KIN 158 Ch. 8

• 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 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.

• 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.

• 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.

• 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 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).

• 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.

• 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.

• 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.

• 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 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"