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Wait a second. Are you saying if I have a cube suspended in space, and I put e.g. a drone inside it that just flies around in a circle inside the cube, the cub

Wait a second. Are you saying if I have a cube suspended in space, and I put e.g. a drone inside it that just flies around in a circle inside the cube, the cube will start to move down? What then? Will the drone crash into the ceiling of the cube because it's independent from the cube itself? My brain disagrees with this, but I'm a hacker, not a physicist.

_whiteCaps_ 7y ago
https://www.newscientist.com/article/dn26807-if-birds-in-a-truck-fly-does-the-truck-get-lighter/ https://www.newscientist.com/article/dn26807-if-birds-in-a-t...
czinck 7y ago
I think the parent poster means a "cube of air" as in a region of air, not something bounded on all 6 sides by glass walls. But to your question, if it was a glass cube, I'm pretty sure the drone would hover in the middle of the cube while gravity pulls it down exactly like if the drone was turned off, based on conservation of momentum. Sure the drone is pushing the air down, but then the air hits the bottom, bounces up to the top, and then gets pushed by the drone again. It's (very literally) a closed system, so nothing happening inside is really noticeable outside, and vice-versa (steady state, if you smack the box the drone will notice).
leetcrew 7y ago
the interior of the cube in your example is a closed system, so ultimately there is no net force in any direction. if the cube is large, the drone might create a slight pressure differential accelerating the cube downwards, but it would eventually be canceled out when the drone hits the top of the cube (I'm assuming zero g, so the drone immediately starts accelerating when it turns on its props). if the cube is sufficiently small, the drone might immediately create too much turbulence to generate sustained thrust in any particular direction.
solarkraft 7y ago
If you just track that cube of air and it's not somehow fixed ... yes, because a downward force is acting on it (with corresponding flow), to keep the drone up. A better analogy one of my favorite kids' shows (I think it was the German Wissen macht Ah) made was putting a heavy boat into a pot of water on a scale - of course the scale reflects the weight of the boat, it has to go somewhere.

_whiteCaps_ 7y ago

https://www.newscientist.com/article/dn26807-if-birds-in-a-truck-fly-does-the-truck-get-lighter/ https://www.newscientist.com/article/dn26807-if-birds-in-a-t...

czinck 7y ago

I think the parent poster means a "cube of air" as in a region of air, not something bounded on all 6 sides by glass walls. But to your question, if it was a glass cube, I'm pretty sure the drone would hover in the middle of the cube while gravity pulls it down exactly like if the drone was turned off, based on conservation of momentum. Sure the drone is pushing the air down, but then the air hits the bottom, bounces up to the top, and then gets pushed by the drone again. It's (very literally) a closed system, so nothing happening inside is really noticeable outside, and vice-versa (steady state, if you smack the box the drone will notice).

leetcrew 7y ago

the interior of the cube in your example is a closed system, so ultimately there is no net force in any direction. if the cube is large, the drone might create a slight pressure differential accelerating the cube downwards, but it would eventually be canceled out when the drone hits the top of the cube (I'm assuming zero g, so the drone immediately starts accelerating when it turns on its props). if the cube is sufficiently small, the drone might immediately create too much turbulence to generate sustained thrust in any particular direction.

solarkraft 7y ago

If you just track that cube of air and it's not somehow fixed ... yes, because a downward force is acting on it (with corresponding flow), to keep the drone up. A better analogy one of my favorite kids' shows (I think it was the German Wissen macht Ah) made was putting a heavy boat into a pot of water on a scale - of course the scale reflects the weight of the boat, it has to go somewhere.