Transcript Stokes’ Law - University High School

Resistance in Fluid Systems
Mr. Andracke
Principles of Technology
Viscosity
Viscosity has the units of (pressure)
(time).
The SI units for viscosity are Pa x s
Fdrag =  x Av/ y
Stokes’ Law
Irishmen George Stokes used viscosity and the
equations of fluid flow to predict the drag force on
a sphere moving through a fluid.
Stokes’ Law applies to objects moving at low
enough speeds that the flow of fluid is streamlined
or laminar.
Stokes’ Law
Fdrag = 6rv
6 =Constant for sphere
R = Radius of object
V = Speed of object
 = Fluids Viscosity
Stokes’ Law
Fdrag = 6rv
Example:
Poiseuille’s Law
Poiseuille was a physician, who
experimented with flowing water, and
learned that the rate at which fluid flows
through a tube increases proportionately to
the pressure applied the fourth power of the
radius of the tube.
Poiseuille’s Law gives the volume flow rate
of a fluid flowing through a tube or pipe.
3 Factors of Resistance
In fluid flow resistance decreases the flow
rate though a pipe. Poiseuille’s Law shows
how this resistance depends on three
factors:
Dependence on Radius
 Dependence on Length
 Dependence on Viscosity

Dependence on Radius
The larger the radius of a pipe, the greater
volume of fluid per second
Fluid resistance decreases as pipe radius
and cross-sectional area increase.
Dependence on Length
Longer pipes have higher fluid resistance
Volume flow rate is inversely proportional
to length
Dependence on Viscosity
Abrupt changes in the direction of fluid
flow can cause turbulence and increase
resistance
Fluid resistance increases as viscosity
increases.
Poiseuille’s Law
V = -  r4  
8 L
R = Radius of Pipe or Tube
 = P2 – P1
 = Viscosity
L = Length of Pipe or Tube
Poiseuille’s Law
V = -  r4  
8 L
Example:
Poiseuille’s Law
Help with Poiseuille's Law
http://hyperphysics.phyastr.gsu.edu/hbase/ppois.html
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