Apollo 13

I’m torn this week. I would normally show 2001: A Space Odyssey at this point in the semester. It’s an absolutely classic film, which has been parodied so many times that even people who haven’t seen it know many of its key scenes. Any real student of physics in film should watch this movie at least once. But this is not to say that most of my students actually enjoy the film; it’s long, it’s slow, and it’s difficult to understand. Since we had to sit through Ad Astra just two weeks ago, I’m going to give you a break this week. Instead of 2001: A Space Odyssey, we’re going to watch Apollo 13. This movie is based on an under-appreciated real episode in the American space program, when the lives of 3 astronauts and the confidence of a nation hung in the balance. The topic is similar to what we would have discussed for 2001: A Space Odyssey, that is, realistic portrayals of space travel in movies.

Assignment: Please describe for your blog readers the correct interpretation of “weightlessness.” Be sure to clarify any common misconceptions using examples and appropriate physics.


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Ch. 4 homework

Problems #2 & 7 of Ch. 4 in Don’t Try this…

Due: Oct. 16

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Reading assignment

Ch. 4 Don’t Try this at Home; Ch. 15 ISMP

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Avengers: Infinity War

Assignment: Although we must be willing to accept some flaunting of the laws of physics to truly enjoy any superhero movie, I still expect you, as students of physics, to be able to spot obvious flaws. For this week’s assignment, I want you to identify 3 scenes in Avengers: Infinity War, where one of the characters blatantly violates one of Newton’s 3 laws of motion. I want you to be specific. State which law is violated and how the character’s action violates it. Let’s not pick on things like psychic, mystic, or magical powers, where you have no way of explaining what actually happened. If you need some help getting started, you could take a look at “The Science of Superheroes” (available in electronic form through the library web page) or “The Physics of Superheroes” (available on reserve in the library). Violations of Newton’s Laws are common calling cards of almost all superheroes, with violations of conservation of mass, energy, and momentum following close behind. Feel free to comment on violations of any of these.

Please post your entry to your blog by Sunday, 6 Oct.

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Ch. 2 homework

Problems #1 & 3 of Ch. 2 in Don’t Try this…

Due: Oct. 7

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Reading assignment

Ch. 2 Don’t Try this at Home; Ch. 5 & 6 ISMP

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To the Stars… Or At Least the Outer Solar System

This week’s feature film was Ad Astra, which, by the way, is a fantastic title for a movie. It’s Latin for “to the stars,” though it is commonly part of a longer phrase, “ad astra per aspera” or “to the stars through hardship.”

Let’s start with the fact that the plot of the movie centers around a mission to answer one of man’s ultimate questions – are we alone in the Universe? That is a question mankind has been pondering in one form or another at least back to the time of ancient Greece, but only in the last century or so have we developed the technological tools to really begin to try to answer it. The movie imagines a “not too distant future” in which that capability has been pushed forward considerably.

The first evidence of this comes in the opening scene of the movie when the main character, Roy McBride (Brad Pitt), climbs out a portal to work on the International Space Tower, a kind of space-station-like platform, but on really, REALLY tall stilts. We’re talking tall enough that the station tower appears to extend at least into Earth’s stratosphere (roughly 30 km above the ground). For reference, the tallest manmade structure in existence today is the Burj Khalifa in Dubai, which is about 830 m tall, less than 3% of the apparent height of the Space Tower. Why anyone would want to build a space tower instead of a space station is never addressed in the movie.

The purported purpose of the Space Tower is to communicate with alien civilizations, should any exist. Why the Space Tower needs to be built so tall isn’t explained, though presumably it’s so that Earth’s atmosphere won’t interfere with the communications. Earth’s atmosphere is opaque to certain types of radiation, such as gamma rays, X-rays, and most ultraviolet radiation, all of which is good for those of us living on the surface of this planet. But that would be bad if alien life forms were trying to communicate with us using these types of radiation. However, NASA and many other space agencies already have telescopes in space monitoring for UV, X-ray, and gamma ray emission, not so much from aliens as from natural sources such as stars, galaxies, and black holes. So, receiving a signal wouldn’t be a problem, but presumably, the Space Tower would give us the capability to answer any messages we receive.

While the Space Tower is there to communicate with extraterrestrials, another mission in the movie, the Lima Project, is designed to actively look for signs of intelligent life beyond our solar system. For reasons that are not very well justified in the movie, this project is stationed out in the outer parts of our solar system, absurdly close to Neptune as it turns out. Why that’s better than somewhere closer to Earth is hard to fathom.

The other thing that this movie, like many others, fails to appreciate is that the vast majority of space exploration missions are unmanned. There are a whole host of reasons for this: people don’t tolerate space very well, people are expensive, people need food and water and have to take breaks from time to time. For monotonous tasks like combing through 300 billion stars looking for evidence of extraterrestrial life, machines just do the job better. And people on Earth can always check what the machines are doing without actually being right next to them. Heck, the Opportunity rover drove around Mars for almost 14 years controlled by commands sent from people back on Earth. Without giving away too much of the plot, the climatic scenes in Ad Astra provide additional reasons why maybe it would be better not to send people on long, lonely missions to the outer solar system.

In a similar vein, the movie never made clear why Roy McBride needed to travel to Mars in order to attempt to contact his father who was on Project Lima out beyond Neptune. One possibility is that the writer assumed that since Mars is further from the Sun than the Earth, then it must be closer to Neptune. This would be true if all the planets were neatly lined up on one side of the Sun, but they generally aren’t. They orbit around, such that the relative distances among the planets depend on where they all are in their orbits. If Earth and Neptune are both on the same side of the Sun with Mars on the opposite side, then, in that instance, it would be faster (and certainly cheaper) to call Neptune directly from Earth.

Another issue with the phone call to Roy’s dear, lost daddy is that it’s going to be a slow conversation. That’s because the light travel time from Mars to Neptune (how long it would take a signal to get there) would be about 4 hours. A reply would take another 4 hours to travel back. If Roy McBride sent his father a message at the start of the workday, then he shouldn’t expect an answer before the end of the day. So, why was everyone standing around like they were waiting for something?

Another common misconception in space movies has to do with how crowded or empty space is. People just don’t seem to appreciate just how BIG space is. Like, REALLY big. Those rings of Neptune? They are probably made up of maybe a trillion particles larger than 1 cm. That sounds like a lot of particles, but they are spread over a volume of roughly a billion billion m3. That means EACH PARTICLE occupies a volume of a million m3, all by itself. Put another way, the average distance between particles (larger than 1 cm) would be about 100 m (a whole football field between each pebble that makes up the rings!). In the scene where Roy McBride needed to fly through the rings, he wouldn’t have needed his shield; he’d have had to be extremely unlucky to hit even one particle.

I’m going to end with a common complaint about so many of the movies we watch in class – why is it that nuclear bombs are the solution to all problems? In the particular case of Ad Astra, Roy McBride uses a kick from a nuclear explosion to propel his ship back home. This is implausible for a number of reasons, but just looking at it from an energetics perspective, a large nuclear warhead from the current U.S. arsenal would have about 1 megaton (4 thousand trillion Joules) of potential energy stored in it. If we are generous and assume ALL of that potential energy gets converted into the kinetic energy of McBride’s ship, then assuming the ship has a mass only three times that of the lunar module the Apollo astronauts rode to the Moon, then McBride’s ship could reach a max speed of 830,000 km/hr, which seems fast, but at that rate, it would take him 215 days or about 7 months to reach Earth – plenty of time to contemplate his (mis)deeds.

In the end, Ad Astra, like so many mass-market movies before it, has played fast and loose with the laws of physics. The sad thing is, it doesn’t have to be that way. There are good movies that also have good science. A short list of examples would include 2001: A Space Odyssey, Contact, Apollo 13, The Martian, and Interstellar.

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OK, there are plenty of things to criticize about this movie from a physics perspective, but at least they get off on the right foot. The opening sequence describes pretty well the event most scientists believe killed off the dinosaurs (they even have the asteroid hitting the Earth in the correct location). It’s also true that something like this will happen again someday. After that, things get pretty sketchy to put it mildly. Of course, there is the obligatory destruction of NY City, even though the likelihood of a few stray meteors hitting one of the most populous cities on Earth are remote to say the least (remember 70% of the Earth’s surface is covered with water, so most likely the meteors are going to hit an ocean). Still, I find this movie entertaining. To me it never appears to take itself too seriously and there are some very humorous scenes. I’ll admit to even getting caught up in the stories of heroism and personal sacrifice. But when it comes right down to it, NASA’s plan to save the world (as portrayed in the movie) is pure bunk; it just wouldn’t work, as we’ll discover in class.

Assignment: Research some of the plans NASA actually has for dealing with future asteroid threats and write a blog post about one of them. Many interesting ideas are currently being or will soon be tested on real asteroids. Your post should be at least a few paragraphs long. Posts should include relevant diagrams, images, figures, and videos (if available).

Please post your response to your blog by Sunday, 22 Sep.

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While I’m gone…

I’m going to be gone on 9/18 and 9/20. Rather than cancel class or have a guest lecturer, I’m going to have you watch two YouTube videos on the topics I would have talked about if I were here.

The first is on the physics of a railgun. Remember, the weapon in the movie Eraser was supposed to be a handheld version of the railgun. While such a thing doesn’t yet exist, the movie was correct that the Navy has been working on developing a railgun for decades and has finally come up with a design that is being deployed on ships. This video explains the basic physics of such guns.


The second video is on the topic of conservation of energy. We’ll do more with this when I get back, but it will be helpful if you’ve covered the basics on your own.


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Reading assignment

pp. 57-82 of Don’t Try This at Home; Ch. 7 of ISMP

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