Physics Help Forum terminal speed of the vessel

 Thermodynamics and Fluid Mechanics Thermodynamics and Fluid Mechanics Physics Help Forum

 Jul 15th 2019, 04:53 AM #1 Junior Member   Join Date: Jul 2019 Posts: 2 terminal speed of the vessel In vacuum and weightlessness, at the bottom of a cylindrical vessel (a cup), there is a layer of solid substance of molar mass μ. This substance sublimes slowly (evaporates from the solid phase into gaseous phase) and pushes thereby the vessel to the opposite direction. Estimate the terminal speed of the vessel. The mass of the vessel M, and the initial mass of the substance m ≪ M; the temperature of the vessel is T; the process can be assumed to be isothermal (cooling due to evaporation and heat radiation remains negligible). The cross-sectional area of the vessel is A.
 Jul 15th 2019, 05:05 AM #2 Senior Member   Join Date: Aug 2010 Posts: 434 If this is "in vacuum and weightlessness" what do you mean by "up"? And why would a gas push the vessel "up"?
 Jul 15th 2019, 05:11 AM #3 Junior Member   Join Date: Jul 2019 Posts: 2 me neither understand that. but the answer should be: V=m/M*sqrt(RT/miu) and I do not know how to get to it
Jul 15th 2019, 07:17 AM   #4
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 Originally Posted by HallsofIvy If this is "in vacuum and weightlessness" what do you mean by "up"? And why would a gas push the vessel "up"?
No where does the word "up " appear ..

The solid is in the base of the cup , subliming , that's to say atoms separate from the solid surface , become gas , the gas can only exit in one direction , the mouth of the cup ,the reaction pushes the cup the other direction.

The terminal speed must be dependent on the speed(temperature) of the gas molecules ... it must be the same as the velocity of gas molecules , because then the molecules cannot impact the surface imparting momentum , the surface is moving away as fast as they are travelling .

Last edited by oz93666; Jul 15th 2019 at 07:28 AM.

 Jul 15th 2019, 09:25 AM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 963 Look at the momentum transfer the momentum of the cup must be equal but opposite to the momentum of the gas molecules. Also look at the ideal gas law. __________________ ~\o/~
Jul 15th 2019, 09:46 PM   #6
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 Originally Posted by Woody Look at the momentum transfer the momentum of the cup must be equal but opposite to the momentum of the gas molecules. Also look at the ideal gas law.
Unnecessary ...

All you need is the velocity of the gas molecules ...
that will be the terminal speed of the container
Because momentum will continue to be imparted to the container, increasing it's speed .. until it is moving away so fast gas molecules can no longer hit the surface sublimating.

 Tags speed, terminal, vessel

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