Physics Help Forum a logical problem regarding pressure and force

 Energy and Work Energy and Work Physics Help Forum

 Feb 4th 2013, 12:12 AM #1 Junior Member   Join Date: Feb 2013 Posts: 2 a logical problem regarding pressure and force Pls help for this simple logical problem? Suppose a 2 metres long pipe.? It's area of cross section is 1 metre squire.The pipe is vertically placed.It's bottom end is closed.Inside the pipe, at the closed bottom end, there is 1 metre cube of air,closed with a disc. This disc is movable, like a piston in the cyllinder.(The cyllinder is the pipe.) Now, I'm pressing on the disc downwards and compressing the air to 5 bar of pressure.(the volume of the air decreased.). Now I'm pouring water on to the disc, 1000kg of water. Now in the pipe,there is the compressed air at the very bottom, then the piston on it, and on the piston, filled with water of 1 metre hight(1 metre cube,ie, 1 tone of weight).). Now, if I release the pressure of the compressed air at the bottom SUDDENLY, to how much height the water colunm will go up(will be shooted up) ? Consider the atmospheric pressure also,which blocking downwards. PLEASE HELP..?
 Feb 4th 2013, 02:23 PM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,321 It's good that you placed this question in the "Energy and Work" area because that's a good hint on how to approach this. What's the work needed to compress the air from 1 bar to 5bar of pressure? It's the force (pressure times area) operating over the distabce of compression of the piston. This will guve you an integral that's not too dificult to evaluate. Set this work equal to the KE of the water after the piston liets fly, and from that you can calculate the height the water goes to. Alll this assumes that air frictio is zero, and that the air pressure under the pistom is a linear functtion of volume (it ignores adiabatic cooling that will occur when the presure is released).
 Feb 7th 2013, 03:51 PM #3 Senior Member   Join Date: Jun 2010 Location: NC Posts: 405 Response... Hi, I started a response to your question. There is a bit more to it. http://www.thermospokenhere.com/wp/0...er_mortar.html I'll try to get back and solve it. Good luck, Jim
 Feb 7th 2013, 10:34 PM #4 Junior Member   Join Date: Feb 2013 Posts: 2 ok Take 1000kg weight , instead of water. Then, if u want, u can neglect the air resistance.Also assume that the piston has no considerable mass, or, 1 kg of mass.
 Feb 8th 2013, 08:55 AM #5 Senior Member   Join Date: Jun 2010 Location: NC Posts: 405 Air/Water Mortar (I named it). Hi Jayan, I'm short on time. Your problem is like Newman's Annihilator (posted at my site). http://www.thermospokenhere.com/wp/0...nihilator.html I'll try to finish your problem soon. Good Luck, Jim

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