- #1
sorax123
- 34
- 0
I was thinking about Walter Lewin's thinking question: "If an astronaut were to try to drink from a sphere of floating juice through a straw in a capsule pressurised at 1 atm, could he drink effectively?"
The answer to this is yes because of the atmospheric pressure. But it got me thinking about the hydrostatic pressure in this situation and also the atmospheric pressure of the air in the capsule.
If this satellite is in free fall (assuming it's orbit is circular, then everything is accelerating uniformly ie. the ball of juice, the air "above" it, the astronaut etc.). Now if we consider the ball of juice, would there be any hydrostatic pressure difference in the middle of the ball compared to the edges (atmospheric pressure)? I would argue not as the hydrostatic pressure of ro g h is due to the weight of the above bearing down on the considered point, but in this case, in freefall, it would seem the weight would not be "felt" as such by the fluid and so the hydrostatic pressure would not be applicable. This could be likened to dropping a 2 litre cola bottle with a few small holes and observing whether the water would stay inside when allowing to free fall, and, having tried it from my roof this afternoon it seems to be true. So this is all very well, if slightly strange, but the weirdest thought comes when considering the atmospheric pressure of the air inside the capsule, and this is where i would appreciate some thoughts to help out here. If the air is also accelerating in freefall, just as the juice which exhibits no hydrostatic pressure is, then how is it that this air can remain at 1 atm?? I feel as though i must be missing something here, but based on the previous reasoning, the atmospheric pressure should be 0, but that surely is wrong. Any thoughts at all from you folks on any of this would be appreciated- very interesting stuff in my opinion and even better if i can get the final puzzle piece in place.
Regards
Dom
The answer to this is yes because of the atmospheric pressure. But it got me thinking about the hydrostatic pressure in this situation and also the atmospheric pressure of the air in the capsule.
If this satellite is in free fall (assuming it's orbit is circular, then everything is accelerating uniformly ie. the ball of juice, the air "above" it, the astronaut etc.). Now if we consider the ball of juice, would there be any hydrostatic pressure difference in the middle of the ball compared to the edges (atmospheric pressure)? I would argue not as the hydrostatic pressure of ro g h is due to the weight of the above bearing down on the considered point, but in this case, in freefall, it would seem the weight would not be "felt" as such by the fluid and so the hydrostatic pressure would not be applicable. This could be likened to dropping a 2 litre cola bottle with a few small holes and observing whether the water would stay inside when allowing to free fall, and, having tried it from my roof this afternoon it seems to be true. So this is all very well, if slightly strange, but the weirdest thought comes when considering the atmospheric pressure of the air inside the capsule, and this is where i would appreciate some thoughts to help out here. If the air is also accelerating in freefall, just as the juice which exhibits no hydrostatic pressure is, then how is it that this air can remain at 1 atm?? I feel as though i must be missing something here, but based on the previous reasoning, the atmospheric pressure should be 0, but that surely is wrong. Any thoughts at all from you folks on any of this would be appreciated- very interesting stuff in my opinion and even better if i can get the final puzzle piece in place.
Regards
Dom