Chapter 4.2 Notes Resistance in Fluids

When one solid object slides against another, a force of friction opposes the motion. 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 When a solid object moves through a fluid, there is a also a force that opposes the motion. This is called drag. Examples: boat, airplanedrag

The drag exerted on an object by a fluid depends on these factors: The drag exerted on an object by a fluid depends on these factors: Speed of the object and fluid Speed of the object and fluid The size and shape of the object The size and shape of the object The physical properties of the fluid (density) The physical properties of the fluid (density)

The flow of a fluid can be either laminar or turbulent. The flow of a fluid can be either laminar or turbulent.

Laminar or Streamlined Flow is a slow, smooth flow over a surface in which paths of individual particles do not cross. Laminar or Streamlined Flow is a slow, smooth flow over a surface in which paths of individual particles do not cross. Laminar or Streamlined Flow Laminar or Streamlined Flow The fluid speed at the surface is 0 and increases speed farther from the surface. 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. Drag is produced by the friction between successive layers of fluid. This is called frictional drag.

Turbulent flow is irregular flow with eddies and whorls causing fluid to move in different directions. 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 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. 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 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. 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. Frictional drag and pressure drag both increase as speed increases.

On a microscopic level, friction is due to electrical forces between atoms. 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. 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. Viscosity is the property of fluid that describes the internal friction.

Force = Force = viscosity (Area x velocity) thickness thickness F = Vis (A x v) F = Vis (A x v) y

Viscosity Demo

Viscosity of most liquids decrease with temperature. Viscosity of most liquids decrease with temperature. Example: cold honey is thick while heated honey becomes watery. 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. As temperature increases, the molecules in the honey become less and less tightly bound to each other.

On the other hand, Viscosity of most gases increases with temperature. 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. Forces between gas molecules are exerted during collisions and when heated, there are more collisions.

Stokes used viscosity to predict the drag force on a sphere moving through a fluid when there was not turbulence. 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. According to Stoke’s Law, the drag force is equal to the product of a constant, the radius, the speed, and the viscosity.

Force = 6 x pi x radius x velocity x viscosity (Stoke’s Law) Force = 6 x pi x radius x velocity x viscosity (Stoke’s Law) F = 6 r v π vis F = 6 r v π vis

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. 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 Viscosity ; page 188 r = d/2 = 1/2 =.5 r = d/2 = 1/2 =.5 F = 6 r v π vis 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.005 =.09 F = 6 x.5 x 2 x 3.14 x 1000 = 18,840 F = 6 x.5 x 2 x 3.14 x 1000 = 18,840

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. 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. 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. The terminal speed of a falling object is the constant speed that occurs when the drag force equals the gravitational force.

Poiseuille’s Law gives the volume flow rate of a fluid flowing through a tube or pipe. 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. 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: Poiseuille’s Law shows how resistance of a fluid in a pipe depends on 3 factors: a. Radius – larger radius = less friction a. Radius – larger radius = less friction b. Length – shorter length = less friction b. Length – shorter length = less friction c. Viscosity – less viscosity = less friction c. Viscosity – less viscosity = less friction