Favorite little-known physics?

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Filip Larsen

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That's interesting. Please direct me to some literature about accelerating away from something in order to get closer to it.
I will keep this short in order not to go too much off-topic. If you think my reply here is inadequate please ask your question again in a new post.

Even though it is fairly simple to explain I have not been able to find a good online description that is not also too technical for most laypersons. Some of the keywords that are relevant are "impulsive transfer orbit" or, more technical, "Clohessy Wiltshire Hill equations". Regarding textbooks there are really many that explains orbital mechanics. If I had to pick a few then Fehse [1] gives a very detailed description of rendezvous and docking dynamics (and it has has a nice trajectory plot on page 51 that illustrates my claim. Unfortunately, I have not been able to locate an online preview of this book that shows this page). However, anyone new to orbital mechanics would probably not want to start with this. For a more general introduction that also contains a bit on rendezvous I strongly recommend Wiesel [2]. It very well written and touches on a ton of interesting topics with regards to the dynamics of launch, orbital maneuvering and reentry of spacecrafts.

[1] Automated Rendezvous and Docking of Spacecraft by Wigbert Fehse from Cambridge University Press.
[2] Spaceflight Dynamics by William E. Wiesel from McGraw-Hill.
 
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So you’re really talking about rendezvous and docking. In that case you don’t accelerate away from something in order to get closer to it, you accelerate away from something in order not to crash into it (because you are already approaching it).
 

Filip Larsen

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So you’re really talking about rendezvous and docking. In that case you don’t accelerate away from something in order to get closer to it, you accelerate away from something in order not to crash into it (because you are already approaching it).
I think you misunderstand. A spacecraft is in the same (circular) orbit around the Earth as a space station, but trailing, say, 100 km behind it. As long as neither of the two maneuvers (i.e. changes orbit) the distance between them stays the same. The spacecraft then wants to perform a single impulsive maneuver that will bring it on a new (transfer) orbit that goes right by the station. The surprise for most people now is, that the spacecraft actually has to accelerate away from the station in order enter this transfer orbit. Of course, later, at just the right time when it passes the station, the spacecraft has to make another maneuver (opposite in relative direction of the first maneuver) if it wishes to stay close to the station.
 
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The elementary discussion I remember can be referenced in the second (extended) edition of the popular selling physics textbook, Fundamentals of Physics by Resnick and Halliday (c1981). This is a big pinkish textbook.

His example 16.7 tells the story of Sally and Igor. I like this example and I wish it was maintained in the later editions. It mentions to catch up the follower should aim the thrusters (not necessarily accelerate forward) in a forward direction (to speed up by getting closer to the Earth). Later to adjust the Speed to catch the leader, the astronaut should aim the thrusters backward, (to slow down by moving outward). All this is oversimplified and Kepler's equation should be solved for a complete understanding of how the circular orbits become ellipses and vice-versa. But this is the main story.
 
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I forgot to say, that In the book Space by James Michener, the astronauts are warned that this counter-intuitive idea will make rendezvous and docking difficult. Presumably, that is why the astronauts conduct simulations.
 

CWatters

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What's your favorite bit of physics that most laypeople are unaware of? It could be something commonplace that doesn't get the attention it deserves,
The fact that no energy is required to hold something up against gravity. Take the humble fridge magnet. A significant percentage of the population would probably argue that a fridge magnet must use energy and will eventually wear out because in their experience it takes energy to carry shopping or hold a book out at arms length.
 
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This maybe more about math than science.

In the random walk in two dimensions (drunkards walk), it is certain that the "drunk" returns to his/her starting point given sufficient time. However,

In the random walk in three dimensions (I call it the random robin), the "drunk robin" does not necessarily return (i.e. fly) to his/her starting point (maybe the robin should walk after drinking).
 
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I think you misunderstand. A spacecraft is in the same (circular) orbit around the Earth as a space station, but trailing, say, 100 km behind it. As long as neither of the two maneuvers (i.e. changes orbit) the distance between them stays the same. The spacecraft then wants to perform a single impulsive maneuver that will bring it on a new (transfer) orbit that goes right by the station. The surprise for most people now is, that the spacecraft actually has to accelerate away from the station in order enter this transfer orbit. Of course, later, at just the right time when it passes the station, the spacecraft has to make another maneuver (opposite in relative direction of the first maneuver) if it wishes to stay close to the station.
I think I see. However, this only makes sense if the distance between the spacecraft are large enough...such as the 100 km you specified.

Orbital mechanics is weird, because unituitively by decreasing your speed, you enter a new orbit which is actually FASTER than the old one. So, by accelerating away from the space station, the spacecraft ends up falling towards the earth and actually GAINS speed in the process, in a new orbit which is overall smaller AND faster than that of the space station. Therefore, it will start to catch up to the space station...but it will also be going underneath it until it completes a full orbit.

Is this the general idea? All those hours spent playing Kerbal Space Program were not for nothing :biggrin:!
 

Filip Larsen

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I think I see. However, this only makes sense if the distance between the spacecraft are large enough...such as the 100 km you specified.
The situation is the same even if there is only 10 m to the station. However, when short distances are involved, like in the final phases of docking, it becomes increasingly more viable for the spaceship to make a faster approach using pulsed or continuous thrust instead of just two small maneuvers separated by around 45 minutes of coasting. The fuel expenditure will be greater though.

Orbital mechanics is weird, because unituitively by decreasing your speed, you enter a new orbit which is actually FASTER than the old one. So, by accelerating away from the space station, the spacecraft ends up falling towards the earth and actually GAINS speed in the process, in a new orbit which is overall smaller AND faster than that of the space station. Therefore, it will start to catch up to the space station...but it will also be going underneath it until it completes a full orbit.
That is correct.

Is this the general idea? All those hours spent playing Kerbal Space Program were not for nothing :biggrin:!
One can hope that with a new generation growing up with KSP the old days where people believe Star Wars physics is correct will be gone (or at least put somewhat in doubt) :rolleyes:
 

CWatters

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Any day now someone is going to publish a long list of all the errors in the film Gravity :-)
 
These are all great. I had never even heard of some of them before.

Another simple one that I think most people never notice is how you can see the difference between phase velocity and group velocity in water ripples. Throw a rock in a lake and you can see that the waves that you watch cross the lake are really made up of smaller ripples that travel from the back of the wave to the front at twice the speed of the overall wave.

It was neat because I read about it a few times before I really had the chance to try it out and almost didn't believe it until I could see it for myself. It's a cool connection between sort of abstract math to a nice bit of physics that you can see with your eyes.

As a side note, Frank Crawford's book on waves from the Berkeley Physics Series is just about my favorite introductory text on anything ever.
 
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No one has contributed for about a month so I thought I might kick things off. Take a book with a tight rubber band around it so that the pages and cover does not flop open(for example). A rigid rectangular block is better. The Block should have three unequal moments of inertia. (For a homogeneous block, you want length, not equal width, not equal thickness). Throw the block in the air gently at the same time rotating the block around the short axis. The book rotates predictably. Now do the same rotating about the longest axis. Again, the book rotates predictably. Finally repeat, rotating about the intermediate (not long, nor short) axis. This time, the block flops around in the air.

This is called the intermediate axis instability in rigid body force-free motion. This is treated in (e.g. Goldstein Classical Mechanics). This is so puzzling I have seen graduate students try this with Goldstein throwing it in the air when they got to this part.

Actually all three dimensional (space) rigid body motion (e.g, the general motion for the heavy symmetric top), as well as force free motion has puzzling aspects.
 

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