What is Fluids: Definition and 518 Discussions

In physics, a fluid is a liquid or gas or other material that continually deforms (flows) under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear force applied to them.
Liquids form a free surface (that is, a surface not created by the container) while gases do not.
Although the term "fluid" generally includes both the liquid and gas phases, its definition varies among branches. Definition of "solid" vary as well, and depending on field, some substances can be both "fluid" and "solid". Viscoelastic fluids like Silly Putty appear to behave similar to a solid when a sudden force is applied. Also substances with a very high viscosity such as pitch appear to behave like a solid (see pitch drop experiment). In particle physics, the concept is extended to include fluidic matters other than liquids or gases. "Fluid" in medicine and biology specifically means any liquid constituent of the body (body fluid), whereas "liquid" is not used in this sense. Sometimes liquids to be taken for fluid replacement, either by drinking or by injection, are also called "fluid" ("drink plenty of fluids"). In hydraulics, "fluid" is a term which refers to liquids of certain properties, and is a broader concept than "(hydraulic) oils".

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  1. E

    Differentiating the Integral Form of the Continuity Equation for Fluids

    Homework Statement I am working on a problem that asks to use the integral form of the continuity equation (for a steady flow) and show that it can equal this (by taking the derivative of it): dr/r + dV/V + dA/A = 0 where V is Velocity and r is the density. Homework Equations What would...
  2. V

    Weight of a Box between two fluids of different densities

    Homework Statement A box with a height L, and a cross sectional Area A is floating between two fluids of densities p1 and p2. Determine the mass of the box using ho, h1, h2, p1, p2, and A. Homework Equations F= pVg The Attempt at a Solution In the file attached I drew my free...
  3. StevieTNZ

    Unraveling the Mystery of Quantum Fluids: A Look into Superfluidity Theory

    Is there a complete theory for quantum fluids (superfluidity) now? In this book (published 1990) it says "Because of the dominant importance of quantum mechanics for He II that fluid is called a quantum fluid. Its theory is still not completed today. Certain features can still not be...
  4. A

    Static fluids; Density; Buoyancy

    Homework Statement A toy dinosaur floats on water in a container and is partially submerged with 90 % of its volume underwater (submerged). (a) What is the density of the dinosaur? I got this answer right. It is 900 kg/m^3 A layer of oil, with a density of 869 kg/m3, is now...
  5. G

    (Presumably) simple fluids problem

    I am trying to figure out the pressure change in a compressed air line. If I've got 120psi air in a 1/4"ID tube regulated to 800scfh and I change the tube size after a gauge or fitting, to a 1/2"ID tube, what is the pressure of the air being delivered at the end of the 1/2"ID tube? Assume...
  6. L

    Solve Daniel's Fluid Problem: Optimal Tube & Orifice Diameters

    Problem Statement- Given the volumes of two different chemicals; One viscous and one non-viscous. Find the optimal diameter of the tube as well as the orfice that the two chemicals exit from to fill up a drum. Known- volume of the two chemicals density of the two chemicals viscosity...
  7. F

    Physics fluids problem- TODAY

    Homework Statement A very large open water tank has a 50 cm diameter output pipe connected 10 m above the water main which is 6 cm in diameter. If the gauge pressure in the water main reads 9.8 E4 Pa what is the speed at which the water is supplied? The answer should be 0.5 m/s but I can't...
  8. N

    Calculating Fluid Flow Rates and Speeds in a Heating System

    Homework Statement Every second, 1.2 cubic meters of water enters a heating system through a pipe of medium width, A, with a cross sectional area of .2 meters squared. The water then flows into a wide pipe, B, with an area of .6 meters squared, and flows out through a narrow peipe, C, with...
  9. A

    How Does Gingerale's Pressure Change in Descending Pipes?

    Homework Statement Gingerale (ρ=1100 kg/m3) moves through a system of pipes in a beverage factory. Initially, it travels through a pipe with a cross-sectional area of 3.80 cm2 at a speed of 4.77 m/s. Gradually, the gingerale descends by 8.49 m while the pipe's cross-sectional area increases to...
  10. N

    Force of Fluids: 123N Frictional Force

    Homework Statement A fresh water pond that is 16.4 m deep is contained on one side by a cliff. The water has eroded a nearly horizontal "tube" through a bed of limestone, which allows the water to emerge on the other side of the cliff. If the "tube" has a diameter of 3.17 cm, and is located...
  11. P

    Fluids (finding the vel of air in a plane)

    An airplane is cruising at an altitude of 10 km. The pressure outside the craft is 0.292 atm; within the passenger compartment, the pressure is 0.97 atm and the temperature is 20°C. A small leak occurs in one of the window seals in the passenger compartment. Model the air as an ideal fluid to...
  12. P

    How Do You Calculate the Diameter of a Pipe Constriction in Fluid Dynamics?

    1. The inside diameters of the larger portions of the horizontal pipe are 2.90 cm. Water flows to the right at a rate of 2.00 10-4 m3/s. Determine the inside diameter of the constriction. 2. P=P0 + pgh a1v1=a2v2 P + pgh + 1/2pv^2 = constant 3. Since the volumetric flow rate...
  13. N

    Dilatant Fluids: Deriving Formulas for Experiments

    Homework Statement Homework Equations The Attempt at a Solution I'm not using the given template because I have a question to ask, to which I do not know the answer... So; My teacher gave me an assignment where we had to do an interesting and exciting experiment for some small kids, and...
  14. M

    How to Calculate the Density of a Doughnut Submerged in Oil?

    Homework Statement A popular brand of doughnut is made in an assembly-line fashion. First, the white, uncooked doughnut is dropped into hot vegetable oil ( ρ= 949 kg/m3) where it browns on one side as it cooks. It is then flipped over and browned on the other side. The final product still has...
  15. M

    Fluids - Using Navier Stokes

    Incompressible fluid of kinematic viscosity v and density p flows in the x direction between two parallel planes at y = +-h, under the action of an unsteady unidirectional pressure gradient -p(G +Qcosnt), where G, Q, n are constants. Verify that unidirectional motion is possible and that...
  16. R

    Can Atoms in Liquids Be Considered Classically?

    Hey Just wondering why atoms in liquids can be considered classically If you compare the deBroglie wavelength with the atomic spacing, I understand that the deBroglie wavelength should be much smaller than the spacing. But how could you show that? If \lambda = \frac{h}{mv} How...
  17. I

    Fluids - Bernoulli's equation - (First and Third Terms)

    Hello, I am having some difficulty in understanding the terms in Bernoulli's equation. Bernoulli's equation is given as follows: P + 0.5(rho)(V^2) + (rho)gh = constant (along a streamline) The first term (P) is the Static Pressure. The second term (0.5(rho)(V^2)) is the Dynamic...
  18. T

    Ballistics, hydrodynamics - tensile cavitation in tissue and fluids

    I have made decent progress with my ballistics model, but I have run into trouble and I'm not quite sure how to solve it. This particular problem deals with penetration and tensile cavitation. Based on the Held equation for cavitation in jet penetration (still haven't found any sites discussing...
  19. K

    Thermal Conductivity in Supercritical Fluids

    Hi there, Please, I would like to know if the Thermal Conductivity of a liquefied gas increases or decreases when becomes a supercritical fluid. Also, I would like to know how is the relation between the molecular mass and thermal conductivity of a liquefied gas and a supercritical fluid...
  20. R

    Fluids doubt water falling from a tap

    Fluids doubt...water falling from a tap When water falls from a tap its area of cross-section is supposed to decrease progressively in accordance with the equation of continuity A1V1=A2V2 in order to maintain the conservation of mass. But using another line of thought, every particle in a...
  21. C

    Calculating Gas Flow Rate in a 180km Stainless Steel Natural Gas Pipeline

    1. The pressure at the start of a 180km stainless steel natural gas pipeline is 4Mpa and at the end is 2Mpa. If the diameter of the pipe is 0.4m, what will be the gas flow rate through the system? (ignore losses other than friciton) (for natural gas, \rho= 40kg/m3, \mu= 11x10-6 Pas2...
  22. C

    Fluids; pipe flow rate problem

    1. The pressure at the start of a 180km stainless steel natural gas pipeline is 4Mpa and at the end is 2Mpa. If the diameter of the pipe is 0.4m, what will be the gas flow rate through the system? (ignore losses other than friciton) (for natural gas, \rho= 40kg/m3, \mu= 11x10-6 Pas2...
  23. J

    Finding Bulk Modulus of Refinery Fluids

    We are studying the waterhammer in a Refinery. Since the bulk modulus is involved in this study, we will like to know where to find the bulk modulus at different temperatures and pressures for several refinery fluids such as kerosene, diesel, atmospheric and vacuum residue. We have found some...
  24. O

    Heat Exchange between Fluids (and similar questions)

    First let me apologize for any spelling mistakes as I am on a mobile device. Second I do have a few questions regarding thermodynamics for a project I would like to begin. 1) is there a way to calculate a volume of water in either the amount of time and the amount of energy required to make...
  25. X

    Fluid in Equilibrium: Internal Motions, Velocity & Beyond

    Suppose I have a fluid of uniform density confined to a roating cylinder of radius r turning at an angular speed of \Omega. Now I am considering the fluid in the rotating frame and I know that the fluid is in equilibrium. What does this imply? If the fluid is in equilibrium are there any...
  26. X

    How Does a Homogeneous Fluid Behave in a Rotating Cylinder?

    Here is the problem: (Question 5-2 in Binney&Tremaine Galactic Dynamics) Consider a homogeneous self-gravitating fluid of uniform density \rho_0 contained within a rotating cylinder of radius R_0. The cylinder and the fluid rotate at angular speed \Omega about the axis of the cylinder, which...
  27. A

    Mechanical engineering help with fluids

    Hi people! I have been doing some fluid questions and got stuck ont he following two; 1) A Casson plastic fluid is subjected to a shear force of 5.5 Pa. If the fluid obeys a law of the form T =1.5 +0.2^0.45u, determine the shear rate and apparent viscosity under these conditions 2)...
  28. T

    Hydraulic press and compressible fluids

    Hi everyone, I did not get a reply in 3 weeks under mechanical engineeting... trying here. Here is practical question mixing compressible and none compressible fluids with a hydraulic press. Can the following actually work ? I believe it can but some experience people in this field looking...
  29. L

    Determination of Grease Viscosity Using Sliding Block Experiment

    Homework Statement A small block is attached on the left of a larger block by a thin layer of grease, 2 micrometres (10^-6) thick. The mass of the smaller block is 0.15 kg and the area of contact is 8x10^-4 metres squared. If the smaller block slides down 5 mm in 2 weeks, calculate the...
  30. G

    Fluids motion HARD. i attached the file as a PDF check picture from Q4

    Homework Statement µ Q.4 An incompressible fluid of density ρ flows steadily through a two-dimensional infinite row of fixed vanes, a few of which are shown below. The vane spacing is a. The velocities V1 and V2 and pressures p1 and p2 are all constant along their respective stations, (1) and...
  31. A

    Spinning Super Fluids: Reach Relativistic Speeds?

    Since there is no friction on a super fluid, can they be spun faster and faster until they reach relativistic speeds? I'm not totally sure the method you would use to spin the fluid, possibly electromagnatism somehow but assuming it were possible.
  32. T

    Hydraulic press and compressible fluids

    Hi everyone, Here is practical question mixing compressible and none compressible fluids with a hydraulic press. Can the following actually work ? I believe it can but some experience people in this field looking at it may prove me wrong. Say we have a sphere, 1 cubic meter in volume. The...
  33. L

    Fluids Physics Question: Calculating Density and Pressure of a Physics Book"

    Homework Statement A physics book has a height of 26.0 cm, a width of 22.4 cm, and a thickness of 3.10 cm. (a) What is the density of the physics book if it weighs 18 N? ? kg/m3 (b) Find the pressure that the physics book exerts on a desktop when the book lies face up. ? Pa (c) Find the...
  34. S

    Fluids Archimedes principle apparent weight question

    Homework Statement Vessel contains water.It is placed on the pan of a physical balance.A solid(specific gravity=1) is suspended from an thread(from an independent support),such that it is immersed completely in the water. What the weight it reads? If the thread was tied to the arm of...
  35. H

    Solids and Fluids: Siphoning Gas

    Homework Statement A siphon tube is filled with gasoline and closed at each end. One end is inserted into a gasoline tank 0.20 m below the surface of the gasoline. The outlet is placed outside the tank at a distance 0.50 m below the surface of the gasoline. The tube has an inner...
  36. H

    How long does it take to pour 4 liters of SAE no. 10 oil through a funnel?

    Homework Statement SAE no. 10 oil has a viscosity of 0.2 Pa· s. How long (in sec) would it take to pour 4 liters of oil through a funnel with a neck 10 cm long and 2.9 cm in diameter. Assume that it is poured in such a way that the oil level is kept just above the top of the tube. Hint...
  37. I

    How can I solve two fluid problems involving a raft and pressure?

    Hey, these two questions are off a take home test I was given. I could use a good grade on them so could anyone give me a hand? thanks A raft with a length of 8.0m, a width of 1.6m, a thickness of 0.11m, and a density of 690.0kg/m^3 is placed in a river. How far below the water's surface does...
  38. J

    Estimating Pressure on a Person Standing on the Ground

    Homework Statement What pressure does a person standing on the ground exert? Homework Equations The Attempt at a Solution This has something to do with fluids, but I'm not sure what! I think I should model the person as a cylinder and find the pressure on the person, then find what the...
  39. D

    Pressure at a centroi thermo fluids

    ive got this q in my homework calculate the pressure at the centroid and hence the force acting plate. the surface is free liquid is water. ive using a formula l x e = y l being length or depth in this case e being value of water which i found to be 1 x 10 to power of 3 kg/m3 y being the...
  40. V

    Reducing Water Velocity Without Reducing Flow: Finding the Right Valve

    Hi all, Need a bit of help with a problem. I have a 4 inch pipe (carrying water). On the end of this 4 inch pipe is a 2" orifice. Now flowing through this orifice is water at 50L/s at a head of 100m (790 GPM and 330 feet). Basically as you can imagine, there is a lot of energy behind this...
  41. M

    Uncovering the Mystery of Fluids with Varying Densities

    I don't know the answer to this because i can't find it on the net or textbook, i need to know a type of fluid that is one where density varies throughout the fluid system? (Eg, a fluid system of oil and water among others) Thank
  42. K

    Fluids Mechanics - Determine algebraic formula for velocity

    Homework Statement A block of weight W is being pulled over a table by another weight Wo, as shown in the figure. Find an algebraic formula for the steady velocity U of the block if it slides on an oil film of thickness h and viscosity μ. The block bottom area A is in contact with the oil...
  43. M

    Programs Learning about fluids as a physics major

    Hopefully I'm posting this in the correct forum but I was just wondering, when would would a physics major learn about fluids? We covered it a little bit in high school but I don't see fluids at all in my university's curriculum.
  44. J

    Calculating Shear Rate in Rotating Cylinder

    Hi, I'm trying to get an expression for the shear rate in a rotating cylinder (as pictured) in terms of the inner cylinder radius, outside cylinder radius and angular velocity. The inside cylinder remains fixed and only the outer cylinder rotates. Assume that the distance x is small. Any...
  45. B

    Calculating Force and Distance in a Hydraulic Lift System

    Homework Statement In a hydraulic lift, the radii of the pistons are 2.50 cm and 10 cm. A car of mass 1000 kg is to be lifted for inspection. b) what force must be applied to the proper piston to be able to lift the car with ease? C) If the smaller piston is pushed by 10 cm, how far is the...
  46. H

    How Does Immersion in Liquid Affect Body Pressure Under Acceleration?

    Hi, I'm trying to investigate how the force exerted onto a human body is affected by the presence of a surrounding liquid. Since the liquid can't compress the pressure is distributed in all directions. If a person is accelerated with this liquid, he/she + the mass of liquid will feel a force...
  47. H

    Equation manipulation, fluids

    I am not entirely sure how some of these steps work. What I have so far (pardon any latex mistakes, I am new to it) K=\frac{\Delta P}{\Lambda Vol /Vol} change in pressure over volumetric strain = K the bulk modulus \rho Vol'= d \rho / dt (\frac{d P}{d t}/\frac{d \rho}{d t}*)\rho=K...
  48. L

    Need MORE Hmwk helpFluid mechanics, Fluids in motion, pressure

    College Physics Homework Statement The figure shows a section of a long tube that narrows near its open end to a diameter of 1.0mm. Water at 20celsius flows out of the open end at 0.020 L/s What is the gauge pressure at point P, where the diameter is 4.0mm...
  49. L

    Need Hmwk HelpFluid mechanics, Fluids in Motion, Pressure

    College Physics Homework Statement The figure shows a syringe with a 4.0-cm-long needle, filled with water at 20celsius. What is the gauge pressure of the water at the point P, where the needle meets the wider chamber of the syringe...
  50. C

    Fluids - Pascal's Principle / Negative vs Positive

    Homework Statement The handle of a hydraulic jack is 15 cm long and is pivoted 2.5 cm from the input piston which has a radius of 0.60 cm. The output piston has a radius of 1.2 cm. What weight could be lifted by the jack if the person pushing on the handle is to exert no more than 110 N of...
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