Pressure Definition and 999 Threads

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure.
Various units are used to express pressure. Some of these derive from a unit of force divided by a unit of area; the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m2); similarly, the pound-force per square inch (psi) is the traditional unit of pressure in the imperial and U.S. customary systems. Pressure may also be expressed in terms of standard atmospheric pressure; the atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer.

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  1. E

    Why is it easier on the heart in a horizontal position?

    The system is closed, and as much blood rises to a certain height, an equal amount descends from the same height, so the two liquid columns create equal but opposite pressures, and their effect on the heart is 'neutralized.' Therefore, if we ignore friction in the blood vessels, it shouldn’t...
  2. R

    B Simulate an altitude at 16000-18000 feet with water depth

    Hi. If we wanna train inside a water pool like just play fighting, how much depth that we need to simulate an altitude at 16000-18000 feet? Thank you.
  3. L

    I Availability at fixed pressure and temperature

    We define ##dA=dU+P_0dV-T_0dS \leq 0##. In my notes it says if you fix pressure and entropy, ##dA=dH##. I don't get this, because at constant T and S, I get ##dA=dU+P_0V##. It seems that somehow, ##P_0=P##. Is this correct, or am I missing something? Second question about this: If ##T_0=T##...
  4. Finchh

    Pigging or Flushing a 15 km Non-Uniform Pipeline

    Hello all, I’m looking for some guidance on cleaning a 15 km non-uniform pipeline that alternates between 4" and 6" Schedule 80 sections, with reducers, bends, and a non-barred tee section (P&ID attached for reference). We’ve calculated a total pressure loss of approximately 30 bar at a flow...
  5. joejoe121

    Two tanks are connected by a valve and line

    I've attached all my work and data table I used to answer the questions but there isn't an answer key so I would like a second opinion.
  6. R

    Pressure versus stress in uniformly charged sphere

    Let the charge density be $\rho$, radius be $R$, total charge be $Q = \rho \frac{4}{3} \pi R^3$. We know from Gauss's law, $E (r) = \frac{Q r}{4 \pi \epsilon_0 R^3}$. We also know from Maxwell stress tensor $\sigma(r) = \half \epsilon_0 E^2$. We can compute the pressure due to the electric...
  7. S

    Pressure question: Weight of a car calculated from tyre contact patch area

    I tried 2 ways to solve it but both yielded different answers. i would like to know which is correct and why the other is wrong. thank you. method 1: consider force of each tyre. 5 x 10 = F = 50N. so 4 tyres = 200N = weight method 2: consider the whole car, total pressure = 4x5 = 20. total...
  8. H

    Maximum exhaust velocity for a De Laval nozzle

    Here is what I did so far. First of all, at ##P_0## (the opposite end of the exit), I supposed ##u_0 = 0##. (Is it correct?) Hence, using Bernoulli's equation for a compressible gas. ##\frac{\gamma}{\gamma -1} \frac{P_0}{\rho_0} = \frac{u^2}{2} + \frac{\gamma}{\gamma -1} \frac{P}{\rho}##...
  9. lost captain

    I Why pressure stays the same when doubling both volume and temperature?

    In thermodynamics we tend to think of pressure as the frequency of collisions with the walls of the container. And we say that the more the collisions the higher the pressure, the less the collisions the lower the pressure. So lets say we have an ideal monoatomic gas enclosed in a cube...
  10. S

    Pressure of car tires

    a) ##P=\frac{F}{A}=\frac{\frac{1500\times 9.8}{4}}{180\times 10^{-4}}=2.04 \times 10^{5} Pa## b) Is the pressure in (a) gauge pressure? My answer for (b) is 2.04 x 10 5 + Po where Po is atmospheric pressure c) (i) it will increase since the size of the tire also increases (ii) I am not sure...
  11. Z

    Chemistry What is "gauge pressure" and how does a manometer work?

    First of all, where does (1) come from? What is an intuitive explanation for the meaning of ##-\rho g##? ##\partial P/\partial z## is the rate of change of pressure relative to position ##z##. Since we have a negative sign on the rhs, it seems that ##z## is being measured from bottom to top...
  12. U

    A Equating Gas Pressure and Projectile Stagnation Pressure

    Imagine a projectile with density p, initial velocity v0 and initial stagnation pressure = 0.5p{v0}^2 being fired into the gas chamber with a final gas pressure given by Pg = nkT, for n being the final number density before (at max pressure of gas) and T the corresponding temperature of the gas...
  13. T

    I Effects on Engine Overheating and Pressure Dynamics

    I tried posting this earlier, but being my first time somehow got it in mechanical and I wanted it in thermodynamics so I am reposting here. Sorry for the confusion: Automotive cooling system Not sure if this is an appropriate forum for this question or not, but I will try. Many current...
  14. T

    Why Did the Temperature Rise in a Sealed Automotive Cooling System?

    Automotive cooling system Not sure if this is an appropriate forum for this question or not, but I will try. Many current automotive cooling systems have a system that is completely filled with coolant and no room for air or expansion, other than slight bulging of the hoses, etc. A pressure...
  15. abrek

    B Does the shape of an object affect the pressure?

    Suppose the speaker has a container into which pressure is pumped, a rod passes through the container with a disk strung on it with one flat side and on the other with a pointed side, will the disk move in any direction under the influence of pressure?
  16. G

    B How the temperature of the Earth's surface affects the air pressure above it

    If the surface is heated, air above it will also heat, its temperature will rise. It will cause two processes: while temperature of gas rise, its pressure will also increase, but, also, as the air molecules get more energy, they will go higher from the earh surface, concentartion of the...
  17. abrek

    B Trying to understand hydrostatic pressure

    Imagine we have a cylinder filled with water with a closing mechanism, the height of the cylinder is quite large to create increased pressure at the very bottom. If the mechanism closes a small part at the bottom of the cylinder, will the pressure decrease or not? Thanks in advance for the answer
  18. abrek

    B Trying to understand hydrostatic pressure with different vessel widths

    will the hydrostatic pressure be the same on the vessels shown with a different diameter of 1 meter and 1 centimeter? and will it be the same in both vessels if the first pipe has a diameter even less than 1 millimeter, 1 thousandth of a millimeter, 1 atom?
  19. S

    B If the source of the pressure distribution around a lifting airfoil is known, why is not explained in textbooks?

    Philosophically, it is impossible to prove a negative, so I will turn to one of the most recognised authorities, Anderson, J. D. (2011) "Fundamentals of Aerodynamics" Fifth Edition. An on-line version can be borrowed from archiv.org at Fundamentals of aerodynamics : Anderson, John D., Jr. (John...
  20. samsanch0803

    B I have a doubt about the pressures at the same level of fluids

  21. M

    Can pressure differential close lateral window?

    I'm trying to think of the least creepy reason I found my small lateral bathroom window closed this morning. I live in a 2 bed/1 bath apartment on the ground level with my two dogs and no one else. I rarely have people over. My bathroom doesn't have a fan and I don't have the best memory, so I...
  22. highschoboy004

    A cylinder has two sides containing gas and water separately

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  23. Steven Bolgiano

    I Gas Pressures and the Resistance of Materials (Biodigester Generates Methane)

    Hi Physics Forum Folks! What an amazing resource of knowledge, ... thanks! The question is on the attached graphic, as its much more clear with an illustration. (Please read questions on this graphics first before looking at the video below,) This video is a very quick look at the actual...
  24. W

    Measuring the water volume in a small tank with a manometer

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  25. L

    Negative Pressure Wave generation due to pipeline leak

    please refer below attached file for further details
  26. saeppi

    I Pressure builds on its own inside this water tank?

    Hey guys, It‘s been a few years since I had my last physics class so please be kind to me. I came here to ask you about a phenomenon I recently discovered in a design of mine. The object in question is a stand with 3 arms, holding a water tank (see pic below). All openings are pretty tightly...
  27. H

    Fluid mechanics: Euler's equation derivation

    For a volume fixed in space with the positive component of the z axis pointing up, I have the force of gravity pointing down. Using the following equation I have to derive the Euler's equation. ##\frac{d}{dt} (\rho \vec{u} \otimes \vec{u}) = \phi + F_v + F_s## Where ##\phi## is the flux...
  28. hraghav

    Calculating work done by a gas

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  29. M

    Is there a pressure ratio limit for choked flow through a c-d nozzle?

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  30. A

    What happens if a negative pressure gradient is applied to a diffuser?

    A diffuser normally decreases the velocity by increasing the area. With assumption of an incrompressible flow, according to Bernoulli, the static pressure increases. What happens if one "forces" the flow through a diffuser by applying a bigger pressure on the inlet than on the outlet?
  31. A

    Simple Static 3-Particle Model

    Diagram Integrate pressure for each particle: Add normal force and weight Sum all the forces in the x and y direction to get:
  32. G

    I What are the properties of a Sound Pressure Wave going down into a Cone?

    I do not remember the correct terms to look this information up on Google. I once read that as a pressure wave goes down into a closed cone every time cross sectional smaller by 1/2 the pressure wave doubles. My cone is 12" x 12" on the open end and 1"x1" on the closed end. Cone is 24" long...
  33. K

    Engineering and pressure related questions

    How did you find PF?: Friend I need to move a 2000 pound weight up twenty feet into the air on top of an eight inch diameter piston (surface area of 50 square inches) how much steady pressure would i need to achieve this and overcome the friction on the piston itself. Im trying to lift a door/wall
  34. joshcitylife

    B Pressure on creation of the Universe

    I had a question today as a part of my studies which was 1) If space is ever expanding what is the pressure on the creating space (universe) upon creation and 2) at what rate might you assume the quantum field is also being created and is it being affected by this pressure of creation. This is...
  35. B

    I Derivation of Fluid Pressure In A Gravitational Field

    Given that ##P = ρgh##, there's obviously a problem with the following derivation of fluid pressure under gravity. Can someone spot the flaw? $$W = mgh$$ $$W = ρVgh$$ $$F \cdot dh = ρVgh$$ $$F \cdot dh = ρ(Ah)gh$$ $$F \cdot dh = ρgAh^{2}$$ $$\frac{d(F \cdot dh)}{dh} = \frac{d(ρgAh^{2})}{dh}$$...
  36. cambeaux

    A Pressure on a Capsule within an Enclosure (Underwater @ 500m)

    For this engineering project I am trying to understand how the pressure would be distributed (if at all) throughout the enclosure. It will be at a depth of 500 meters (4888 kPA). It is designed with a stainless steel enclosure which can withstand the external pressure at this depth. Inside is...
  37. L

    Atmospheric pressure and water pressure - Boyle's law

    I am a bit confused on the marking scheme as attached above. P1V1 is a constant by Boyle's Law. If the volume increases by a factor of 3, then the pressure decreases by a factor of 3. This means that the pressure at the top is 1/3 the pressure at the bottom, right? The pressure at the top is...
  38. as919461

    How best to install a pressure sensor inside a pipe?

    Dear memebers, I have a test setup with a pipe (25 mm dia) having certain holes with a suction fan at the end of pipe. I need to check the suction pressure generated in the holes. What type of sensors can i use to install inside the pipe (above the hole in the mainstream) which doesn't disturb...
  39. A

    A vertical U tube rotates about the vertical axis -- Find omega to cause this height offset

    I thought I could use force balance as $$V\rho g=m\omega^2r$$ But that gives two different values and neither correct $$(Ab)\rho g=(Ab\rho)\omega^2 (b) \text{ on the left}$$ $$(A×3b)\rho g=(A×2b×\rho)\omega^2(2b) \text{ on the right}$$ Just in case, the answer in the book in B
  40. A

    B Pressure in fluid (liquid)

    Let’s imagine a glass of water. Now, we press on the free surface of water with a spoon (just for example), will the pressure in the entire glass increase equally?
  41. C

    I Air pressure question for two different water columns

    If you had pressure sensors (red) inside closed containers (black). The one on the left having more air volume than the one on the right. They are inside a bucket (green) that is filled with water. The water will rise higher in the left container. Will the pressure sensor readings be identical...
  42. N

    Effect of pressure due to change in diameter of pipe

    The pump has an outlet size of 4 inch 1) In case 1, 4 inch pipe is connected at length of 20 m 2) In case 2, 2 inch pipe is connected using reducer In both cases the pump outlet conditions are same( having same flow rate at a given pressure) What happens to pressure at the end of the pipe in...
  43. D

    Engineering Quantitative Risk Assessment of a Chlorine storage tank

    Storage tank leaks are caused by the failure of the pressure relief valve, tank body, or tank piping. If the pressure relief valve fails, can't it contribute to tank body and/or piping failure from overpressure? But the tank body and tank piping can fail by other mechanisms (mechanical...
  44. yecko

    Superheater - pressure and temperature distribution

    Quoted from the O&M manual of an electrical steam superheater: "The Steam Superheater has eight thermocouples of the NiCr-Ni type, known as "K," attached to the heating element sheath. Four of these thermocouples are positioned near the steam outlet nozzle, where the highest temperature is...
  45. chevywaldo

    I Why Does Differential Pressure Vary As The Square Of The Flow?

    Can someone explain to me Bernoulli's principle as to why the pressure across a restriction varies as the square of the velocity? I'm looking for an understanding (conceptual) as to why this is without a gazillion math examples please. Thanks.
  46. patric44

    Calculate the average force on the side walls of a container

    Hi All, in the following problem: the book solution I don't understand why he added the term $$ \rho g h_{z} A_{y} $$ shouldn't it just be : $$ F = 1/2 \rho g h_{y} A_{y} $$
  47. aboredperson

    Chemistry Where Did I Go Wrong in My Gas Law Calculation?

    PV=nRT P*1L = 6 moles * 0.0821*298 (I added up all the moles and solved for pressure) P =146.79 atm 146.79 atm * V = 2 moles Ar * 0.0821* 298 (I plugged in the moles for argon and solved for volume) V= 0.333 L Answer key says the answer is 1 Liter. Where did I go wrong?
  48. lost captain

    I Hydrostatic pressure inside an inverted tube

    When we learn about atmospheric pressure in physics class, there's this classic experiment with a straw (second drawing). So i wanted to ask about the hydrostatic pressure in that particular experiment. Is the total pressure at point 1' equal to Patm + ρgh ? So is the total pressure at point 1'...
  49. J

    B Ideas regarding gravity and entropy

    I've never had any physics class before so please bare with me on my lack of understanding. I've been thinking about gravity and its relation to entropy lately and was wondering if my thinking is correct. Entropy seems to be an opposing force to gravity. where gravity is creating gradients...
  50. tellmesomething

    Why Does Surface Tension Allow Bubbles to Form Despite External Pressure?

    The solution manual says that before water enters the hollow So here an air bubble of radius R will escape from the sphere. The excess of pressure inside the bubble is 2T/R. The excess of pressure prevents the penetration of water into the sphere. Water begins to penetrate when the excess of...
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