What is Pressure: Definition and 1000 Discussions

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. I

    B Rebound Height vs Air Pressure: A Puzzling Relationship

    Hi all I have a very quick question I'm trying to understand why a graph of Rebound height (m) vs Air Pressure (x axis) doesn't have a line of best fit which goes through the origin. I understand a basketball's bounce can be compared to the compression of a mass-spring system but then why...
  2. curiousPep

    Hertzian contact pressure & shear stress

    Hello, I am trying to get some intuition about the direction of the shear stress caused by the Hertz contact pressure. Once I exert some pressure downwards on a spherical object the direction of the Hertz pressure will be upwards. However, this case some shear stress to exist, but I can't see...
  3. CherryB

    B Hydrostatic pressure vs. gravity

  4. Borek

    I Second Tonga shockwave - why a drop in the pressure?

    I already posted this image in GD as a curiosity, but there is one thing that I don't get. This is pressure recorded by my weather station near Warsaw in Poland after the Tonga eruption (the description says '24 h' but it is wrong, I forgot to modify the script generating the plot). Around 20:05...
  5. M

    B Coulomb pressure and concentric spheres

    Suppose you have a sphere of radius a of positive charge, and a concentric shell from a to b of negative charge. The positive charge is equal to the negative charge. (non-conducting, uniform density) Is there an outward pressure at a of kqq/a2/(4πa2) - with pressure decreasing with radius...
  6. mastermechanic

    Measuring the total pressure of a centrifugal compressor

    Hello everyone, Nowadays I am planning to redesign my centrifugal compressor which I designed for a subsystem in my graduation project. It's a 3D printed and brushless motor driven centrifugal compressor and its about a hand size. However, due to its relatively small size and relatively low RPM...
  7. philo

    How much pressure in PSI was needed in the Ancient World to cut stones?

    Was watching a video on the the Great Pyramid of Giza and it got me wondering how one might cut one of the stones. Doing a couple google searches, I thought that a Venturi pump would likely be the means by which the stones were cut. The problem being how would they get a large enough psi...
  8. M

    Engineering Thermodynamics: Determine pressure in the tank of a manometer

    I was watching a YT video getting prepared for class this coming semester, and in the video, this diagram of a manometer was drawn (see image at end of message). The professor stated that the pressure in the tank (Pt) is = to the pressure at point A. Following this, he said that technically...
  9. weirdoguy

    Chimney effect and pressure difference

    Kind of similar question was once on polish high school ("matura") exam, and the solution that the authors gave was: Pressure at the bottom of the chimney inside of it is ##p_{top}+\rho_1gh##, and outside ##p_{top}+\rho_2gh## where: ##p_{top}## is the atmospheric pressure at the height of ##h##...
  10. J

    A Problem solving forces with pressure integration

    Angle theta is different for every place at airfoil surface, so it can't be one theta from leading edge to trailing edge. Can please someone explain pressure integration in depth, step by step?
  11. J

    How calculate lift/drag using static pressure taps in wind tunnel test?

    An airfoil section in a wind tunnel has many static ports/holes/taps on its upper and lower surface. These static ports can only read static pressure which acts perpendicularly to the local airfoil surface. In place A are static ports that read a pressure value of relative -100 Pa . This...
  12. L

    Pressure drag and the Reynolds number

    Ok, I am kind of lost in my fluid dynamics course on a specific topic. I understand the reynolds number and how it works. What i do not get:: if Re = VD/v, and if we increase V(velocity) which in turn will increase the inertial effects of the number, why is it called a pressure drag? pressure...
  13. freddie_mclair

    Scaling pressure drop to other temperatures

    Hi, I have some measurements for pressure drop of Helium at room temperature and I would like to scale it to other temperatures. Taking into account that, i) the flow is turbulent, ii) the pressure drop, ##\Delta p##, happens always in the same piping and iii) there is only variation on the...
  14. Jonathon

    How to factor atmospheric pressure into relative humidity calculations

  15. J

    Calculate lift from pressure distribution

    v=50m/s ∆Pg= -981Pa ∆𝑝𝑑 =490Pa c=1m 𝜌 = 1,225 kg m3 𝐹3 = ∫ ∆𝑝3 ∙ 1 ∙ 𝑑𝑥 𝑐 2 0 = ∆𝑝𝑔 ∫ 2∆𝑝𝑑 𝑐 𝑥𝑑𝑥 𝑐 2 0 = 2∆𝑝𝑑 𝑐 ∙ ( 𝑥 2 2 ) c⁄2 | 0 = 2∆𝑝𝑑 𝑐 ∙ 𝑐 2 4 ∙ 2 = ∆𝑝𝑑 ∙ 𝑐 4 = 490,5 ∙ 1 4 = 122,625 N Lift per unit of span = (F1) + (F2) +F3 + F4= 490.5+245.25+122,25+122,625=981N :confused: Pressure...
  16. B

    Find hole size for allowable leak in pressure vessel

    Hello, all. I am completely new here so please be gentle. I have a small, cylindrical, sealed ("no leak") pressure vessel with an approximate volume of 27 cm3. It has a constant supply of air pressure at 8"WC. I'm trying to figure out how big of a hole it can have if there is an allowable...
  17. Dr Wu

    High gravity vs. high pressure: ref. Dragon's Egg?

    Having re-read "Dragon's Egg" recently, I recall that the cheela, being composed of degenerate matter, would explode if removed from their high-gravity environment and placed in freefall.* (For those who haven't read the story, the environment here is a half-solar mass neutron star with a...
  18. O

    Relationship between belt tension and pressure it exerts

    I have following setup where a non-elastic belt is wrapped around an elastic object, like leather belt around waist. When I pull the belt with force F, it tightens around the elastic object and the belt exerts force to the elastic object. I would like to know what is the relationship between the...
  19. Galexy

    Atmospheric pressure inside a protoplanetary disc

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  20. J

    I Why does a Pressure Washer clean better than a Garden Hose?

    In Bernoulli's equation, you have static, dynamic, and hydrostatic pressures equal to a constant. In one sense, I'm wondering if the dynamic part of the equation explains this since -> Pressure = (1/2*rho*velocity^2) and pressure can be expressed as a force / unit area, therefore, an increase in...
  21. D

    Find the pressure drop and power loss of pipe when elevation changes

    Summary:: I can’t get the correct answers, I think I haven’t fully understand the theory behind them The answers are given in the picture
  22. S

    Designing an orifice plate to increase the pressure in a pipe

    say for 3.013bar absolute pressure pressure ratio : P/Po = 14.7psi/43.707psi = 0.33 which means its chocked flow now i want to find diameter or area of orifice : so could anyone help me
  23. J

    Working out steam velocity with only pressure difference

    I am currently doing an assignment on nuclear power and in the turbine, the steam is moving from pressure of 6Mpa to 0.008 Mpa. is there any way to work out the velocity of the steam when moving between these pressure differences?
  24. SpectraPhy09

    Pressure due to a Liquid on container Wall

    I have just started this topic so i Don't have much clearity about it . In our school it was tought that Pavg = (Pi + Pf)/2 ...(i) If we have a cuboidal shape container then it should be pgh/2 ryt ? But also Pavg = Ftotal on the wall /A ...(ii) =...
  25. Stonestreecty

    Tire pressure Sensor Troubleshooting

    Good day, all I have a K1600 motorcycle. Will an after market tire pressure sensor work on a 2014 GTL? Dealers are crazy expensive and was told today that they are on back order. After search the TPMS Sensor Guide, I've new tires ready to be mounted, but a little confused. Any help will be...
  26. Dario56

    I Bernoull's Equation Derivation From Work - Energy Principle

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  27. B

    The Pressure-Entropy Relationship for a Photon Gas

    This is from Problem 7.45 of Thermal Physics by Daniel Schroeder.
  28. questionmonkey123

    Constant pressure process (piston) on gas violates Newtons 2nd law?

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  29. D

    Manometer -- determine height from relative pressure

    The correct answer is 5.6m, not sure where I made the mistake.
  30. V

    B Air escaping out of can with hole

    In the first figure, air goes forward and the can goes backward. My question is why does the air exert the pressure gradient force on the can. Can I have a hint for why the can doesn't move when air is being sucked in in the second photo.
  31. B

    Changes in pressure, temp, & entropy of ideal gas in atmosphere

    So for a collection of particles each with mass m, the pressure beneath them, ##p(z)## should be higher than the pressure above them ##p(z + \Delta z)##. This is a change in force per unit area (force per unit volume I suppose) times a volume to equate with the gravitational force $$ \frac...
  32. RawPotatoes

    I Issue calculating water pressure due to the constriction of water flow

    For an experiment where I must be able to control the pressure i attached to a syringe a 'chamber'. The inner diameter of the syringe is 19.3 mm and i will fill water to 17 cm of the syringe bottom (i call this P1), the spout has a inner diameter of 2 mm and a length of 2 mm (i call this P2) and...
  33. es4

    Calculation of pressure & volume both isothermally & adiabatically

    Hi, here is my attempt... Feel free to correct me if I am doing it wrong. v1 = .8 m3 p1 = 250 or 250000 n/m2 p2 = 7000 or 7000000 n/m2 Cp = 1010 Cv = 7321. isothermally p1v1 = p2v2 [Boyle equality law (isothermal process)] 250 000 * .8 = 7000000 * v2 7000000 * v2 = 250 000 * .8...
  34. M

    External Pressure vs. Internal Pressure when considering work

    Summary:: Basically the title: It seems that one of the formulas I use considers the pressure in PdV to be internal, when as far as I know, it is external. So to my understanding, in w = PdV, the pressure is the external pressure. However, I get tripped up because in my textbook, for a...
  35. Pim_Verwoerd

    Engineering Pressure after a water turbine

    For an engineering project we have to make an energy storage system. We thought of something like a hydro dam. We were having a discussion about the pressure after the turbine in the hydro dam. The water immediately flows into the atmosphere ( so it falls down into a lake ). Can we then safely...
  36. ZdMh

    I What is the real meaning of a pressure between 0 and 1 bar?

    When we increase the pressure value to for ex 3, 5, ...100 bars the pressure increases. When the pressure is 1 we called it atmospheric pressure. My question is what's happening when the pressure is between 0 and 1; at 0 it's a vacuum. The idea is that I can't imagine it: if I am in a room...
  37. D

    Tubing Pressure drops with Compressible Fluids

    I am a poor, dumb EE often stuck with the odd plumbing calculation. I am often asked questions like: "what size tubing do I need to convey 10 SLPM of 20 PSIG oxygen 200' with no more than 2 PSI pressure drop?" I generally treat the fluid as in-compressible and use Darcy-Weisbach (I like Bellos...
  38. S

    Magnetic energy density, and pressure due to magnetic force

    Hi, The problem I am working on requires me to work out the the pressure on the outer conductor of a coaxial cable due to the current on the inner one. This cable carries a dc current of 5000 Amps on the inner wire of radius 2 cm. The outer cylindrical wire of radius 5cm carries the return...
  39. H

    Exploring Pressure Variations of CO2 Gas at 200°C

    A container of 0.5l containing 6 moles of CO2 are heated to 200 ° C on is interested in explaining the pressure variations between the pressure of a perfect gas and the actual value measured on a pressure gauge. The coefficients of the real gas a=3.59 [atm l2/mol2-] b=0.0427 [l /mol] indicate...
  40. T

    A Relation of ionisation voltage vs. pressure for hydrogen

    Suppose we have specific amount of hydrogen gas enclosed inside a metallic chamber and that is connected to a very high positive voltage source. As the voltage is positive and that's so high that all the molecules inside the chamber lost their electrons and there is nothing but a nuclei gas is...
  41. I

    How to determine the pressure drop in pipe diameter sizing

    Hi all, I'm a tinkerer with no formal engineering background, but enjoy read about physics and engineering in general. I'm reading up on fluid flow and I'm a bit grasping at the concepts. As far as I understand it, flow rate, pressure drop and pipe size are all interrelated, where one affects...
  42. E

    B How to determine force based on pressure

    Hello In a pharmaceutical lab,a certain pressure is applied to IV bags using the equipment shown below.We need to calculate the force acting on the bags based on the applied pressure.I know the formula is F=PxA.But i am not sure what the surface area is.Should i take the whole surface area of...
  43. .Scott

    Impact of alcohol and reduced cabin pressure on flights?

    In a recent news article, a Frontiers Airline passenger became very unruly after two alcoholic drinks. On an airline flight, the only ones instructed on the combined effects of alcohol and altitude are the cabin crew. This is what the FAA has to say to them: Since issues related to alcohol...
  44. PainterGuy

    I The pressure exerted by radiation

    Hi, I was reading about the pressure exerted by radiation and need help with few points. I'd appreciate if you could help me. Source: Proceedings of the American Academy of Arts and Sciences, Volume 38, By American Academy of Arts and Sciences , 1903 You can check this capture for more...
  45. S

    Diesel Injector and Water Tube Boiler Back-Pressure

    Diesel injector: How much pressure relative to the maximum combustion chamber pressure during the sharpest phase of detonation does an injector need to be able to overcome to inject its full charge? How much overlap is typical between injection and detonation? Water-tube boiler: Are the water...
  46. P

    Pressure change in pipe due to sudden closure of valve

    Water is flowing in the pipe with velocity v0. Upon sudden closure of the valve at T, a pressure wave travels in the -ve x direction with speed c. The task is to find ##\alpha##, where ##\Delta P = \rho_0 c (\Delta v) \alpha##. The 1st step is to set up an equation using conservation of mass...
  47. B

    Saturated vapour pressure vapour quality and T-S diagram location

    Hi all., Just hoping to get a better fundamental insight into a few things. If we start with this: http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/vappre.html so we have a closed container at a given temperature, then we can find it's saturation pressure. All good so far. In the following...
  48. jjirving

    I Grip Strength: Comparing Pressure Needed to Hold 20kg Weight Plate

    I am interested to compare two scenarios relating to grip strength and how much horizontal pressure will be required to hold the weight. Scenario 1: You have a 20kg weight plate in one piece. You hold the plate between your fingers. Scenario 2: You are holding a small 2X4 block of wood with a...
  49. tmalcolm

    Force pressure variance after hole is plugged?

    There is a tall cylinder filled with water. And there is a 3 in diameter hole near the bottom and water is gushing out. (assume the cylinder is continually being re-filled from the top) You work to plug the hole with a 10 inch long cylinder that is exactly the perfect diameter fit to plug the...
  50. agnimusayoti

    How to Measure Very Small Differential Pressure in a Subsonic Wind Tunnel

    I assume that air have ##1 kg/m^3## density. Therefore, using Bernoulli equation, on upside and downside of my test object, there is a differential pressure ##\Delta P##: $$\Delta P=0.5*(v_2^2 - v_1^2)$$ From cases: (a) ##v_1 = 1 m/s## and ##v_2 = 2 m/s##, then ##\Delta P = 1,5 Pa## (b) ##v_1 =...
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