Need help with understanding this solution (fluid pressure problem)

  • Thread starter Thread starter bigmike94
  • Start date Start date
  • Tags Tags
    Pressure
AI Thread Summary
The discussion centers on understanding fluid pressure calculations, specifically at point B in a system involving oil and water. The pressure due to water is considered negative while that of oil is positive, as measurements are taken from the oil-water interface to determine differences in pressure. The method involves calculating hydrostatic pressure as one moves down through oil and then up to point B, with the atmospheric pressure serving as a reference. The calculations can be visualized similarly to a U-shaped manometer, emphasizing the importance of the interface for accurate pressure measurement. Clarifying these concepts aids in grasping the overall pressure dynamics in the system.
bigmike94
Messages
99
Reaction score
61
TL;DR Summary
I dont fully understand the solution
So here’s the question (I am only talking about the pressure at point B, the other 2 I can understand.)
94326FD6-597D-474F-94C3-9BCBC33DF58B.jpeg


And here is the solution
3C602BA6-4E95-403A-8973-BFE9E6853BB6.jpeg

B86A26B6-2AE9-4037-AB6F-1F0DF5EF36DD.jpeg


Here is what I am not understanding, why is the pressure due to the water negative and the oil positive, and why are all measurements only made from where the oil meets the water?

i must be missing something really obvious
 
Engineering news on Phys.org
bigmike94 said:
why are all measurements only made from where the oil meets the water?
Because that's how we find the difference between ##p_{atm}## and ##p_B##
Going down contributes a positive ##\Delta p## and up a negative ##\Delta p##.

One can just as well take the bottom as the turning point: the ##\Delta p## of the extra 1.25 m down is cancelled by ##\Delta p## going up the same 1.25 m

##\ ##
 
  • Like
Likes Chestermiller and hutchphd
BvU said:
Because that's how we find the difference between ##p_{atm}## and ##p_B##
Going down contributes a positive ##\Delta p## and up a negative ##\Delta p##.

One can just as well take the bottom as the turning point: the ##\Delta p## of the extra 1.25 m down is cancelled by ##\Delta p## going up the same 1.25 m

##\ ##
Thank you that helped a lot. I attempt the problems quite a bit after I learnt about that method of finding the pressure. Hopefully I’ll remember it 😃
 
I think of the problem as moving along a path from the external reference surface to the destination, at point B.
1. Start at the surface with atmospheric pressure. Specify absolute or gauge pressure for the reference.
2. Compute the increasing hydrostatic pressure as you move down through the oil, to the oil-water interface surface.
3. Compute the reducing hydrostatic pressure as you move up through the oil to the destination at point B, on the water-air interface.
 
bigmike94 said:
Here is what I am not understanding, why is the pressure due to the water negative and the oil positive, and why are all measurements only made from where the oil meets the water?

i must be missing something really obvious

This arrangement can be calculated like a U-shaped manometer, if you can imagine the bend located just underneath the central partition.

Please, see:
https://pressbooks.online.ucf.edu/osuniversityphysics/chapter/14-2-measuring-pressure/
 
How did you find PF?: Via Google search Hi, I have a vessel I 3D printed to investigate single bubble rise. The vessel has a 4 mm gap separated by acrylic panels. This is essentially my viewing chamber where I can record the bubble motion. The vessel is open to atmosphere. The bubble generation mechanism is composed of a syringe pump and glass capillary tube (Internal Diameter of 0.45 mm). I connect a 1/4” air line hose from the syringe to the capillary The bubble is formed at the tip...
Thread 'Physics of Stretch: What pressure does a band apply on a cylinder?'
Scenario 1 (figure 1) A continuous loop of elastic material is stretched around two metal bars. The top bar is attached to a load cell that reads force. The lower bar can be moved downwards to stretch the elastic material. The lower bar is moved downwards until the two bars are 1190mm apart, stretching the elastic material. The bars are 5mm thick, so the total internal loop length is 1200mm (1190mm + 5mm + 5mm). At this level of stretch, the load cell reads 45N tensile force. Key numbers...
I'd like to create a thread with links to 3-D Printer resources, including printers and software package suggestions. My motivations are selfish, as I have a 3-D printed project that I'm working on, and I'd like to buy a simple printer and use low cost software to make the first prototype. There are some previous threads about 3-D printing like this: https://www.physicsforums.com/threads/are-3d-printers-easy-to-use-yet.917489/ but none that address the overall topic (unless I've missed...
Back
Top