The transmission of energy from an object passing through a fluid to the fluid is known as fluid resistance. The resistance of an object passing through a fluid increases as the speed of the object increases and as the viscosity of the fluid increases. Fluid Resistance Contact Forces

Surface drag is a result of the friction between the surface and the fluid. The fluid closest to the object (boundary layer) rubs against the object creating friction. Kyle (1989) reported that wearing loose clothing can increase surface drag from 2% to 8%. Surface and Form Drag Contact Forces

Van Ingen Schenau (1982) reported a 10% reduction in surface drag when a speed skater wears a smooth body suit. Surface Drag Contact Forces

Form drag occurs when air is driven past an object and is diverted outward creating a low pressure region behind the object. high pressure low pressure Form Drag Contact Forces

The orientation of the object will affect the frontal area and will play an important role in the amount of form drag. Low form drag High form drag Form Drag Contact Forces

frontal area.5m 2 (upright).42m 2 (touring).34m 2 (racing) The second cyclist can ride within the low pressure zone of the first cyclist and thus lower the pressure difference and the drag. This is called drafting. Contact Forces

At low velocities laminar flow occurs. The boundary layer remains attached to the surface. During separated flow the boundary layer separates toward the back of the object and a low pressure region is formed. During fully turbulent flow the boundary layer becomes turbulent and the size of the pocket is decreased. laminar separated fully turbulent Flow Type Contact Forces

Factors Affecting Flow Type size shape surface roughness viscosity of the fluid flow velocity Contact Forces

The particles following the path from D1 to D2 will be more spread out than particles following the path from C1 to C2 because of the greater distance. This creates a low pressure region above the airfoil. Airfoil Bernoulli’s Principle, 1738 Contact Forces

direction of movement Lift always acts perpendicular to drag. F drag = 1/2(C d A  v 2 ) F lift = 1/2(C l A  v 2 ) F air resistance F drag F lift Lift Contact Forces

The lift-to-drag ratio is critical (i.e. the larger the ratio, the more effective the airfoil is in flight). L/D ratio is dependent on the angle that the airfoil makes with the incoming air (this is called the ANGLE OF ATTACK). Increasing the angle of attack increases the L/D ratio to a point; beyond that point the angle becomes too steep and the airfoil stalls typical angles of attack: airfoil - below 15 o javelin - 10 o Contact Forces

Adapted from Aerodynamic Factors Which Influence Discus Flight, Ganslen. Lift to drag ratios for the discus. Contact Forces

Rotating objects can also create a pressure difference. low pressure zone high pressure zone Direction of air flow Rotating Objects Magnus Effect, 1852 Contact Forces

low pressure zone high pressure zone intended direction of flight actual direction of flight Contact Forces

The golf club imparts backspin on the golf ball and increases the length of the drive. Contact Forces

From The Mechanics of Sport, E. Bade. Dimples on a golf ball increase the velocity of the boundary layer and can dramatically influence the length of a drive. Contact Forces

Terminal Speed An object falling through a fluid reaches its terminal speed when the drag force is equal to its weight. This results in a net force of zero and thus no further acceleration takes place. weight drag force Contact Forces

Estimated Terminal Speeds of Selected Spheres C D : coefficient of drag  : fluid density D: sphere diameter W: weight of sphere V T : terminal speed Adapted from Sport Science by Peter J. Brancazio. Contact Forces