1. Resistance in Fluid Systems
4.2

2. Drag
The force that opposes the motion when a solid object moves through a fluid
Occurs only when there is relative movement between an object and a fluid

3. Laminar Flow (Streamlined)
Slow, smooth flow over a surface in which the paths of individual particles do not cross
Fluid speed at the surface is zero.
Fluid moves in theoretical layers, or laminates, with increasing speed away from the surface.
Frictional drag is produced by the friction between successive layers of fluid.

4. Turbulent Flow
irregular flow with eddies and whorls causing fluid to move in different directions
produced by high speeds, by shapes that are not streamlined, and by sharp bends in the path of a fluid
produces wakes
Pressure drag is produced because the pressure difference in the wake is less than the fluid pressure in the streamlined flow and causes a force to act on the object in the direction opposite its relative velocity.
Frictional drag and pressure drag both increase as speed increases.
Frictional drag is directly proportional to the increase in speed. Pressure drag (and turbulence) increases as the square of the speed.

5. Viscosity property of a fluid that describes internal friction
normally how slowly a fluid moves because of internal bonds
Greek letter η (eta) represents viscosity.
Syrup and ketchup have relatively high viscosities; water and air have relatively low viscosities.

6. Viscosity
Fdrag = F = η (Av/Δy) (Drag force = viscosity X area · velocity divided by the change in the thickness of the fluid layer)
SI Units—N/m2·s or Pa·s—English Units—lb/ft2·s or psi·s
ηwater = N/m2·s = poise = centipoise
Viscosity for Common Fluids—p. 188
Viscosity of most liquids decreases as temperature increases—ex: honey
Viscosity of most gases increases as temperature increases—more collisions per second as temp increases

7. Stokes Law Predicts the drag force on a sphere moving through a fluid
Low speeds, so only laminar flows—no turbulence, just frictional drag.
Drag force acts in the direction opposite the object’s velocity.
Fdrag = 6πrvη-Drag force = 6 X pi X radius X velocity X viscosity

8. Terminal Speed When drag equals the weight of the object.
Speed becomes constant.
Terminal speed of baseball—about 40 m/s
Terminal speed of basketball—about 20 m/s

9. Poiseuille’s Law
A. gives the volume flow rate of a fluid flowing through a tube or pipe
B. V = -(π/8)(r4ΔP/ηL)—Volume flow rate (change in volume with respect to time) = (pi divided by 8) X ({radius4 X change in Pressure} divided by viscosity X length)

10. Factors Affecting Flow Through a Pipe (pp. 193-195)
radius of pipe
length of the pipe
viscosity of the fluid
R = -ΔP/V—Fluid Resistance = -change in pressure divided by the Volume Flow Rate