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. V

    Bouyency and Water Discplacement of 2 Fluids

    Homework Statement A boat floating in fresh water displaces water weighing 37.3 kN. What is the difference between the volume of fresh water displaced and the volume of salt water displaced (Vfresh - Vsalt)? F(bu) = 37.3E3 kg/m^3 (rho)fresh water = .998E3 kg/m^3? (rho)salt water =...
  2. A

    Physics and Fluids: Accelerating Particles in a Fluid

    Homework Statement Very small particles moving in fluids are known to experience a drag force proportional to speed. Consider a particle of net weight W dropped in a fluid. The particle experiences a drag force, Fd = kV, where V is the particle speed. Determine the time required for the...
  3. G

    Velocity & Volume Change in Ideal Fluids

    Homework Statement In an ideal fluid, the pipe length is doubled, while radius is decreased by factor 2 what will have to the velocity and volume that is flowing? Homework Equations Q=Av = pi*r^2*v The Attempt at a Solution From this the volume that will pass, in ideal fluid cases...
  4. A

    Find Oil Density in Open Tank with Horizontal Bottom

    An open tank has a horizontal bottom. It is divided into two parts by a vertical gate. The gate is rectangular, 3m high and it is hinged at its top. Water of density p,=1000kg/m3 in one part of the tank is acting on one side of the gate, and oil of density p, in the other part of the tank is...
  5. C

    Understanding Buoyancy Force in Static Equilibrium: Explained

    Hi - I'm having trouble with the bouyancy force. I understand that in static equilibrium a force from below a submerged object (not resting on the floor of a container but in the liquid) equals the overlying liquid plus the weight of the object. So that the bouyancy force is the difference of...
  6. P

    How Does a Hydraulic Lift Work with Different Piston Sizes?

    Okay, so I'm stuck on the last part of the problem : A hydraulic lift has two connected pistons with cross-sectional areas 15 cm2 and 430 cm2. It is filled with oil of density 620 kg/m3. -------------------------------------------------------------------------------- a) What mass must...
  7. M

    Plasma: MHD; what about neutral fluids?

    When I have an ionized fluid (plasma) and I describe it in a continuum approach I can use the MHD equations (the continuity equation, the equation of motion, the induction equation and the energy equation). However; I am modeling a certain model of a sunspot and there is also an unionized...
  8. B

    Divergence Theorem and Incompressible Fluids

    Homework Statement Hi, I'm trying to follow the proof for the statement \nabla . u = 0 I'm basing it off this paper: http://delivery.acm.org/10.1145/1190000/1185730/p1-bridson.pdf?key1=1185730&key2=4151929021&coll=GUIDE&dl=GUIDE&CFID=25582973&CFTOKEN=82107744 (page 7, 8) In...
  9. M

    Question regarding ideal fluids vs. non-ideal fluids

    I understand that in an ideal fluid, when the cross-sectional area of the pipe is increased, the pressure that the fluid exerts on the walls of the pipe also increases. Also, when the cross-sectional area of the pipe is decreased, the pressure that the fluid exerts on the walls of the pipe...
  10. B

    Need help with fluids and pressure problem

    [SOLVED] need help with fluids and pressure problem Homework Statement The L-shaped tank shown below is filled with water and is open at the top. (a) If d = 4.0 m, what is the force on face A due to the water? N (up) (b) What is the force on face B due to the water? N (right)...
  11. M

    How Deep Can a Diver Go with a Snorkel Based on Lung Pressure Differential?

    Homework Statement THe human lungs can operate w a pressure differential of abt 1/20 of an atmosphere. If a diver uses a snorkle for breathing, about how far below the water level can they swim? Homework Equations P=P(i) + pgh where p is ro, density. The Attempt at a Solution...
  12. J

    What is the difference in height between two liquids in a U-tube?

    Homework Statement A simple U-tube that is open on both ends is partially filled with a liquid of density (491 kg/m^3). Water is then poured into one arm of the tube, forming a column height of (8.2 cm). The density of the heavy liquid is (1000 kg/m^3). What's the difference , h, in the...
  13. N

    Understanding Fluids: The Weight of Steel Balls in Alcohol and Oil

    Hello all, I have a very basic question about fluids. Here it is. There are two containers, one has alcohol in it and the other has oil. A steel ball is held down inside of each. A scale will weigh the steel ball as it is held down in the alcohol and then in the oil. Will the ball be...
  14. G

    I'm inventing my own physics problem for extra credit thermodynamics and fluids

    1. A man falls into a tank at 40 degrees C. His body temperature is 98.6 degrees Fahrenheit. I want to ask if the man will freeze to death, but I don't know at what temperature that happens. Can I ask them to figure out what his final body temperature is, if say he doesn't get rescued in 10...
  15. C

    Buoyant force of pool (fluids)

    Homework Statement An above-ground backyard swimming pool is shaped like a large hockey puck, with a circular bottom and a vertical wall forming its perimeter. The diameter of the pool is 5.2m and its depth is 1.4m. Find the total outward force exerted on the vertical wall of the pool...
  16. N

    Differential equation and fluids

    Homework Statement Hi guys Please take a look at the second question in 10.20 (the question with the diff. equation)...
  17. L

    Fluids in motion (Bernoulli's equation)

    1. The time required to fill a bucket with water from a certain garden hose is 30.0 s. If you cover part of the hose's nozzle with your thumb so that the speed of the water leaving the nozzle doubles, how long does it take to fill the bucket? 2. Homework Equations : None required. I hope...
  18. S

    Solve for Mass and Density of a Floating Cylinder in Fresh Water

    Homework Statement A solid cylinder 350mm in diametre and of total length 750mm floats upright in fresh water to a depth of 500mm determine (a) the mass of the cylinder.(b)the density of the material in the cylinder. Homework Equations mg=pgha The Attempt at a Solution area= lxd...
  19. G

    Drag can happen in extremely low-viscosity fluids

    First, let me introduce myself. I am new to these forums, but not new to sharing ideas that cause argument. If you think anything I say is wrong, please speak up. The reason I joined was because of this outrageous 2+ year old thread: https://www.physicsforums.com/showthread.php?t=70227 The...
  20. P

    Solve Fluids & Forces Problem | Physics Forum

    [SOLVED] Fluids and Forces Hi, I'm new to this site, so I'm not sure if this is the right place to post. I found a year-old entry with the same question that I'm having trouble with, found here: https://www.physicsforums.com/showthread.php?t=153538" For Part a, I followed the same method...
  21. C

    Fluids: Buoyancy Homework - Prove Archimedes' Principle

    Homework Statement Using Archimedes' Principle and the diagram shown: (diagram is a "mass" of something in a liquid with an immersed portion (Vb) and an Un-immersed portion (Va)). A. prove that if an object is floating in a liquid, the fraction of the object's total volume that is...
  22. A

    Fluids at Rest: Calculating the Force Required to Hold Wood Underwater

    Fluids at rest problem help! Homework Statement How much force does it take to hold 0.5 m^3 of medium density (specific gravity = 0.77) wood under water? The Attempt at a Solution density of wood is 770 kg/m^3. Fb is the force of buoyancy that pushes the wood up. F-Fb = mg...
  23. N

    Solve Fluid Problems on Titan and Underwater Colony | AP Physics Worksheet

    Hi, I am having some trouble with a couple problems from an AP Physics worksheet. I don't really think that they are actually that hard overall but I'm justhaving a hard time tryng to figure out where to start. Here they are: A 90,000 kg structure is built on stilts on the surface of Saturns...
  24. I

    How quickly will a can sink in water with a small hole at the bottom?

    Homework Statement A cylindrical can of height h and base area A is immersed in water to a depth ho and left to sink down. A small hole of area 'a' exists at the bottom of the base of the can. Determine how quickly the can sinks. http://img80.imageshack.us/my.php?image=fluids1wb7.jpg...
  25. G

    How Does Terminal Velocity Arise in Fluid Dynamics?

    heheh...I need some help understanding something. If there was an object, such as a sphere, in water, gravity would be pushing it downwards, while a resistive force R = -bv would be pushing it upward (b as a constant). that would imply that the net Force Fy would be Fy = mg - bv = ma =...
  26. I

    Calculating Maximum Depth for a Sphere Dropped into Water

    Homework Statement A small sphere 0.75 times as dense as water is dropped from a height of 11 m above the surface of a smooth lake. Determine the maximum depth to which the sphere will sink. Neglect any energy transferred to the water during impact and sinking. for clarity's sake, I'm...
  27. S

    Static Equilibrium in Fluids: Pressure and Depth

    A cylindrical container 1.2 m tall contains mercury to a certain depth, d. The rest of the cylinder is filled with water. If the pressure at the bottom of the cylinder is 1.9 atm, what is the depth d? Does anyone have any ideas on how to approach this problem, better yet,solve it!?
  28. I

    I with a static fluids question

    [SOLVED] i need help with a static fluids question Homework Statement A block of wood of 1.8 kg mass floats on water with 64% of its volume submerged. A lead block is placed on the wood and the wood is then fully submerged. Find the mass of the lead block. Homework Equations Fb =...
  29. P

    Fluids - Head Loss complicated

    Homework Statement Heated air at 1 atmosphere and 35 deg. Celsius is to be transported in a 150 meter long circular plastic duct (smooth) at a rate of 0.35 cubic meters/sec. If the head loss in the pipe is not to exceed 20 meters, determine the minimum diameter of the duct. Homework...
  30. N

    How Does Temperature Variation Affect Surface Tension and Flow in a Liquid Film?

    could not solve this exam problem completely please help: Variations in surface tension can cause a flow in a liquid.One of the chief causes for variation of surface tension is temperature.Consider a thin film of liquid on a flat surface.The liquid film also has a free surface where the...
  31. K

    Calculating Pressure from Stacked Bricks: Fluids Homework Solution

    Homework Statement A brick weighs 15.0 N and is resting on the ground. Its dimensions are 0.203 m 0.0890 m 0.0570 m. A number of the bricks are then stacked on top of this one. What is the smallest number of whole bricks (including the one on the ground) that could be used, so that their...
  32. Y

    Pressure Calculation for a Container with Water and Olive Oil

    I have solved this problem but am not sure where I'm going wrong here... Suppose you pour water into a container until it reaches a depth of 20cm . Next, you carefully pour in a 9.7cm thickness of olive oil so that it floats on top of the water. What's the pressure at the bottom of the...
  33. B

    Calculating Safe Diving Depths: Understanding Pressure Injuries

    Homework Statement In scuba diving, a regulator is used so that the pressure of the air the diver breathes is close to that of the ambient water. A reckless swimmer decides to use a hose sticking out of the surface to breathe underwater while diving in a lake. When the air pressure in the lungs...
  34. J

    Fluids: Water runs into a fountain

    Homework Statement Water runs into a fountain, filling all the pipes, at a steady rate of 7.52×10−2 m^3/s How fast will it shoot out of a hole 4.53cm in diameter? Homework Equations A1V1 = A2V2 The Attempt at a Solution A1V1 = 7.52x10-2 m^3/s = A2V2 Divide by A2, which...
  35. C

    Fluids and pressures (artery question)

    Homework Statement The diameter of a certain artery has decreased by 22% due to arteriosclerosis. (a) If the same amount of blood flows through it per unit time as when it was unobstructed, by what percentage has the blood pressure difference between its ends increased? (b) If...
  36. M

    How Do You Calculate the Radius of a Pipe in Fluid Mechanics?

    Homework Statement Water (n = 1.00 10-3 Pa·s) is flowing through a horizontal pipe with a volume flow rate of 0.029 m3/s. As the drawing shows, there are two vertical tubes that project from the pipe. From the data in the drawing, find the radius of the horizontal pipe. Homework...
  37. S

    Fluids Viscosity: Calculating with a Flat Plate Viscometer

    Homework Statement A flat plate viscometer consists of a disc of diameter 50mm rotating at 300rpm in close proximity to a fixed horizontal plate. When the gap between the plates is 0.1mm, it is found that the torque required to drive the disc is 0.2Nm. What is the viscosity of the liquid...
  38. C

    Fluids problem - gauge pressure

    Homework Statement Water flows steadily from an open tank. The elevation of point 1 is 10.0 m, and the elevation of points 2 and 3 is 2.00 m. The cross-sectional area at point 2 is 0.0480 m^2; at point 3 it is 0.0160 m^2. The area of the tank is very large compared with the cross-sectional...
  39. K

    Modeling Heat Transfer for Different Shapes and Fluids

    Hi, I have nothing to do this summer so I figured I would like to model the heat transfer for different shapes and fluids and compare with experimental values.. :) Anyway I searched on google and found nothing. What are the main problems? For example is it acceptable to assume that the...
  40. 6

    Easy Fluid Problem: Find Stream Function & Pressure

    Homework Statement a. Given potential flow, find stream function and velocity components. b. Compute pressure along the x-axis and plot it. Homework Equations Psi = 2axy The Attempt at a Solution So, I got the psi from phi (above) but i don't understand how I am to go from...
  41. E

    Fluids and pressure with kinematics

    [b]1. There is a cylinder fitted with a piston, the piston has a mass m1 of 0.50 kg and a radius of 2.50 x 10^-2 m. The top of the piston is open to the atmosphere. The pressure underneath the piston is maintained at a reduced (but constant) value by means of the pump. As shown, a rope of...
  42. F

    Quick Questions on Fluids: Pressure, Density, and Hydraulic Lifts Explained

    1. Which of the following is not a unit of pressure? (Points: 1) atmosphere N / m Pascal mm of mercury Well, I would say it's N/m, but I thought it would not hurt to check. 2. When atmospheric pressure increases, what would hapen to the absolute presure at...
  43. F

    Solids and Fluids pressure homework

    Homework Statement In a sample of seawater taken from an oil spill, an oil layer 4.0 cm thich floats on 55 cm of water. If the density of the oil is 0.75 x 10^3 kg/m^3, what is the absolute pressure on the bottom of the container. p= 0.75x10^3 kg/m^3 g = 9.8m/s^2 h=.55m p=? pa =...
  44. D

    Calculating Fluid Velocity in a Constricted Tube

    1. A tube of radius 5 cm is connected to tube of radius 1 cm as shown above. Water is forced through the tube at a rate of 10 liters/min. The pressure in the 5 cm tube is 1×105 Pa. The density of water is 1000 kg/m3. Assume that the water is nonviscous and uncompressible. 2. P=F/A, Q=v1A1...
  45. M

    What is the role of Pauli's exclusion principle in superfluidity?

    Super fluids are fascinating. Got this from Wikipedia, but I have also read it at other sites/books.. "Superfluidity is a phase of matter or description of heat capacity, in which, supercondictivity and "unusual" effects are observed when liquids of typically helium-III or hydrogen overcome...
  46. T

    Question involving a stream of fluids

    When a stream falls from a fosset it thins out near the bottom. Why is that? This eems to apply to any falling fluid.
  47. B

    Computational problems in Fluids and Fields

    Hi there - I'm new to the forum... I need help! I have to research an interesting problem in the topic of fluids and fields which can be solved/investigated through Computational methods. To do this we're supposed to use google - but it's just not coming up with anything. Any suggestions...
  48. S

    Fluids in Rigid Body Rotation

    I have found many cases of fluids entering rigid body motion where the gravity vector is purely down the rotation vector. I am curious if there is a soultion for where the gravity vector is in another direction. I've attempted to solve this myself for a particular rotation, but it is so fast...
  49. T

    Fluids: Nozzle Force Question

    Homework Statement A nozzle has a radius of 25mm. Water emerges at the rate of 750L/min. Find the force with which the nozzle must be held.Homework Equations F = m \times a \Delta m / \Delta t = pAv A_1 \times v_1 = A_2 \times v_2 R = vA The Attempt at a Solution I've so far been able to...
  50. Astronuc

    Continuum Mechanics - Solids and Fluids

    I started reading a great textbook and thought I would share some of its finer points. It is a great introduction to mechanics of solids and liquids, although the title explicitly states Earth and Environmental Sciences. http://www.science.mcmaster.ca/~geo/faculty/emeriti/middleton/index.html...
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