# The characteristics and effects of high speed rotating domes

I'm currently designing a world for a story that I'm writing that is based on a system of magic that deals with barriers, specifically domes. To get to the point one key characteristic is that these dome barriers tend to rotate at very high speeds. I'm not too sure what this means in terms of how they behave with relation to the sound barrier. For the sake of simplicity the surfaces of these domes are smooth, and have a surface which feels like thick glass. They are for the most part shaped like perfect spheres. In some cases half of the body is above ground while the other is below (and in some cases they are nothing more than half spheres, but still perfectly spherical in geometry. There are three scenarios that I would like clarification on:

1) If one of these barriers were to spin faster than the speed of sound, how would they behave? Would there be a sonic boom, a specific sound, any unique behaviors of the dome at this speed, etc? I'd like to add that structurally these barriers are completely sound in this regard, there need not be any consideration given to the barrier "failing" due to any physical factors involving its rotation.

2) If an object were stuck to the surface of one of these barriers as it spun (horizontally of course) would this impact the behavior once it reaches supersonic speeds? What would likely happen to a human stuck to the surface in this scenario? 2.1) If this rotation were to occur vertically (meaning the barrier is a full sphere with half above ground and half under) would any differences arise in the characteristics assuming that the terrain it is in is capable of being moved (i.e. dirt, sand, etc.)?

3) If one were to inadvertently walk into a barrier spinning at such speeds, what would happen to them? Would this even be possible, and if not what would be a reasonable explanation to try and explain this? Also in the structural sense would a barrier rotating at such speeds be more or less likely to break if something with ordinarily sufficient force struck it?

I know this is a decent slew of questions, but I would appreciate any help and clarification on this subject, thanks you!

• I submitted an edit to change "the the point" to "to the point" and the second above to below, as that seems to have been your intent based on context. Commented Sep 20, 2019 at 3:18
• Per your statement, there are too many questions in a single post, while we have a strict one question per post policy.
– L.Dutch
Commented Sep 20, 2019 at 5:09
• @L.Dutch unless they all apply intrinsically to the same subject, which in this case, they do (the domes). The OP describes the domes and asks for the effects of sonic barrier, adhered objects, and contact. I have seen you answer much broader multi-part questions. This question is pretty straightforward and all interrelated. If the OP had divided it into parts, you would have labeled it a duplicate. Commented Sep 20, 2019 at 5:14

1) they would not break the sound barrier. A sonic boom is generated when the object moves through air and builds up a critical "bubble" of air pressure in front of it. As your spheres are rotating in place, this would not occur.

2) it depends on the mass of the object vs that of the dome (and the force propelling it to spin). As I'm envisioning city or castle-sized domes, a small object like a person would likely have negligible effect. The person (or object) would be subjected to incredible centrifugal force, making the blood flow to outer extremities and subjecting them to extreme g-force (exact amount depends on radius of your spheres and their speed). If the sticking force wasn't extremely strong, they would fly off like a bullet.

3) Pink mist. Most bullets travel at much less than the speed of sound and have virtually no mass compared to the domes you describe. You did say they were smooth like glass, but unless the surface was completely frictionless, energy transfer would occur and POOF the mist formerly known as Sir Reggie the Blunderer.

3.5) The domes' motion/inertia would lend it much greater structural integrity. Consider that typhoons/hurricanes/tornados frequently drive intact straws (yes, the yellow crinkly dry grass you coat a barn floor with) through solid concrete without destroying or even wrinkling the straw. Those straws are not even approaching the speed of sound. Now, if a blow DID have enough force to overcome the inertial protection of the sphere/dome . . . the resulting explosion of energy release would be catastrophic.

4) (you didn't ask but . . .) Consider making at least the inner surface of your domes (especially the spheres) frictionless. Otherwise, the above mentioned energy transfer will cause the domes to scoop up the puter layer of soil/ground and carry it up the sides with centrifugal force until it is all spun out of the center.

• In terms of no sonic booms being produced would there be any other behaviors that would exist? I initially thought that a perfectly spherical surface rotating at very high speeds would do nothing besides appear to be sitting still, but I'm unsure if anything from a slight hum to a gentle wind, or whatever else, would take place. In terms of size you've hit the nail right on the head for the most part, but what would the effects be on a smaller scale? Something along the lines of a bubble intended to fit one person, and in some cases several? Commented Sep 20, 2019 at 3:44
• @Bacom15 this again depends on the surface friction of your domes. Anything short of frictionless is going to generate wind around it and thus noise. If they are spinning at the speed of sound, the air will initially be propelled at that speed, but air in air decelerates and dissipates enegry quickly, but you're still looking at high storm winds (or extreme low friction may generate lesser winds). Again, if your domes have any friction at all, their grinding against the soil is going to generate a constant sliding noise that will probably resonate into a rumble/whirring. Commented Sep 20, 2019 at 3:50
• Also, if your domes ARE frictionless, then someone walking into/touching one would not even be able to tell it was spinning. No pink mist . . . unless they struck it hard enough to force a momentary contact with the surface. Then, mist (if they didn't hit it hard enough to beak the dome) or kaboom (if they did). Commented Sep 20, 2019 at 3:53
• @HAHarvey I thought most bullets travel faster than the speed of sound, And the Straw through concrete was simply a myth. Commented Sep 20, 2019 at 5:34
• @HAHarvey: My basis is that once upon a time I actually took a course on the strength of materials. Please provide a reliable citation. Commented Sep 20, 2019 at 6:34
1. I believe it would create a vortex of some sorts around your city. As the Dome spins, it will drag the air in contact with the surface with it, accelerating it close to the speed of sound. I say close because chances are you will have Air particles hitting the dome, bouncing off faster than the speed of sound, colliding with subsonic particles and creating a very turbulent flow around the done itself. This air would likely shoot outwards due to the spinning motion which will pull colder air in from above due to the semi-vacuum state it will create. Basically I believe it will create a semi permanent Tornado around the dome.

2. Objects stuck to the surface of this Dome will likely be thrown off or torn apart. Anything stick on the dome will be flung outwards due to the circular motion, Battered by the winds and somehow still held down to the dome. Travelling at roughly 1200KM/H would tear apart most bodies, even if it was made of steel. The object will likely travel slightly faster than the speed of sound, if the surface of the dome is travelling at the speed of sound. And the resulting sonic boom will likely aid in the destruction of anything stuck to the surface.

3. If you were to walk into the dome, it would be a miracle at first due to the raging wind currents this dome would produce. Even then, with even the slightest amount of friction on the domes surface, it would literally be like putting your hand on a sanding machine but worse. The surface would chew through anything pushing into it, but also throw it outwards at incredible speeds. I believe if you were to touch this, it would fling your arm outwards, causing you to fall into the dome, lose half your face and be launched into the air, landing several 10's or 100's of meters away.

Of course... this is all just guess work.