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[link] shows how viscosity is measured for a fluid. Two parallel plates have the specific fluid between them. The bottom plate is held fixed, while the top plate is moved to the right, dragging fluid with it. The layer (or lamina) of fluid in contact with either plate does not move relative to the plate, and so the top layer moves at v size 12{v} {} while the bottom layer remains at rest. Each successive layer from the top down exerts a force on the one below it, trying to drag it along, producing a continuous variation in speed from v size 12{v} {} to 0 as shown. Care is taken to insure that the flow is laminar; that is, the layers do not mix. The motion in [link] is like a continuous shearing motion. Fluids have zero shear strength, but the rate at which they are sheared is related to the same geometrical factors A size 12{A} {} and L size 12{L} {} as is shear deformation for solids.

The figure shows the laminar flow of fluid between two rectangular plates each of area A. The bottom plate is shown as fixed. The distance between the plates is L. The top plate is shown to be pushed to right with a force F. The direction of movement of the layer of fluid in contact with the top plate is also toward right with velocity v. The fluid in contact with the plate in the bottom is shown to be in rest with v equals zero. As we see through the layers above the one on the bottom plate, each show a small displacement toward right in increasing order of value with the topmost layer showing the maximum.
The graphic shows laminar flow of fluid between two plates of area A size 12{A} {} . The bottom plate is fixed. When the top plate is pushed to the right, it drags the fluid along with it.

A force F size 12{F} {} is required to keep the top plate in [link] moving at a constant velocity v size 12{v} {} , and experiments have shown that this force depends on four factors. First, F size 12{F} {} is directly proportional to v size 12{v} {} (until the speed is so high that turbulence occurs—then a much larger force is needed, and it has a more complicated dependence on v size 12{v} {} ). Second, F size 12{F} {} is proportional to the area A size 12{A} {} of the plate. This relationship seems reasonable, since A size 12{A} {} is directly proportional to the amount of fluid being moved. Third, F size 12{F} {} is inversely proportional to the distance between the plates L size 12{L} {} . This relationship is also reasonable; L size 12{L} {} is like a lever arm, and the greater the lever arm, the less force that is needed. Fourth, F size 12{F} {} is directly proportional to the coefficient of viscosity , η size 12{η} {} . The greater the viscosity, the greater the force required. These dependencies are combined into the equation

F = η vA L , size 12{F=η { { ital "vA"} over {L} } } {}

which gives us a working definition of fluid viscosity     η size 12{η} {} . Solving for η size 12{η} {} gives

η = FL vA , size 12{F=η { { ital "FL"} over { ital "vA"} } } {}

which defines viscosity in terms of how it is measured. The SI unit of viscosity is N m/ [ ( m/s ) m 2 ] = ( N/m 2 ) s or Pa s size 12{N cdot "m/" \[ \( "m/s" \) m rSup { size 8{2} } \] = \( "N/m" rSup { size 8{2} } \) "sorPa" cdot s} {} . [link] lists the coefficients of viscosity for various fluids.

Viscosity varies from one fluid to another by several orders of magnitude. As you might expect, the viscosities of gases are much less than those of liquids, and these viscosities are often temperature dependent. The viscosity of blood can be reduced by aspirin consumption, allowing it to flow more easily around the body. (When used over the long term in low doses, aspirin can help prevent heart attacks, and reduce the risk of blood clotting.)

Laminar flow confined to tubes—poiseuille’s law

What causes flow? The answer, not surprisingly, is pressure difference. In fact, there is a very simple relationship between horizontal flow and pressure. Flow rate Q size 12{Q} {} is in the direction from high to low pressure. The greater the pressure differential between two points, the greater the flow rate. This relationship can be stated as

Q = P 2 P 1 R , size 12{Q= { {P rSub { size 8{2} } - P rSub { size 8{1} } } over {R} } } {}

where P 1 size 12{P rSub { size 8{1} } } {} and P 2 size 12{P rSub { size 8{2} } } {} are the pressures at two points, such as at either end of a tube, and R size 12{R} {} is the resistance to flow. The resistance R size 12{R} {} includes everything, except pressure, that affects flow rate. For example, R size 12{R} {} is greater for a long tube than for a short one. The greater the viscosity of a fluid, the greater the value of R size 12{R} {} . Turbulence greatly increases R size 12{R} {} , whereas increasing the diameter of a tube decreases R size 12{R} {} .

Questions & Answers

Calculate the work done by an 85.0-kg man who pushes a crate 4.00 m up along a ramp that makes an angle of 20.0º20.0º with the horizontal. (See [link] .) He exerts a force of 500 N on the crate parallel to the ramp and moves at a constant speed. Be certain to include the work he does on the crate an
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What is thermal heat all about
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why uniform circular motion is called a periodic motion?.
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mistake, there is enough information to calculate an average velocity
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It is the opposite of kinetic friction
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static fiction is friction between two surfaces in contact an none of sliding over on another, while Kinetic friction is friction between sliding surfaces in contact.
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I don't get it,if it's static then there will be no friction.
author
It means that static friction is that friction that most be overcome before a body can move
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It is a force a body must overcome in order for the body to move.
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Muhammed
force is something which is responsible for the object to change its position
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more technically it is the product of mass of an object and Acceleration produced in it
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energy is distributed in any medium through particles of medium.
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we have to first figure out .... wats a particle accelerator first
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The resistive force of surface.
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on both a string and peldulum?
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spring*
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teach him SHM init
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how.an.equipotential.line is two dimension and equipotential surface is three dimension ?
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definition of mass of conversion
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Force equals mass time acceleration. Weight is a force and it can replace force in the equation. The acceleration would be gravity, which is an acceleration. To change from weight to mass divide by gravity (9.8 m/s^2).
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how many subject is in physics
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the write question should be " How many Topics are in O- Level Physics, or other branches of physics.
effiom
how many topic are in physics
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If u are doing a levels in your first year you do AS topics therefore you do 5 big topic i.e particles radiation, waves and optics, mechanics,materials, electricity. After that you do A level topics like Specific Harmonic motion circular motion astrophysics depends really
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Yeah basics of physics prin8
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yh I need someone to explain something im tryna solve . I'll send the question if u down for it
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hallow
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please send the answer
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I don't know please give the answer
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Practice Key Terms 5

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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