving faster than the headlights! Awful! :)

Still, this accident report is practicing what I wrote earlier about the propensity of news editors and other "official-type" reports of using the phrase "rate of speed", when all they meant was just "speed". If all they meant was that the vehicle was moving fast, then "rate of speed" is the wrong expression, because this is acceleration. An object could have an instantaneous speed of 0 and yet, still have the highest "rate of speed" it will have in its motion (example: oscillating mass on a spring).

More media writers need to have better physics education. Unfortunately, based on this news report, so do people in a Department of Inland Fisheries and Wildlife

Zz.

More Bad Physics - Part 2

Again, this is possibly nitpicking. But really, something this elementary should not be done incorrectly. You have read previously how someone mistaken energy as mass*velocity. We have another one of the same degree.

I think this is nothing more than an advertisement, which makes it worse since it shows that whoever is peddling this doesn't know much about what he/she is talking about. There are two very strange physics in here and they both occur in the same paragraph.

Being Able to slam dunk is almost as much in the mind than it is in the legs but in the end it comes down to one thing – power. In physics power is strength x speed. Strength is the amount of energy that your legs can push up with and speed is how long it takes to push out the strength.

Close, but no cigar.

In physics, Power = Energy/time, or Force*speed. And "speed" isn't "how long it takes to push something", which is really a quantity of time, not speed. It is really the time rate to move something.

Again, this is basic, intro physics that first year college kids, or even high school kids, would have come across. The terms "power", "energy", and "speed" are very well-defined at this level. One simply can't just mix things up as one pleases. So for this person to invoke "in physics...." and then proceeded to mess things up is really astounding.

I don't know about you, but I certainly won't buy whatever it is that is being peddled here. :)

Zz.

The Physics of Sailing

Ah, to sail out in the open waters with the warm breeze in my face .... right before I throw up due to motion sickness... :)

Anyhow, this is an article on the physics of sailing in the Feb. 2008 issue of Physics Today. There's a lot of physics involved here, obviously. But one would think that such things are only being considered when designing a vessel, rather than when one is actually sailing it.

Zz.

Rate Of Speed

Have you ever heard of people using the phrase "rate of speed" before? I have, mainly on TV during one of our local news. Usually it is during a description of some vehicular traffic incident, and some vehicle was described as moving at a "high rate of speed". What they really want to say is simply that the vehicle was moving very fast, but somehow, they think saying "high rate of speed" sounds "sexier".

This, of course, is rather inaccurate. Typically, when say say "rate of something", we usually mean the time rate of change. In calculus, it is d/dt of something, i.e. the time derivative. So when one say "rate of speed", one is actually saying ds/dt, where s is speed. This is ACCELERATION!

Now there's nothing wrong with this if the newscasters actually did intended to say acceleration (which begs the question on why they don't just say "acceleration"?). But more likely, they wanted to say "speed". So really, transposing "speed" into "rate of speed" is not only non-economical in terms of words to say, it is also no longer correct.

So, if you write for some news broadcast, and you want to say that a vehicle moves very fast, just say "high speed" and NOT "high rate of speed". If your producer or proof reader disagree, ask him/her to open a physics textbook.

Zz.

More Problems With MOND

There are more challenges to the Modified Newtonian Dynamics (MOND) theory.

[Note: a few of the links may require free registration to gain full access to the article]

First of course was the observation from the collision in the Bullet cluster from a year ago. The result from this strongly favors the presence of Dark Matter. In fact, many astrophysicists even proclaimed that MOND is dead after this observation. Of course, things don't die off that easily (and that fast) in physics. The MOND advocates came back with a scenario that could be consistent with the Bullet cluster observation without any need for Dark Matter. This was met with major skepticism by others in the field, as can be read at the end of that article in the link.

Now comes another publication that might seriously challenge MOND's analysis of the Bullet cluster.

I. Ferreras et al., "Necessity of Dark Matter in Modified Newtonian Dynamics within Galactic Scales", Phys. Rev. Lett. v.100, p.031302 (2008).

Abstract: To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of ~10^-10 m/s^2. This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory].

This lensing issue was brought up in one of the article that I linked above, so obviously, this is a serious issue with MOND. Let's see if they can dig out of this one.

Zz.

Law Round the Bend, Says Fined Physicist

Y'know, I kept thinking that it's only going to be a matter of time before something like this happens, and it has (at least it is the first time I've seen reported in the news media). A physicist is challenging his speeding fine that was based on a "speed camera".

Dr Fielden, 41, said: "Speed cameras are designed only to work on a straight line. As a physicist, I know that radars, which the cameras use, travel in straight lines. If you set one up on a curve, it is going to be inaccurate."

I wish the news report had more details on this "speed camera", i.e. how it determines the speed of a vehicle. If all it does is detects either "incoming" or "outgoing" motion, then there's something not quite right with this physicist's argument. Let's say it has a radar-type detection that can only detect how fast something is coming or going away from it. Now, if a vehicle is going at 30 mph but going around a bend, the speed detected will ALWAYS be less than 30 mph. One can see this if one looks at a vehicle moving in a circular motion. Only when it is moving in a tangential path towards where the detector is will it register the actual speed. At other locations along the circular path, the speed that the object is either moving towards or away from the detector will always be less than the actual speed.

So if this physicist was caught with a speed of 36 mph while going around in a bend, then if the scenario that I've argued here is correct, it means that he was actually going FASTER than 36 mph, which wouldn't help his case. But like I have said, it depends on how the speed is detected here. Without knowing anything more, I'm just making speculation on how this is done. Does anyone else know how such a "speed camera" work?

Zz.

Sheep Collisions: the Good, the Bad, and the TBI

I seldom try to "advertise" a paper on arXiv that is yet to be published, but I just found this a fascinating read and an amusing one as well.

The authors of this preprint are trying to show how an analysis in Halliday and Resnick on why 2 sheep can survive a violent head-butting without suffering any brain injury is not actually correct. The subject matter is rather amusing (at least to me), but there's some rather good basic mechanics issue here being presented, especially for motion without the assumption of a constant acceleration. So I think any physics undergraduate that have had sufficient calculus can follow this.

Now, anyone willing to read it carefully enough to see if they've done the physics correctly? :)

Zz.

Schrodinger's Kittens Enter The Classical World

This is a rather fascinating angle on the quantum to classical transition. The traditional explanation on the cause of the difference between quantum world and the classical world is the onset of decoherence, where the system interacts with its environment. That interaction with the large degree of freedom causes the emergence of our familiar classical world. We have seen several experiments that showed that the onset of such decoherence gave us back the familiar classical description. In fact, it has been shown that even with just ONE interaction, a single-particle system can quickly lose its quantum coherence.

However, a new theoretical research has taken a different angle. Two physicists in Austria has published a paper[1] showing that the emergence of classical observation can be also be obtained without having any decoherence effect, but rather due to the "coarse-grained" measurement that we make. A review of this work was reported in Nature Daily News (the link may be available for a limited time and may require registration and/or subscription).

Johannes Kofler and Časlav Brukner of the University of Vienna and the Institute of Quantum Optics and Quantum Information, also in Vienna, say that the emergence of the 'classical' laws of physics, deduced by the likes of Galileo and Newton, from quantum rules happens not as objects get bigger, but because of the ways we measure these objects. If we could make every measurement with as much precision as we liked, there would be no classical world at all, they say.

We know that "size" isn't the issue here, especially with the recent SQUID experiments of Delft and Stony Brook. However, the conventional thinking is that the larger the size, the more difficult it is to maintain coherence of all the parts of the system. What the new approach here has tried to explain is that with the larger size, the precision of our measurement also tends to get worse. Unfortunately, their proposal to measure and detect the quantum effects on large system appears to be rather daunting, if not almost-impossible.

Kofler says that we should be able to see this transition between classical and quantum behaviour. The transition would be curious: classical behaviour would be punctuated by occasional quantum jumps, so that, say, the compass needle would mostly rotate smoothly, but sometimes jump instantaneously.

But watching such quantum jumps between life and death for Schrödinger’s cat would require that we be able to measure precisely an impractically large number of quantum states. For a 'cat' containing 1020 quantum particles, say, we would need to be able to tell the difference between 1010 states – too many to be feasible.

Still, I wouldn't put it past some experimentalists coming up with an ingenious way to test this.

Zz.

[1] J. Kofler and C. Brukner, Phys. Rev. Lett. v.99, p.180403 (2007).

The Physics of Shamu

Hey, if you have gone to Seaworld parks, and have seen the Shamu show, this article on the physics of Shamu might interest you.

I need to time my jumps perfectly in order to get the greatest amount of lift. Right before Shamu’s forward and upward momentum peaks, I flex my knees a bit and spring forward. Flying through the air feels free and effortless. Heading toward the water’s surface, my inertia is about to be halted rather abruptly. An object in motion tends to stay in motion, with the same speed and direction, unless acted upon by an unbalanced force. I aspire to a perfect arch before plunging into the water. That isn’t always the case, however. Belly flops, windmills (picture arms flailing) and back slaps are part of any Shamu trainer’s experience. The law of inertia is all too evident as I hit the water. Shamu, it seems, does not experience the same phenomenon.

The laws of physics are very evident in killer whales, to be sure.
Energy. Force. Magnitude.

I just wish that the author didn't write about Newton being "the father of modern physics". That's a bit misleading since "modern physics" is normally associated with quantum physics and special/general relativity.

Zz.

Could the Space Shuttle Return to Earth Slowly and Skip Heat Shields?

Good question! This article attempts to answer that.

You can actively slow your descent, but that needs a rocket to slow you down.

What sort of a rocket do you need to make your speed constant? Well, it would have to do pretty much what the rocket did to get you up into orbit in the first place.

To me, the best way of educating the public of science is to illustrate how it is used in things they see everyday. So questions and answers like this are some of the most effective means of science education.

Zz.

Tacoma Narrows Bridge Is Falling Down

So I'm sure everyone has seen the video footage of the infamous Tacoma Narrows Bridge collapse. It became a case study of engineering design, and also a lesson in my physics classes.

This news article describes the detailed event leading up to that collapse, involving a few people who were present right before the historical event. While there were no human fatalities, the incident did kill Tubby, a pet dog who was stuck in one of the vehicles left on the bridge that day.

Zz.

Terminal Velocity

I wrote a while ago about the paper on how physics is handled in Hollywood blockbusters and the limited lessons that they can give. Strangely enough, in the current online issue of The Answer Man (September 20, 2007) at the famous film critic Roger Ebert's website, there was a rather interesting lesson on terminal velocity between a person and a bullet after both have left an airplane.

Q. It is foolish of me to wonder about the physics of a movie that contains skull-piercing carrots and bullet-propelled merry-go-rounds, but in "Shoot 'em Up" would there be any point to shooting down at Mr. Smith when he is falling from the plane? He should be traveling at terminal velocity and wouldn't the bullets also be going that fast, too? Hence, they couldn't catch him?
Alex Kincade, St. Joseph, Mich.

A. According to Hypertextbook.com, "If an object falls with a larger surface area perpendicular to the direction of motion, it will experience a greater force and a smaller terminal velocity. On the other hand, if the object fell with a smaller surface area perpendicular to the direction of motion, it will experience a smaller force and a greater terminal velocity." A skydiver has a larger surface area than a bullet; also, the skydiver is falling, but the bullet is propelled by an explosive charge.

While certainly the cross-sectional geometry would dictate a amount of drag force exerted on the object, one also cannot ignore the mass of the object here, no? For example, take 2 object of the same shape by different mass. While they would fall at the same rate in vacuum, the one with the large mass has a larger gravitational force, and requires a larger drag force (i.e. at a higher velocity) to cause it to reach a terminal velocity.

But the other aspect of it is also interesting. While it is true that the bullet was propelled out of the gun (i.e. initial velocity is some value) while the person fell out of the plane (initial velocity is approximately zero), would the bullet still reaches the same terminal velocity? I say it would, because if it is moving faster than the terminal velocity, the drag force is larger than the gravitational force on it, and will slow it down until they both are equal. So whether the bullet will catch up to the person or not depends how far that person has started his fall.

In other words, what happened as described in the movie isn't impossible physically. But it isn't necessarily what can happen all the time since a few other facts are involved.

So what do you think? Did I analyze this correctly myself?

Zz.

Fictitious Forces

Here's an explanation of "fictitious forces", given by no less than 2004 Nobel laureate David Politzer.

Zz.