Exercise, Sport, and Materials Science

Introduction Tremendous advances have been achieved in the design of equipment and implements Theses improvements have been a result of our improved understanding of how the laws of mechanics need to be meshed with what we know about human performance

Resistive Forces Friction: how to use it Friction occurs when two surfaces interact, resulting in a force that inhibits the motion of one surface to another

Friction: How to Use it The coefficient of friction is the resistance to movement between two surfaces Friction force; the force parallel to the surface Normal force; the force oppose the friction force (e.g. weight of body when runs along the ground)

Static coefficient of friction (while impending motion) Kinetic coefficient of friction (while is motion); Rolling friction Sliding motion Static friction >> kinetic friction

Frictional forces in cycling Axle friction Optimal lubricant Chain friction Due to chain and sprocket is small; except chain slightly worn and lubricated with a light oil Tire/ground friction Tire thickness Inflation pressure Wheel diameter Roughness of the surface Wind resistance

Fluid Dynamics Drag A force –resisting movement and acts on any object moving through a fluid (air or water) Aerodynamic drag: cyclists, speed skaters, runners … Hydrodynamic drag: swimmers, rowers, kayakers … Magnitude of these drags dependent on Fluid density Frontal area of the moving body; moving body head on The drag coefficient; shape of the object (e.g. streamline) The velocity of movement; air resistance increases as square of the velocity

Skaters crouch to reduce drag.

The two perpendicular forces components of fluid resistance: lift and drag

Swimming style

Body positions during cycling by using aero bar to improve streamlining thus reducing drag

Two body positions during swimming streamlined body position reduces drag (upper panel) and poor body position (lower panel) Form drag The resistive force resulting from poor body positioning in the water Poor alignment of the body increase form drag More body surface area, the speed slower Faster the speed, the greater form drag To streamline the body by decrease the frontal area expose to the oncoming water

surface drag The interaction between the body surface and the water The factors that affect hydrodynamic drag Water temperature Water viscosity body surface area Velocity

Bernoulli’s principle Lift acts perpendicular to the direction of motion and opposite in direction to the drag component The wing is curved, air travels faster over the top than over the bottom of the wing, a region of lower pressure on the top and higher pressure on the bottom and resultant difference in pressure tends to make wing rise Bernoulli’s principle Pressure is inversely related to the velocity of the fluid

The spoiler on a racing car acts to decrease lift, increasing the frictional force between the surface and the tire Rear spoiler 擾流板

Clothing Materials Most attempts to design special clothing for the athlete have been made to reduce the drag of the incoming drag Friction drag Pressure drag

The mechanics of Sports Implements Projectile implements in Track and Field The most aerodynamic is the javelin (torpedo-like shape; 7 times more aerodynamic than the discus; the discus is 10 times more aerodynamic than the hammer; the hammer is 10 times more aerodynamic than shot

Bicycle

Bicycle is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. The basic shape and configuration of a typical upright, or safety bicycle, has changed little since the first chain-driven model was developed around 1885. But many details have been improved, especially since the advent of modern materials and computer-aided design. These have allowed for a proliferation of specialized designs for many types of cycling.

Boomerang A boomerang is a thrown tool, typically constructed as a flat aerofoil, that is designed to spin about an axis perpendicular to the direction of its flight. A returning boomerang is designed to circle back to the thrower Boomerang回飛棒

A returning boomerang has two or more airfoil wings arranged so that the spinning creates unbalanced aerodynamic forces that curve its path so that it travels in an elliptical path and returns to its point of origin when thrown correctly. While a throwing stick can also be shaped overall like a returning boomerang, it is designed to travel as straight as possible so that it can be aimed and thrown with great force to bring down game. Its surfaces therefore are symmetrical and not uneven like the aerofoils which give the returning boomerang its characteristic curved flight.

Rowing and Sailing Competitive Rowing Rowing is a sport in which athletes race against each other in boats, on rivers, on lakes or on the ocean, depending upon the type of race and the discipline. The boats are propelled by the reaction forces on the oar blades as they are pushed against the water. Racing boats (often called shells) are long, narrow, and broadly semi-circular in cross-section in order to reduce drag to a minimum. There is some trade off between boat speed and stability in choice of hull shape. They usually have a fin towards the rear, to help prevent roll and yaw and to increase the effectiveness of the rudder.

Rowing Boat

Rowing Oars Oars are used to propel the boat. They are long (sculling: 250–300 cm; rowing 340–360 cm) poles with one flat end about 50 cm long and 25 cm wide, called the blade. Classic oars were made out of wood, but modern oars are made from more expensive and durable synthetic material, the most common being carbon fiber.

Competitive Sailing Sailing, also known as yacht racing or yachting, is a sport in which competitors race from point to point, or around a race course, in sail-powered vessels. Fiber glass – hull weight and strength Carbon fiber and Mylar – boat structure Mylar and Kevlar – sail material Shape and waterline – speed Lift and trail- speed

Pole Vault Potential energy and Kinetic energy

Tennis For the first 100 years of the modern game, rackets were of wood and of standard size, and strings were of animal gut. Laminated wood construction yielded more strength in rackets used through most of the 20th century until first metal and then composites of carbon graphite, ceramics, and lighter metals such as titanium were introduced. These stronger materials enabled the production of over-sized rackets that yielded yet more power. Meanwhile technology led to the use of synthetic strings that match the feel of gut yet with added durability.

The sweet spot is the area that, when hit by a tennis ball, imparts the greatest amount of kinetic energy (mass x velocity) to the ball. The sweet spot is not the center of the racquet face. the sweet spot slight distance outboard, in the direction away from the grip on the racquet face

Increasing the Sweet Spot Stringing Modern tennis racquets have larger sweet spots, accomplished by increasing the face area, but also by increasing the frame stiffness using expensive carbon fiber construction found in high performance aerospace equipment.

Golf the modern game of golf originated in 15th century Scotland A golf course consists of a series of holes, each with a teeing ground that is set off by two markers showing the bounds of the legal tee area, fairway, rough and other hazards, and the putting green surrounded by the fringe with the pin (normally a flagstick) and cup

Golf ball When a golf ball is hit, the impact, which lasts less than a millisecond, determines the ball’s velocity, launch angle and spin rate, all of which influence its trajectory (and its behavior when it hits the ground). A ball moving through air experiences two major aerodynamic forces, lift and drag. Dimpled balls fly farther than non-dimpled balls due to the combination of these two effects. Most golf balls on sale today have about 250–450 dimples, though there have been balls with more than 1000 dimples

Golf Golf Clubs Driver and fairway Perimeter weighting Grooves of clubface

Athletic Footwear Attributes of an athletic shoe include a flexible sole, appropriate tread for the function, and ability to absorb impact. Impact force (Newton) Midsole hardness (durometer)

Shock Attenuation Shock attenuation Impact shock 2-3 time of body Heavy percussion wave cause injury Ethyl vinyl acetate and polyurethand (PU) for shoes

Stability High-end marathon running shoes will often come in different shapes suited to different foot types, gait etc. Generally, these shoes are divided into neutral, overpronation and underpronation (supination) running shoes to fit the respective foot strike of the runners.

Analyzing Materials and Equipment Design Shoe test Shock absorption: Foot torsion Flexibility: torque to measure Torsion扭轉; 扭曲

Shoe’s flexibility tester Test of shoe’s grip

Force – deformity relationship Energy store in a given materials: force -deformity relationship Force loading unloading Deformity

Foot Torsion Twisting action of the foot Ground contacts period: rear foot and then forefoot 1. Rear foot contact (pronation) 2.heel lift off the ground; forefoot continues to pronate 3. while rear foot start to supinate Footwear is designed to allow torsion of the foot and not to restrict it.

Finite Element Analysis (FEA) Biomechanics Research Area of high stress in equipment like golf, bobsleds, bicycle … can be located with a technique known as (FEA) Allows engineers to enhance their equipment design via computer software in place of physical testing 25000 times accurate than physical testing and can decrease development time by 50% Computer program can perform volume, area, mass…calculations

Discussion What is friction, how it can be used in athletic training. In your opinion, what is the most important factor for athletic footwear in a given sport (select one). How you could to test its competence. The Magnus effect results from the spin imparted to a golf ball at impact, what factor can enhance the effect.