Tuesday, October 31, 2017

Trick or treat? See for yourself

It's been a little quiet around here for a variety of reasons. So let's shake off some dust for ... Halloween? Why not?

Four drops of food coloring are added to the center of a half-filled "skinny fish tank" (a.k.a., Arbor Scientific Laser Viewing Tank) resting on a low-friction turntable (Pasco's Rotating Chair platform).

The tank is then given a spin. And, well..?

It made me say those magic words all people in science prize: "That's funny!" I didn't know what to expect, so in some ways, this wasn't overwhelmingly surprising. Still though...

I get the parabolic surface. But what's going on in the dye? I'm sure there's a lovely, simple explanation that I should know. But I don't.

If you do, kindly leave it in a comment. I am your student.

Tuesday, October 03, 2017

Don't Ask Smithsonian

My in-laws gave me a subscription to the Smithsonian magazine a few years back. I don't read it as regularly as my Sky and Telescope and Aviation Week subscriptions but I do get around to reading most of them. I enjoy history, especially the small forgotten details that have unforeseen important impacts. The Smithsonian is full of such stories. The most recent issue I read was September 2017. I read about Chick Parsons, a pivotal figure in the guerilla war in the Philippines during WW II, speculation that a disagreement over smallpox inoculation estranged Ben and Deborah Franklin, and Benjamin Lay, one of the earliest Quaker abolitionists. There usually is a science or technology article. This issue had an excerpt from Scott Kelly's new book about spending almost a year in space.

At the end of each issue is the "Ask Smithsonian" page. Readers send in questions that are answered by a bullpen of Smithsonian-connected scientists. It is hard to believe that a column like this still exists in this age of instant Internet searching, but it is entertaining to read. Perhaps the people who are most likely to send in a question to a magazine are the least likely to know how to do a key word search. In the September issue the first question was "In how many ways can snake venom kill humans?" Hopefully this was not a time sensitive query. The reply by Matt Evans, an assistant curator at the National Zoo is thorough and encourages further research with the brief mention of "complex venoms". At the end of each column is an invitation to submit queries to Smithsonian.com/ask. 

The first exhibit in support of the harsh title of this post is a question in the November 2015 Smithsonian from Stan Pearson, Newport News, Virginia. Stan asks, "Why do astronauts aboard the International Space Station seem to float? The ISS is only about 200 miles above Earth—where, according to Newton, gravity is almost as strong as it is here on the ground." Stan's question was answered by Valerie Neal, curator of space history at the National Air and Space Museum. Dr. Neal starts off well when she replies, "They experience weightlessness not because of a lack of gravity but because the ISS, and they, are orbiting Earth in constant free fall." 

Invoking free fall to explain the sensation of weightlessness is helpful. It connects the questioner to experiences they have had like jumping off a diving board or jumping on a trampoline. There is no difference between those experiences and orbiting except that your path intersects the Earth's surface while that of the space station does not. I prefer to use the expression "apparent weightlessness" because I define weight to be the force of gravity on an object. Not everyone uses this definition. Some define weight to be whatever a scale reads. Therefore, I won't list this as a mistake. 

Dr. Neal continues, "They’re falling toward Earth and moving forward at about the same velocity." 

It can be useful to consider an orbiting object to be falling toward Earth but this statement is just plain wrong. In a quarter orbit it moves "below" its initial position by a distance equal to the radius of the orbit. In the same time it moves a quarter of the orbit's circumference. This distance is pi/2 greater so the velocities are not the same. Since the directions are different, the term speed should have been used, but making something wrong less wrong is of questionable utility. 

Dr. Neal then adds, "Because the downward and forward forces are nearly equal, the astronauts are not pulled in any specific direction, so they float." 

There are no forward forces in a circular orbit. Neglecting drag, there is only the force of gravity and it is acting down. An elliptical orbit would have a component of the force of gravity tangent to the orbit. Since this question is about the ISS and its circular orbit, that is not applicable nor would this help Dr. Neal. This statement is nonsense and makes me wonder where she learned about gravity and orbits. 

My next exhibit for why asking Smithsonian can be problematical is from the March 2017 issue.  This one can't currently be found online but I have the print version. Joseph A. Leist from Hamilton, New Jersey asks "Why doesn't Saturn's gravity pull its rings crashing down to its surface?" Matthew Holman, senior astrophysicist at the Harvard-Smithsonian Center for Astrophysics answers, "Saturn's rings are composed of billions of particles of rock and ice from broken-up comets and asteroids that are orbiting the planet like so many tiny moons. Those particles, orbiting at speeds of 20,000 to 40,000 mph, sometimes collide with one another, but they don't come crashing down onto Saturn's surface because the centripetal acceleration of their orbits balances out the planet's gravitational pull." 

He muddles the issue with the mention of collisions. It is certainly possible that a collision would send a particle crashing onto the surface but since nearby particles have similar velocities, the collisions are not violent enough to result in a large enough change in velocity. Dr. Holman veers away from the laws of physics when he tries to explain why the particles stay in their orbit regardless of any collisions. Acceleration and force are related but they are different quantities. They cannot ever be equal. It also is puzzling how they would balance each other out when they are in the same direction. Centripetal means toward the center and the force of gravity from Saturn is toward the center as well.

One explanation for why objects stay in orbit is that the centrifugal force is balanced by the force of gravity. This is a very common explanation because it is brief. However, for people that know little about physics it is devoid of information. This explanation is from the accelerating frame of reference of the orbiting object. If you were in this frame of reference you might believe there is no acceleration and conclude that the sum of the forces on you must be zero. Since you know there is gravity pulling down, you would postulate an opposing force in the outward, or centrifugal direction that balances gravity. There is nothing wrong with this point of view but analyzing motion from an accelerating frame of reference is not a trivial exercise. It is best to first understand orbital motion from a non-accelerating frame of reference. The ring particles are accelerating because the direction of their velocity is changing. Their orbits are stable because they have just the right velocity and distance from Saturn that results in an acceleration that is equal to the force of gravity from Saturn divided by the particle's mass. Put a particle at this velocity closer to Saturn and it would start to move toward the surface because the force would be more than what is needed to cause the centripetal acceleration.

Dr. Holman was given a second chance to answer this question when readers responded to his original answer in the May 2017 issue. Their response was summarized by the Smithsonian as "Some readers thought the March “Ask Smithsonian” should have stated that the acceleration of Saturn’s rings was “centrifugal,” not “centripetal,” as printed." He boots this opportunity to correct himself by responding, "Perhaps I should not have written ‘balances out,’ because it is easy to misinterpret. But the force is indeed centripetal. An object in circular motion with a constant speed v and a radius r about the center it is orbiting has an acceleration a=v^2/r that is always directed toward that center. That acceleration is centripetal.”

It appears he didn't bother to read his initial answer. He didn't mention centripetal force in his original response. Why would he say "the force is indeed centripetal"? He appears to cite a quickly searched definition of centripetal acceleration and once again confuses force and acceleration. If you put the entirety of both responses together, his answer is meaningless.

None of this is surprising to high school physics teachers. We are used to scientific publications and websites getting things wrong, especially on the subject of orbits. Sometimes this is because the person does not understand orbits. However, the author is often someone who does know better and the frequency of these erroneous explanations is too high to explain it away to incompetence. I think it is due more to laziness. It is far easier to say the centrifugal force is balanced by the gravitational force than to explain it from a non-accelerating frame of reference. When writing an article about the space station it is easier to use the terms zero gravity and weightlessness than to explain how astronauts are actually in free fall. This is understandable for an author that is frequently writing about such topics. The flow of the article will be affected if they have to explain everything from basic principles every time. Perhaps that is what motivated Phil Plait, the Bad Astronomer, to write a short article about his views on centripetal Vs centrifugal force.

Most physics teachers would disagree with Dr. Plait's position that there is no difference between forces and fictitious forces. The latter arise in accelerating frames of reference and can be useful for analyzing motion, especially circular motion. But they are not real forces. Here is a counterpoint example using linear motion. I am sitting in an airplane as it starts to accelerate down the runway. I might explain the sensation the following way. I think I am not accelerating because I can look around the plane and see that the other passengers are not moving relative to me. Since I feel the seat back pushing forward on me, there must be some other force pushing on me in the other direction to balance this force, resulting in my zero acceleration. I will call this force the "ventilabis retro" force and say it is as real as any force I have ever encountered. This is exactly what Phil Plait writes to make the case that the fictitious outward force on a passenger in a car when taking a turn is real. The key step is to mistakenly conclude there is no acceleration. Thinking there is no acceleration in a turn is more common because many people do not realize that acceleration can result from changing direction, not just changing speed. If the car passenger knows this, they think the car door is pushing in on them causing an acceleration, just like any airplane passenger knows there is only the seat back pushing forward on them, causing an acceleration during takeoff.

I understand why Ask Smithsonian gets things wrong when answering questions like these. They feel compelled to ask an expert with impressive credentials to answer the questions. This often works but not when it comes to explaining something from introductory physics. An astrophysicist from the Harvard-Smithsonian Center for Astrophysics is far removed from his introductory high school physics class. They are unlikely to have ever been asked to explain basic physics to a lay person. They speak from authority and are uncritical of what they write because they believe they will not be challenged. I don't fault them for this. I fault the Smithsonian and their tendency to put credentials above experience. It would be OK to ask Smithsonian if they would ask someone more qualified to answer their physics questions, a high school physics teacher.

If you are a high school physics teacher, I suggest you give one of the questions to your students. Select the best 3-4 answers and mix them up with the response from the expert in Ask Smithsonian. Hand them out and have your students try and pick which response was from the space history curator or the astrophysicist. The results will be surprising, or perhaps not.