Will Decreasing Stem Diameter Affect Seal Point?

[mido]

Hi All,

Me and my friend are having a dispute on whether or not, decreasing the diameter of the stem from 30 to 15mm will have any impact on the sealing point as shown on the picture attached. Can you please help if you can?

 

[A.T.]

Me and my friend are having a dispute on whether or not, decreasing the diameter of the stem from 30 to 15mm will have any impact on the sealing point as shown on the picture attached. Can you please help if you can?

So, what are his and your arguments?

 

[mido]

He is saying basically, decreasing the diameter will have no impact on the force against the seal, I disgree

 

[A.T.]

He is saying basically, decreasing the diameter will have no impact on the force against the seal, I disgree

That's your conclusions. But what are the presented reasons for either position?

 

[mido]

That's your conclusions. But what are the presented reasons for either position?

I don't think you understand what's going on

 

[A.T.]

I don't think you understand what's going on

If that was your only argument, no wonder your friend was unconvinced.

 

[russ_watters]

I don't think you understand what's going on

I don't think YOU understand what is going on. We don't spoonfeed here. We want you to lead us through the entire line of logic. Then we'll help with bumps back to the right path if it goes astray.

 

[mido]

If that was your only argument, no wonder your friend was unconvinced.

You keep asking irrelevant questions, all i wanted to know whos right and wrong, force is presssure x area, the larger the area the higher the force, do u want me to solve this question for u ?

 

[A.T.]

force is presssure x area

Only for a flat surface. For arbitrary surfaces you have to integrate the pressure over the surface. See equation 2.17 here:

http://www.ae.metu.edu.tr/~ae244/docs/FluidMechanics-by-JamesFay/2003/Textbook/Nodes/chap02/node13.html

 

[mido]

This is strange to be honest, I would never imagine a 1mm and a 10mm of water height will exert same amount of pressure. I know this is very basic physics principles but still doesn't make sense to me :(

 

[A.T.]

This is strange to be honest, I would never imagine a 1mm and a 10mm of water height will exert same amount of pressure. I know this is very basic physics principles but still doesn't make sense to me :(

Not sure what you mean. But regarding your plug: The outer part of what you call "stem" has the same pressure from both sides, so the forces on it cancel, and thus reducing that outer part doesn't change the total force from the fluid on the plug. If the seal was at the circumference of the "stem", the force on it would depend on the size of the stem.

 

[mido]

OK, let me explain this further, let's call the shaded area (the cap) and the bit inside it (the stem), the stem is connected by a float which is not showing, however, we fill the cap with water which leaves the cap through till floats drag the stem up to the point when the base of the stem (30mm) touches the seal and create a static pressure inside. the distance between the top of the diameter to the inner side of the cap as shown is almost 1 mm. I neglected that area when I have done my calculation since the amount of water between the 2 surfaces where the seal sits is almost nothing, hence the argument.

 

[A.T.]

the distance between the top of the diameter to the inner side of the cap as shown is almost 1 mm. I neglected that area when I have done my calculation since the amount of water between the 2 surfaces where the seal sits is almost nothing, hence the argument.

Okay, now I see the 1mm you mean. It doesn't matter if it's 1mm or 1m. If it has the same pressure as the opposite side, then the forces on the equal areas on each side are equal but opposite, so they cancel.

You could float a battle ship on a thin water layer around it.
http://www.physics.montana.edu/demonstrations/video/2_fluidmechanics/demos/battleshipinbathtub.html

 

[mido]

top guy, thank you so much...i guess i have to admit I'm wrong :(