Confused about the pressure formula

Click For Summary

Discussion Overview

The discussion revolves around the different formulae for calculating pressure in fluid mechanics, specifically addressing the inclusion of initial pressure (atmospheric pressure) in one formula and its absence in another. Participants explore the implications of these formulae in practical scenarios, such as determining the maximum safe depth for a submarine.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • Some participants note that the formula p = p0 + rho*g*h includes initial pressure, while p = rho*g*d represents pressure due to the fluid alone.
  • One participant suggests that to calculate the full pressure, the initial pressure must be added to the pressure due to the fluid.
  • A later reply questions whether to use the formula with or without atmospheric pressure in a specific scenario involving a submarine's window.
  • Another participant mentions the ambiguity in whether the inward force from the water or the net inward force should be considered, suggesting a preference for the formula without atmospheric pressure due to lack of information.
  • One participant expresses a personal preference for a more cautious approach, referring to it as the 'pessimistic' version.

Areas of Agreement / Disagreement

Participants express differing views on whether to include atmospheric pressure in calculations, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

There is uncertainty regarding the interpretation of forces acting on the submarine and the specific context of the pressure calculations, which may affect the choice of formula.

jayadds
Messages
42
Reaction score
0
Hi,

I've come across two different formulae for pressure:

p = p0 + rho*g*h (p0 = initial pressure)

OR

p = rho*g*d

My question is, why does one of the formulae includes the initial pressure (initial pressure is often the atmospheric pressure = 1.013*10^5 Pa) in the formula whereas the other one doesn't? Do they actual calculate the same thing?

Many thanks.
 
Physics news on Phys.org
The second version just gives the pressure due to the fluid (density rho). If you want the full pressure, you need to add the pressure above the fluid, p0.
 
Doc Al said:
The second version just gives the pressure due to the fluid (density rho). If you want the full pressure, you need to add the pressure above the fluid, p0.

Having said that, then for the following question should I use the formula with or without the atmospheric pressure?

A research submarine has a 20 cm diameter window. It can withstand forces up to 1.0 x 10^6 N. What is the submarine's maxiumum safe depth in water?
 
Depends. Do they mean the inward force from the water or the net inward force? (Presumably, the pressure inside the submarine is atmospheric.) Lacking additional information, I would use the one without the atmospheric pressure.
 
Personally, I'd use the 'pessimistic' version. But I may be just a wimp.
 

Similar threads

  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 14 ·
Replies
14
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 4 ·
Replies
4
Views
1K
Replies
1
Views
2K
  • · Replies 16 ·
Replies
16
Views
7K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 6 ·
Replies
6
Views
1K
  • · Replies 13 ·
Replies
13
Views
21K