# Calculating Force on seal

1. Mar 17, 2015

### 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?

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2. Mar 17, 2015

### A.T.

So, what are his and your arguments?

3. Mar 17, 2015

### mido

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

4. Mar 17, 2015

### A.T.

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

5. Mar 17, 2015

### mido

I dont think you understand whats going on

6. Mar 17, 2015

### A.T.

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

7. Mar 17, 2015

### Staff: Mentor

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.

8. Mar 17, 2015

### mido

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 ?

9. Mar 17, 2015

### A.T.

10. Mar 18, 2015

### 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 :(

11. Mar 18, 2015

### A.T.

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.

Last edited: Mar 18, 2015
12. Mar 18, 2015

### mido

OK, let me explain this further, lets 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.

13. Mar 18, 2015

### A.T.

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

Last edited: Mar 18, 2015
14. Mar 18, 2015

### mido

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

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