Fluid Dynamics

•  All athletic events take place in a fluid environment
• water (swimming), air (cycling), both (water polo)
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• Drag and lift forces are perpendicular to each other, produce different effects, and are affected by different factors

 

 

 

 

 

• Flow velocity –

 

Fluid Drag Forces

• Two types:

 

• Both skin friction and profile drag are proportional to relative flow velocity, cross-sectional area, shape of object, smoothness of surface, and density of liquid

Skin Friction Drag

•  Caused by the fluid tending to rub (shear) along the surface of the body
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• The layer of fluid next to the skin sticks to the body; however, the next layer is towed along and therefore slides relative to the innermost layer
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Laminar flow

• The smooth, layered, flow pattern of a fluid around an object with no disturbance
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Profile Drag

• The main form of drag in skiing, cycling, running , all projectiles events, and swimming
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• Characterized by turbulent flow in which the pressure on the leading surface of a body is greater than the pressure on the trailing surface 

Turbulent flow

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• This causes a large, turbulent, low-pressure zone to form behind the object
• The region of low pressure increases the amount of work done on the object
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Fluid Lift Forces

•  Always directed perpendicular to the flow velocity
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• More air flows over the upper curved surface than the lower flat surface, such that the difference in velocity across the surfaces results in a pressure difference between the two sides
• The external force resulting from the pressure difference is perpendicular to the direction of flow velocity, and can change the motion of the object
• Bernoulli’s principle –

 

 

Angle of Attack

• Refers to the tilt of an object relativeto the flow velocity
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• Function of the shape of an object and the flow velocity
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  If the angle of attack increases too much, it approaches a critical maximum angle (stall angle), beyond which the lift force decreases as the drag force becomes dominant

 

The Magnus Effect

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• The changes in flight path are always perpendicular to the flow velocity of the projectile

 

Why a Curveball Curves (topspin, under spin, sidespin)

Body Balance and Stability Control
 

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• Generally depends upon the location of the centre of mass and how stable that centre of mass is
• The stick figure model is useful

 

 

Equilibrium

• The state of a system that is not experiencing any change in its direction or speed
• Static Equilibrium:
• Dynamic Equilibrium:

 

Balance

• Process whereby the body’s state of equilibrium is controlled for a given purpose
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• If the line of gravity passes through some part of the body’s base of support, the body will be balanced

Stability

• A measure of the difficulty with which equilibrium can be disturbed
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• There is a trade-off between maximizing stability and acquiring speed off a mark

 

 

 
Increasing Stability in Static Equilibrium

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• Increase the inertia of the body
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• Increase the distance between the point where a vertical line from the centre of mass intersects the base of support

Increasing Stability in Dynamic Equilibrium

• Enlarge the body’s base of support in the direction of the external horizontal forces
• Adopt a starting position in which the centre of pressure is close to the edge of the base of support whenever a quick acceleration is important
• When slowing down or reversing directions in running, lean backwards but keep normal frictional forces high to prevent slipping
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