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Almost all of the non-star things we can see from Earth are gravitationally bound to our Sun. The other planets, the asteroids and comets, moons, etc are all part of our solar system, which means they are bound to the Sun. Most of the planets have nearly circular orbits, so they have very repeatable, normal patterns, while some of the comets and asteroids have highly elliptical orbits (spend a little bit of time close to the Sun moving fast, but spend most of their time far away from the Sun, moving slow). It's these comets with highly elliptical orbits that have these odd patterns you're mentioning. Probably the most famous comet, Halley's Comet has a very high eccentricity (of 0.96. 1 is the max eccentricity, Earth's is 0.016), meaning it can be up to 35 AU (1 AU is the average distance from the Sun to the Earth), and down to 0.5 AU.

What does it mean to be gravitionally bound? One way of thinking about it is that the total energy of the system (system being object orbiting and the object it's orbiting) is negative. How is it negative? Traditionally we consider gravitational potential energy to be negative- and it gets more negative the closer you get to it. Kinetic energy is positive, increasing with speed. So, if the sum of the kinetic energy + potential energy is negative, then the object is "gravitionally bound" to the object it's orbiting. This is how you calculate escape velocity, and another way of saying it is that the comets are traveling at less than the escape velocity of the Sun.

Most “microbot” innovations require magnetic fields to run them. How far are we from truly autonomous microscopic robots?

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What are the important characteristics of the bacterial component to make this system work? Anything beyond motility and engineerability?

Is the surface display of biotin attachment peptides limited? Do you think you could see significant gains by using other bacteria, like clinically derived probiotics that are better adapted for nonpathogenic colonization? I'm thinking Lactobacillus here.

This is incorrect and neglects the entire fields of colloids, non-Newtonian fluids, rheology, and possibly others.

Jelly is a perfect example of a substance that is both fluid AND solid and is regarded as a colloid possessing the thixotropy property.

how much we need these "microalgae and bacteria-based microrobots" in our body to defeat cancer? Can they reproduce? And how will you manage their reproduction?

What's to stop Putin from using them for assassinations?

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What are the complications with those baceteriabots attacking good cells/tissue?

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A good (reasonably not bad) example of the rheology of the mantle is Silly Putty.

If you put a ball of it on a desk and hit it with a hammer, it shatters. A good analogy for earthquakes (brittle response)

If you just leave it on the desk it will become nice flat putty pancake (ductile response)

Two different responses to two different stress/strain regimes. Particularly effective in class

There's no aerodynamic reason they shouldn't be able to do that. Fighter pilots and aerobatic pilots have been performing maneuvers that involve deliberately stalling the wing since the very early days of flight. But perhaps a description of a very simple form of stalled-wing maneuvering will help you visualize how it works.

The first thing we have to do is correct the commonly held misunderstanding that a stalled with "loses lift." That's a very poor way to describe what happens because a stalled wing still produces lift proportional to the square of airspeed, it just does it with more drag and at a higher AOA. If you doubt that, consider that a paper airplane wing is essentially stalled all the time. A better way to think of it is that stalling the wing results in a sudden shift to a lower lift to drag ratio. The wing can still produce 1 g of lift (or however much lift you want), but at much higher drag than the same wing when it is not stalled, and at higher airspeed.

Probably the easiest stalled-wing maneuver to understand is the technique used by bush pilots to minimize damage in an off-field forced landing. The pilot flies the airplane into a stall and holds it there, with aft stick, while maintaining stability with the rudder (not with the ailerons). This results in a glide with a much steeper than normal descent angle but with a low vertical speed, because the airspeed is low. (Glide angle is inversely proportional to L/D.) The pilot can then fly the airplane to a smaller clear area on the ground than they could hit with a normal glide, because of the steeper glide path. Damage on impact is minimal because of the low airspeed and low vertical speed. In fact, with a good STOL airplane, very short landings can be made on a normal runway with no damage at all. I have done this may times in a Druine Turbi, stopping in well under 200 ft from the runway threshold.

The trick with a fighter jet is to have enough control authority to usefully maneuver the jet in yaw and roll with the wing stalled without having surfaces so large that they compromise un-stalled performance. It's also helpful to have loads of low speed thrust so you don't lose more energy during that maneuver than necessary.

What’s your prediction if large amounts of these nanobots end up in waste water and the impacts of this on the environment ?

And knowing that cats were the last animal to be domesticated it makes sense we wouldn't have developed as much resistance against them, imho.

Thanks for clearly stating what I was trying to express somewhat sloppily. This is largely why in discussions of rheology (for rocks at least), talking about them as either "solid" or "fluid" is uncommon and instead you tend to see them described just as "materials", i.e., when texts introduce useful analogues for thinking about the stress-strain or stress-strain rate relationship (i.e., the various combinations of a sliding frictional block, spring, or dashpot that would produce some sort of equivalent stress-strain or stress-strain rate response) they tend to do so in terms of just materials, e.g., "Maxwell materials" or "Voigt materials" etc. Not all geology texts are good about this though.

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Do you believe that biological nanobots will be more useful for medical purposes than hypothetical artificial nanobots?

How do you use magnets on E.coli? Did you use the sensory systems of magnetotactic bacteria?

This is not really the definition of fluid as there actually is no strict boundary between what is and is not a solid. Indeed as others have noted but incorrectly commented on, things like the mantle, pitch, and jelly are examples of substances that have a dual nature in both being solid AND liquid.

 

To quote George Batchelor (taken from An Introduction to fluid dynamics), "The distinction between solids and fluids is not a sharp one, since there are many materials which in some respects behave like a solid and in other respects like a fluid." ... "But, even supposing that these two definitions could be made quite precise, it is known that some materials do genuinely have a dual character.".

 

What this really means, and what fluid dynamicists recognise, is trying to constrain a substance/object into being a solid or a fluid has more to do with humans desire to define things in discrete buckets and less about the actual physical world.

One way to think about the fact that “the mantle is a solid but it flows” is to think about ice. Ice is a solid, just look at an ice cube, and yet a kilometer-thick glacier flows.

Like ice, the mantle isn’t perfectly solid. But if you compare it’s viscosity to that of a glacier, you find that the mantle is roughly a million times more solid than ice.

https://www.nature.com/articles/s43247-022-00385-x/figures/2

(Ice is also a non-Newtonian fluid, so a precise comparison isn’t possible.)

Hi hocam! My question is how microrobots behave after colonized? (idk right term) Do the bacteria colony behave different than u programmed for? I mean is it emergent or it is behaving similar to computer program, it behaves exactly what you programmed to? Congrats hocam!

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Liquids (and solids) are much less compressible than gases, but they are still compressible. Constant volume (incompressibikity) is just (sometimes) a useful simplifying assumption. (In other contexts like sound/seismic wave speed, it would be, well, complicating to say the least, given that would result in an infinite wave speed.)

If you have a tall enough, a column of metal, or even rock, it will deform under the pressure from its own weight. A penny is just very small and light, and deformation is negligible. Like a solid, a liquid will also not deform without without some force being applied, but the type of deformation is different.