Timeline for Will a ball thrown straight up in a train land in same spot (in real world)?
Current License: CC BY-SA 3.0
9 events
| when toggle format | what | by | license | comment | |
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| Sep 29, 2011 at 17:55 | vote | accept | Dave | ||
| Sep 23, 2011 at 18:46 | comment | added | Ron Maimon | @dmckee: you are absolutely right, of course, but I looked up the viscosity, and the naive decay rate due to viscosity of a size 100 ft circulation in air is 10^8 seconds! This cannot be right. I suppose the reason winds in cars decay is due to boundary layer friction, but I haven't sorted it out. | |
| Sep 23, 2011 at 17:09 | comment | added | dmckee --- ex-moderator kitten | @Dave: Your everyday experience can be a guide here. Wouldn't you notice if there we on-going significant winds blowing around a car or buss or airplane every time it started or stopped? Also, think about length and time scales here. If the air was moving fast enough to effect the ball (order of m/s) how long could it possible go in the confines of a railroad carriage? | |
| Sep 23, 2011 at 16:53 | history | edited | Ron Maimon | CC BY-SA 3.0 |
remove viscosity--- this is negligible
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| Sep 23, 2011 at 16:48 | comment | added | Ron Maimon | @Dave: the pressure in air transmits the force to all the air at the speed of sound, which is always much greater than any speed of the train. The acceleration at the beginning is smooth, so the air settles to its moving profile adiabatically, without producing sound waves (no flow in the middle). If you accelerated the train with an atomic explosion, then you would get pressure waves inside the train from the air in the middle bunching up in the back, but this is incredible acceleration. | |
| Sep 23, 2011 at 16:17 | comment | added | Dave | To expand on the last comment, are boundary conditions the way to think about this? Again, it's the air immediately around a train or car that is affected. Similarly, I'd suppose that air in middle of train is less affected by the moving train as it accelerates. | |
| Sep 23, 2011 at 16:15 | comment | added | Dave | Thanks Ron. I believe the part about "everything settles down", but what's the basis for saying "except for a brief period at the beginning". First of all, what is brief? Milliseconds? Minutes, Hours? It seems reasonable that the air would continue to move until brought to a stop by drag from the surfaces (walls, floor) of the car and other air molecules. I guess I don't have a "gut" feel that this would happen super fast. Also, are you sure about "a pressure force to all the air". You wouldn't argue that the air in front of the train is affected by the train once you get away from train. | |
| Sep 23, 2011 at 16:09 | history | edited | Ron Maimon | CC BY-SA 3.0 |
expand, remove errors
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| Sep 23, 2011 at 16:04 | history | answered | Ron Maimon | CC BY-SA 3.0 |