Transcript 4.2 Fluid Friction Notes

Chapter 4.2
Notes
Resistance in Fluids
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When one solid object slides
against another, a force of
friction opposes the motion.
When a solid object moves
through a fluid, there is a also
a force that opposes the
motion. This is called drag.
Examples: boat, airplane
The drag exerted on an object
by a fluid depends on these
factors:
 Speed of the object and fluid
 The size and shape of the
object
 The physical properties of the
fluid (density)
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The flow of a fluid can
be either laminar or
turbulent.
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Laminar or Streamlined Flow is a
slow, smooth flow over a surface in
which paths of individual particles do
not cross.
The fluid speed at the surface is 0 and
increases speed farther from the
surface.
Drag is produced by the friction
between successive layers of fluid.
This is called frictional drag.
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Turbulent flow is irregular flow with
eddies and whorls causing fluid to
move in different directions.
Turbulence is created by high speeds,
irregular shapes, and sharp bends
in the path of fluid.
Turbulence produces the visible wake
behind moving boats and the
invisible wake behind a moving
airplane or car.
The fluid pressure in the wake is less
than the fluid pressure in the flow.
 The lower pressure in the wake causes
a force to act on the object (boat or
car) in the opposite direction to its
velocity. This pressure difference in a
wake is called pressure drag.
 Frictional drag and pressure drag both
increase as speed increases.
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On a microscopic level, friction is
due to electrical forces between
atoms.
 These forces create internal
friction in the fluid, which causes a
resistance to movement.
 Viscosity is the property of fluid
that describes the internal friction.
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Force =
viscosity (Area x velocity)
thickness
F = Vis (A x v)
y
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Viscosity Demo
Viscosity of most liquids decrease
with temperature.
 Example: cold honey is thick
while heated honey becomes
watery.
 As temperature increases, the
molecules in the honey become
less and less tightly bound to each
other.
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On the other hand, Viscosity of
most gases increases with
temperature.
 Forces between gas molecules
are exerted during collisions
and when heated, there are
more collisions.
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Stokes used viscosity to predict
the drag force on a sphere
moving through a fluid when
there was not turbulence.
 According to Stoke’s Law, the
drag force is equal to the product
of a constant, the radius, the
speed, and the viscosity.
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Force = 6 x pi x
radius x velocity x
viscosity (Stoke’s
Law)
 F = 6 r v π vis
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What force is needed to move a
sphere at a speed of 2 m/s through
blood if the sphere has a diameter of
1 meter. Next through lava.
 Viscosity ; page 188
 r = d/2 = 1/2 = .5
 F = 6 r v π vis
 F = 6 x .5 x 2 x 3.14 x .005 = .09
 F = 6 x .5 x 2 x 3.14 x 1000 = 18,840
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When an object moves through a fluid, the
drag force on the object increases as the
speed increases until at some point the
upward drag equals the weight.
 At this point, the forces acting on the
falling object are balanced and the object
no longer accelerates; the speed becomes
constant.
 The terminal speed of a falling object is
the constant speed that occurs when the
drag force equals the gravitational force.
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Poiseuille’s Law gives the volume flow rate
of a fluid flowing through a tube or pipe.
The fluid layer in the center moves the
fastest, and layers near the wall move the
slowest.
Poiseuille’s Law shows how resistance of a
fluid in a pipe depends on 3 factors:
a. Radius – larger radius = less friction
b. Length – shorter length = less friction
c. Viscosity – less viscosity = less friction