Resistance In Fluid Systems. Principles of Technology. All content was received from Physics In Context. When one solid object slides against another, a force of friction opposes the motion When a solid object moves through a fluid, there is also a force that opposes the motion. Examples
Principles of Technology
All content was received from Physics In Context
When a solid object moves through a fluid, there is also a force that opposes the motion.
Boat moves through water
Airplane moves through air
You can feel drag when you stand in high wind
Or when you put your hand out the window of a moving carDrag
Speed of the object (or fluid)
Size and shape of the object
Physical properties of the fluid
These factors make it difficult to calculate drag exactly.
You can Approximate
Simplest approximation is to ignore drag forces when they are small
Ignore drag for an object moving slowly in fluids such as air or water
Although very slow speeds
produce significant drag in fluids such as motor oilLaminar & Turbulent Flow
Laminar (streamlined flow)
is a slow, smooth flow over a surface, in which the paths of individual particles do not cross
Fluid speed at surface is zero
Frictional Drag: Drag is produced by friction between layers of fluid
Is irregular flow with eddies and whorls causing fluid to move different directions
Pressure Drag: This pressure difference causes a force to act on the object in the direction opposite its relative velocity.
(Doubling speed = Doubling frictional force)
The drag force on a car increases as the car’s speed increases
(eta) to represent viscosity
Top plate is pulled to the right at a constant speed v
Layer of fluid of thickness
Bottom plate held in place
The viscosity of a fluid can be measured by pulling a plate at constant speed across a layer of the fluid.
Pg. 188 Chapter 4
SAE Viscosity recommendations for various climates
These oils were chilled to -35 degrees C for 16 hours. The photo was taken 30 seconds after the caps were removed from the containers.
The terminal speed of a falling object is the constant speed that occurs when the drag force equals the gravitational force.
The terminal speed of a baseball is about 40 m/s, but the terminal speed of a basketball is only about 20 m/s.
Which ball has a greater drag force at any given speed?
Layers nearer the wall move more slowly
Fluid in contact with the wall does not move
The fluid layer at the center moves the fastest
= the change in pressure of the fluid as it flows the length L
Is negative therefore V is positive
Volume flow rate is inversely proportional to length
Due: April 15, 2008
At the beginning of class
Problems 1-15 (show work)