# Question regarding underwater pressure

1. Dec 12, 2013

### smth

Hi, I am having some argument with my friend over the following elementary physics problem:

Assume there is a pressure sensor located at position C underwater and deep enough, and assume also the water is completely still. If there is a helicopter or hovercraft idling above the water, will the pressure sensor give different readings when the helicopter or hovercraft moves from somewhere else to the position right above C?

2. Dec 12, 2013

### ShayanJ

Well,if you take a large enough amount of water,for example an ocean,then I think there will be no difference.But as you decrease the amount of water,a difference between those cases develops and so the assumption that there is no change in the reading of the pressure sensor can only be an approximation that becomes worse as the amount of water decreases.
But I still should say that,I think,even when the approximation is bad compared to the case of considering the ocean,it is still a good approximation.
I also think the approximation becomes better as you increase the depth at which the sensor is placed.

3. Dec 12, 2013

### smth

Thanks Shyan, so you mean theoretically speaking (like an ocean) the pressure applied by the helicopter on the water will be equivalently distributed to the buttom of the ocean, am I understanding it correctly?

Thanks:)

4. Dec 12, 2013

### ShayanJ

Yes,that's what I mean.
And as I'm thinking right now,I see that it is really safe to take the assumption that there will be no difference in almost all applications.

5. Dec 12, 2013

### smth

Okay thanks :)

6. Dec 12, 2013

### anandparasar

Am I right the above explanation given by shayn is based on pascal theorem for static fluids. ...

7. Dec 12, 2013

### anandparasar

Hey smith r u der ?

8. Dec 12, 2013

### smth

hi anandparasar, sorry for the late reply. I have the same feeling that this should be an example of the pascal theoerm (or so-called pascal's law) for static fuilds.

9. Dec 12, 2013

### anandparasar

Thanks for clarification smth....

10. Dec 12, 2013

### ShayanJ

No,I didn't use Pascal's law!
Pascal's law says that the difference of pressure between two points inside a fluid,depends only on the difference in their heights.It doesn't speak about whether there is something floating on the surface of the fluid or not.
My reasoning was based on the fact that when water molecules are placed together,they don't act individually and almost become a single thing,because of the interactions between the molecules.So if you put something on the surface,not just the molecules under it experience the force but also molecules far apart from that region and that's because of the bonds between molecules.Because of that,the pressure on the surface is distributed among it and as it propagates downward inside the fluid,it becomes more and more uniform!

11. Dec 12, 2013

### smth

Hi Shyan, what you descibed, like the "pressure on the surface is distributed...", sounds very like the pascal law (at least in some form) which states that the static pressure in a fluid acts equally in all directions...

12. Dec 12, 2013

### ShayanJ

Well,the different part is when I considered an object on the surface.Pascal's law can't be used there!

13. Dec 12, 2013

### smth

You may be right:-<, perhaps pascal law is not quite the law we really need, though they are likely related.

Thanks!

14. Dec 12, 2013