KIN 158 Ch. 8
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- Force due to immersion of an object in a fluid
- Always acts vertically upward
- The unit of measurement is a Newton (N)
Equal to the weight of something. Displaced- "water has moved." Take an object and place in water,
"the magnitude of the buoyant force equals the weight of an equal volume of water displaced by the object."
- The ratio of the weight of an object to the weight of an equal volume of water displaced by the object.
- Specific gravity of water is 1.0.
- No unit of measurement.
- Ratio of mass to volume.
- Unit of measurement is a kilogram per meter cubed (kg/m3)
- Density of water is about 1000 kg/m3. Density of air is only about 1.2 kg/m3.
Sinking and Floating
- Sinking or Floating is determined by volume of the object immersed and weight of an equal volume of water displaced by the object.
- Less than 1.0 will float, greater than 1.0 will sink.
Buoyancy of the Human body
- Definition- weight of fluid that the object has emerged and displaced.
- Body Composition- Muscle/Bone is greater than 1000 kg/m3. Fat is less than 1000 kg/m3.
- Therefore, body composition is a major factor in determining your tendency to sink of float to the surface.
Dynamic Fluid Forces
- The buoyant force is the vertical force exerted on an object immersed in a fluid and is present whether the object is at rest or is moving relative to the fluid.
- When an object moves within a fluid (or when a fluid moves past an object immmersed in it), dynamic fluid forces are exerted on the object by the fluid.
- When analyzing dynamic fluid forces, you must consider: velocity of the object and velocity of the fluid.
- Relative velocity is used to represent the effects of these two absolute velocities.
- Relative velocity is the difference between the object's absolute velocity and the fluid's absolute velocity.
- Unit of measurement is m/s.
Dynamic Fluid Force
- Is commonly resolved into two components: drag force and lift force.
- Unit of measurement for either force is a Newton (N).
The component that acts in opposition to the relative motion of the obect with respect to the fluid.
Drag Force (2) continued...
Drag forces are produced by two different mechanisms: surface drag and form drag
- Is the sum of the friction forces acting between the fluid molecules and surface of the object. (must be tight fitting and smooth)
- It is also called skin friction or viscous drag.
- The coefficient of drag is influenced by several factors associated with surface drag: Roughness of the surfance and viscosity of the fluid (how thick the fluid is).
- Is the sum of the impact forces resulting from the collision between the fluid molecules and the object.
- It is also called shape drag, profile drag, or pressure drag.
- Type of fluid flow influences the magnitude of form drag: Laminar Flow (smooth, flowing in one direction) and Turbulent Flow (can't predict the flow).
Form Drag (2) continued...
- Factors influence the magnitude of form drag: shape of the object and surface texture.
- At low velocities, an object with a rougher surface will experience greater form drag. Sometimes, however, a rougher surface will actually decrease form drag.
- Less than 20 mph is surface drag. More than 20 mph is form drag.
Strategies for Reducing Drag Force
- Reduce Fluid Density- high altitude, warm-weather, low humidity.
- Reduce the Coefficient of Drag- make body surfaces and clothing smoother.
- Reduce Cross-Sectional Area for Drag- reduce area being hit by the fluid. Streamline the shape of the body or equipment.
Strategies for Reducing Drag Force (2) continued...
- Reduce the Relative Velocity
- Because this term is squared, it has the greatest effect on drag force, so
- It is the most important variable that the athlete can control
- The method for reducing relative velocity is known as drafting.
Strategies for Reduce Drag Force (3) continued...
- Form drag- faster velocities and Surface Drag- slower velocities.
- If greater than 10 m/s or 20 mi/hr, go for the streamlined shape which reduces the cross-sectional area for drag.
- Otherwise, try to reduce the coefficient of drag.
- The dynamic fluid force component that acts perpendicular to the relative motion of the object with respoect to the fluid.
- Examples: race car, boat, skying, etc.
- faster-moving fluids exert less pressure laterally than do slower-moving fluids.
- The "Airfoil"- the lateral pressure exerted by the faster-moving molecules is less than that exerted by the slower-moving molecules. An upward force is generated.
- Air moves from high to lower pressure.
Lift for Object Shapes other than Airfoils
- Lift is caused by the lateral deflection of fluid molecules as they pass the object.
- The object exerts a force on the molecules that causes this lateral deflection.
- According to Newton's 3rd law, an equal but opposite lateral force is exerted by the molecules on the obejct.
- This is the lift force.
- Lift: change of Direction, Drag: change of speed.
- Lift and Drag is caused when angle of attach occurs.
- The Magnus Effect
- Lift forces are also genearted by spinning balls. These lift forces are known as "Magnus Forces"
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