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Pressure question

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Pressure question

Unread postAuthor: kjjohn » Sun Feb 14, 2010 4:53 pm

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I have a question on a hybrid piston valve design I have been working on, but I ran into a problem. Say you have a piston assembly such as the one shown above. (gray being the pistons and the rod connecting them, and black being the chamber. i didn't draw o-rings, but both pistons are sealed and airtight) Say you have equal pressure on each side, and the area in the middle is not pressurized. Would the pistons stay in the same place, or would they move to the right due to a high surface area on side A?
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Unread postAuthor: Gun Freak » Sun Feb 14, 2010 5:14 pm

Surface area won't effect how much force is on an object, so I'm pretty sure you should be fine. If the pressures on each side of the pistons are equal, then it SHOULD remain stationary. Hope this helps.
Good luck.
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Unread postAuthor: Gippeto » Sun Feb 14, 2010 6:20 pm

Gun Freak wrote:Surface area won't effect how much force is on an object, so I'm pretty sure you should be fine. If the pressures on each side of the pistons are equal, then it SHOULD remain stationary. Hope this helps.
Good luck.


Bak ta clas wid u. :)

Force = Pressure * area

Moving on...

"I have a question on a hybrid piston valve design I have been working on, but I ran into a problem. Say you have a piston assembly such as the one shown above. (gray being the pistons and the rod connecting them, and black being the chamber. i didn't draw o-rings, but both pistons are sealed and airtight) Say you have equal pressure on each side, and the area in the middle is not pressurized. Would the pistons stay in the same place, or would they move to the right due to a high surface area on side A?"


The spool will move toward "side B"
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Unread postAuthor: D_Hall » Sun Feb 14, 2010 6:50 pm

What Gippeto said.
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Unread postAuthor: Gun Freak » Sun Feb 14, 2010 7:01 pm

I beg to differ.
Force = Mass x Acceleration.
Pressure = Force / Area
Which would give you the psi if you are using inches and pounds.
Pressure = Pounds of force / Square inches
or
Newtons / Square centimeters.
A larger surface area will have less pressure but an EQUAL amount of force, therefore it WILL remain stationary.
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Unread postAuthor: D_Hall » Sun Feb 14, 2010 7:10 pm

You're an idiot, GunFreak. I don't say that because you're wrong. I say that because you're wrong, you've been corrected, and you refuse to learn. Anybody can be wrong. It takes an idiot to not learn from it.

First off, F = ma isn't relevant to the conversation at hand. It also isn't entirely accurate as long as you want to bring it up in a penis contest. Try F = d(mv)/dx on for size. Granted, with a constant mass system that simplifies to F=ma but I figure if you want to bring up irrelevant stuff so can I.

Strawmen out of the way....

Yes, Pressure = Force / Area. Gee, let's reorder that...

Pressure = Force / Area
Pressure * Area = Force
Area = Force / Pressure

Those are all the same equation, you're just rewriting them in different ways. Welcome to Beginning Algebra.

A larger surface area will have less pressure but an EQUAL amount of force, therefore it WILL remain stationary.

Hello... In the original post it was stated that the pressure was equal. This means that the forces will not be equal and that the piston will move to the right.
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Unread postAuthor: Gun Freak » Sun Feb 14, 2010 7:26 pm

D_Hall wrote:Those are all the same equation, you're just rewriting them in different ways. Welcome to Beginning Algebra.


I took algebra last year, THANKS VERY MUCH.
And I'm now in Geometry. HONORS. Bet thats better than you did at the age of 13? I'm sorry if I'm wrong, but that's what I remember from physics class. I have e-mailed my physics teacher... lol... so we'll see what she says.
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Unread postAuthor: D_Hall » Sun Feb 14, 2010 7:59 pm

What I did at 13 (or even what you've done) is irrelevant to the question at hand. That said, do yourself a favor and learn from those who have more knowledge and experience.

Oh, and I suspect I've had as much or more training in Newtonian physics than your teacher has. Put it this way: the military pays me very well for my knowledge of things that go boom (including guns). And yet, I still learn things from the people on this forum. Sometimes even teenagers.

This is not one of those times.
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Unread postAuthor: Davidvaini » Sun Feb 14, 2010 8:10 pm

yeah don't try to fight D-hall when it comes to math or gases

(especially gases) (he's a stinky fellow... jkjk)

He is the creator of the GGDT (gas gun design tool), found here:
http://thehalls-in-bfe.com/GGDT/

and
http://thehalls-in-bfe.com/HGDT/

It is the premiere tool for gas gun design.

Now I understand that its annoying when you are proved wrong.. but you have been... enough has been said about that...


As far as the piston.. I think it will move right, but as far as how fast.. that might not that quick.
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Unread postAuthor: btrettel » Sun Feb 14, 2010 8:12 pm

Gun Freak, draw a free body diagram of the piston assembly. Find the resultant force and note its direction. Plug that into F = m * a to find what direction it accelerates towards.
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Last edited by btrettel on Sun Feb 14, 2010 8:17 pm, edited 2 times in total.
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Unread postAuthor: Gun Freak » Sun Feb 14, 2010 8:12 pm

ALL HAIL D-HALL!

lol. I'll shut up now... :D

While I have Mr. Hall's attention..

Why can't I run GGDT?
It gives me this message.
I run Windows 7.
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Unread postAuthor: btrettel » Sun Feb 14, 2010 8:16 pm

Try a Google search for mschrt20.ocx site:spudfiles.com . There's a solution floating around.
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Unread postAuthor: velocity3x » Sun Feb 14, 2010 8:22 pm

Gun Freak wrote: I have e-mailed my physics teacher... lol... so we'll see what she says.


For the sake of your scholastic future, it might be wise to hide it from your current teacher.
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Last edited by velocity3x on Sun Feb 14, 2010 8:24 pm, edited 1 time in total.
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Unread postAuthor: Gun Freak » Sun Feb 14, 2010 8:24 pm

Too late 8)
You missed the unquote by the way.
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Unread postAuthor: MrCrowley » Sun Feb 14, 2010 8:26 pm

I have e-mailed my physics teacher... lol... so we'll see what she says.

No need, you answered your own question as D_Hall pointed out. In your first post, you mixed up "force" with "pressure". You were closer to the answer with your second reply. If the forces are to be equal, than the larger surface area side will have less pressure than the smaller surface area side.
Gun Freak wrote:If the pressures on each side of the pistons are equal, then it SHOULD remain stationary

Gun Freak wrote:A larger surface area will have less pressure but an EQUAL amount of force, therefore it WILL remain stationary.

And in the original post we've already assumed that both sides have equal pressure therefore, uneven forces.
kjjohn wrote:Say you have equal pressure on each side


But thanks for the laugh anyway.

edit: Damn, forgot to refresh the page after half an hour :D
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Last edited by MrCrowley on Sun Feb 14, 2010 8:34 pm, edited 2 times in total.
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