200 Flashcards
(34 cards)
What drives sport evolution
athletes, society, regulations, technology?
- technology certainly is the most powerful motor of sport evolution
_ what has become more performant
↳sailors or the boats
↳ skiers or the skis
Sport’s technology is not only about science; it should also comprehend and consider the needs and feelings of the athlets. Athlets and technology have to adapt to each other.
Cycle of sport evolution
Technology innovation-Technology improvement
- Athletes’adaptation to new technology
- Evolution of rules & regulations
=Technical innovation
Innovation
Technological innovations Motor sports: Invention of internal combustion engine Tennis, pole vaulting, etc: Discovery of composite materials Alpinism: Invention of alpine crampons Time measurement: Invention of photo-finish Skiing: Invention of carving skis with heightened bindings Sprint: Invention of the starting blocks
Technological improvement & Evolution of regulations
Engine ajustment (mix air-fuel, type of fuel, etc.)
Fabrication process to obtain best possible fibers’alignment (improved resistance & energy storage capacity)
Evolution of binding, position of points and materials
Adaptation of skis to new aerodynamic conditions to maximize lift-to-drag ratio
Introduction of chest-first rule Limitation on the height of the plates Approval of technique
Admission of the starting blocks
Technical innovations Ski jumping: Invention of the V-technique High jumping: Introduction of the Fostbury technique Sprint: Invention of the
Why does physics of sport matter?
Sport administrators
– Understand rules & regulation and adapt policy
– Set rules & regulations with full knowledge of the issues prevailing
Equipment suppliers
– Understand mechanical limits & possibilities to see where and what technical and / or technological improvement is still possible
– Technology intelligence
• Coaches
– Help correct errors and improve sport performances of athletes
– Improve approach to coaching
– Foresee changes and anticipate their integration (technology foresight)
• Athletes
– Improve technique
Physics of running
Action = Reaction. Newton’s third law says that when one object exerts a force on a second object, the second object always exerts a force equal to, but in the opposite direction of the original force.
Runner’s injury. When a runner’s foot strikes the ground with the full force of the runner’s weight (F=Mg: Newton’s second law), the ground also exerts a force on the runner’s foot, which moves up the runner’s leg joints to their spine, again due to Newton’s third law.
Physics of running
Impulse Momentum (p=Ft) § Cushioning is used in training and trial shoes to decrease the amount of force felt by the runner's feet and joints. This is accomplished by increasing the amount of time it takes for the force to reach the runner's feet. Instead of transferring it directly, the shoe cushion absorbs part of the energy and releases it on a longer time than with hard soles which transfer the force instanatneously. § The more the cushioning, the longer it takes for the runner to feel the force caused by striking the ground. The increased amount of time t and the decreased amount of force F (at constant impulse p) helps to minimize the wear and tear on the runner's body.
Impact management techniques
Although the total force felt by the body might not change, the force applied at one instant in one place of the body can be significantly reduced by
– extending the time-frame
– Extending the area of application of this force.
§ This principle applies to most sports where catching or landing is involved, i.e. any impact situation:
• Use of gloves and « sliding into home base » technique in baseball • Entering the water vertically rather than horizontally in diving
• Flexing of legs when landing in ski jumping or gymnastics
• Break-fall technique in judo
• Soften Body checking by standing against the ice ring
• Legendary rope-a-dope of Mohamed Ali in boxing • However, cushioning impacts on efficiency
Work efficiency
The efficiency h of a work is defined as the ratio of effective energy to consumed energy (or effective power to power produced). It is normally expressed in %, whereas the energy is measured in Joule or Kcal (1 J=4.18 cal) and the power in Watt (=J/s).
The efficiency is 100% when no energy is lost, that is when the energy consumed is entirely transformed into effective (useful) energy. This can never be achieved by physical activities, nor by any machine.
If different efficiencies are involved in an activity, these multiply up. For instance, the efficiency of cycling is given by the product of the human machine efficiency, the shoe efficiency to transfer power to the bike, and the bike efficiency to transfer mechanical energy into kinetic energy.
Efficiency of various activities
All means of transportation do not require the same amount of energy to travel the same distance. This is due to the different weight and different efficiency of these transport systems.
• The bicycle is from far the most energy efficient means of transportation. It can be up to 5 times more efficient than walking (you will need 5 times the same amount of energy to walk a given distance than to cycle it).
• The difference between the energy necessary to cycle and to drive by car is enormous. One hundred calories can power a cyclist for 5 km, but it would only enable a car to drive 85 m!!!
Surface characteristics
Elastic vs hard surface
• An elastic playing surface such as grass feels springy to run on and produces fewer injuries than more rigid surfaces such as concrete.
• But the time spent rebounding is higher on a springy surface, slowing the runner down.
• The best surface for athletics is one that is absorbent enough to limit injuries but firm enough to give athletes the best chance of achieving optimum results.
Deformable surfaces
• The least efficient surfaces are those where the deformation is plastic and not elastic, such as snow, water or sand, as a large amount of the running energy is consumed for the deformation of the surface.
• Not only does this energy expenditure not contribute to the forward movement, but it is also not restored to the body when the feet move up.
• This is why, for instance, beach volley ball is so much more energy consuming than indoor volleyball, and why jogging in the water is so demanding.
Impulse
Impulse is not used only to slow down and stop
• In various sports, the duration of application of a force is crucial, e.g.
– Javelin throw – Karate
Frictional forces
Static friction
Occurs between to surfaces at rest
Shoe outsoles should have static friction as large as possible to avoid slippage. The better the grip, the shorter the time the runner’s feet are in contact with the ground, the quicker they can run their legs over. Spikes, for instance, increase the grip.
No static friction, no movement whatsoever!
Static friction
Occurs between to surfaces in movement relative to each other. Kinetic friction is always opposed to the direction of the movement of the moving surface, thus slowing it down.
Skis should have kinetic friction as low as possible to maximize speed, but not to high to avoid loss of control.
No kinetic friction, no change in direction, and thus no control whatsoever !
Frictional forces: examples
Tennis. Tennis shoes should prevent slippage at start of the run (high μstat), but should enable sliding at the end of the run end (low μkin). Sand stack in the shoe patterns modifies their surface roughness and thus decreases greatly the static friction coefficient. Tennis players thus hit their heels with the tennis racquet on clay surfaces to unstuck the sand.
• Basketball. Basketball shoes reflect the need to address the difference between surface types (indoor wooden court vs. outdoor concrete court), as well as the understanding need differ in different areas of the outsole, depending on the movement (no slippage at traction, no sliding at stopping, good slippage at pivoting).
• Ski, Snowboard, Windsurfing, etc. The friction coefficient does not generally depend on the area of the surfaces in contact. However, in sport where a deformation of the surface is generated (snow, water, sand, etc.), the greater the surface in contact (at equivalent weight), the smaller the pressure, the smaller the surface deformation, and thus the smaller the frictional force. This is why snowboard is better suited than skies in deep snow.
• Climbing shoes are designed to offer maximal friction. Best performances are obtained at cooler temperatures.
Frictional forces: rolling friction
• Motor racing. As long as the wheel rolls, the friction is static (the wheel is moving but instantaneously the point of contact wheel-road is not). Should the wheel slide, then the friction becomes dynamic. Reminding that Fstat > Fkin, this explain why :
– The racing cars and motor bikes avoid sliding. This makes them lose both time and control.
– Because of friction, the car tires become smoother at each lap, in turn reducing friction and lap time, until it becomes worth changing tires although it costs a few seconds.
– Friction depends on the nature of the two surfaces in contact. Thus tire profiles and materials will differ on dry road, wet road, snow, etc.
– Friction depends on the temperature of the tires and is optimised for when they are warm. On the warm-up lap the cars zigzags in order to generate lateral friction and thus heat up their tires.
– The ABS braking system enables to break without sliding (the wheel remains in static friction at all time), which reduces the breaking distance compared to just blocking the wheel and allow static friction to stop the car.
Air resistance
Not only friction with the ground plays a role in sport, air resistance does too. This effect is actually dominant for number of activities.
• When projectile motion is treated in basic physics courses, the influence of air resistance is often neglected in the calculations and the trajectory of a projectile becomes a parabola where the horizontal velocity component is constant and the vertical component is subject to gravity.
• However, it appears that the motion of the ball is governed not only by gravity, but also by air resistance, thus making the real path much shorter.
• More air in front implies more pressure, similarly to running in the rain.
• Air resistance being a frictional force, it also applies opposite to the motion, thus reducing the travelling speed.
CFD is used in many sports where the aerodynamic question is relevant. CFD is of course not confined to sport activities but is found in many R&D sectors of activity.
• Situation is more complex for sport where presence of other competitor affects the own aerodynamic (car racing, sailing, etc.) or when external conditions modify the aerodynamic behaviour. Real-time adaptable design would be the solution, but this often reveals too complex
Aerodynamics
The aerodynamic drag on a object moving in a fluid is always opposed to direction of motion and is given by:
D=1/2pACdv2 (r o fluid density
A o effective frontal area of the object
CD oDragcoefficient
v o relative velocity (= velocity of object if fluid is at rest)
The drag coefficient CD depends on the shape and surface roughness of the object, as well as on viscosity of the fluid
Surface roughness. Generally speaking, the rougher the surface of the object, the more turbulent the fluid flow, and the greater the drag. However, at certain speed, a roughened sphere may experience less drag than a polished one. The dimples in the golf ball enables it to travel 30% faster than a smooth one.
Air resistance
Without air resistance, free-fall acceleration is constant (=gravity acceleration g=9.81m/s2), and free-fall speed thus increases proportionally with time: v=gt (and the time frame for your impact on the ground is very very short…)
• In reality, air resistance is so great that it counter-balance gravity. Free-fall speed thus becomes constant.
Slipstream in cycling ( drafting)
Another important aerodynamic effect in sport in the « slipstream phenomenon », which generates a forward suction effect.
To efficiently get protected from the air resistance, cyclists position themselves according to the relative direction of wind
Triathlon. It is forbidden in triathlon to ride in another rider’s slipstream. Therefore, a minimal distance (10m axial or 2m lateral) must be kept between the cyclists so that no one can benefit from suction. Surprisingly, no such rule prevail during the swimming section, although the slipstream phenomenon can be quite significant.
• Final cycling sprint. The second cyclist tries to overtake at the very last second to beneficiate as long as possible from suction effect and thus save power for the very end, while the first cyclist keeps on zigzagging to try and get the follower away from his slipstream)
Slipstream in car racing
Slipstream & speed. In car racing, the slipstream phenomenon begins to be significant from 60 km/h upwards. The higher the speed, the greater the effect.
• Principle. The strategy consists in catching up with another car at the beginning of a straight line and to get as close as possible behind it, in order to no only be protected from the air resistance, but also to be situated in a small zone, a few meters long, where no turbulences occur, as eddies produced by the turbulences carry energy away and tend to slow vehicles down. Furthermore, the suction effect comes from the fact that the pressure in front of the car may be lower than that at the back, which generates a forward push, conversely to the car in front.
• Speed & power. Both cars drive at the same speed, but the second one needs less power. Therefore, when it will filter to overtake, the second car still has enough power left to accelerate, whereas the first car does not.
• Overpowered engine. The suction phenomenon results in an increase of the maximal speed, which may in turn result in overpowering the engine. To avoid this problem, the sixth gear is extended.
Computational Fluid Dynamic (CFD)
Visualisation: pressure distribution cannot be physically visualised. Furthermore, the equations that govern aerodynmics are highly non-linear and cannot be solved analytically for complex bodies moving in fluid. Numerical models are therefore necessary.
• Computational Fluid Dynamics (CFD): Equations such as Bernouilli or Navier-Stokes are programmed into computers. The computers provide solutions to the problem of external airflow over vehicle shapes. The body of the configuration and the space surrounding it are represented by clusters of points, lines and surfaces; equations are solved at these points. CFD is divided into three steps. Grid generation, numerical simulation and post-process analysis.
Air resistance in altitude
As the soccer ball moves through the air, the air in front of it experiences a rise in air pressure and pushes the ball in the direction opposite its motion.
• The higher the air pressure was to start with, the greater its rise in front of the ball and the stronger the backward push of air resistance. Thus if you were to play soccer in the Rocky Mountains, where the air pressure is much less, you’d be able to kick the ball significantly farther.
Positive effect: Air resistance greatly reduced
• Negative effect: Oxygen reduced => aerobic capacity greatly reduced (muscles burn sugars to produce ADP through an oxydation process)
• Insignificant effect: variation of weight
Altitude is thus well suited for sport demanding anaerobic effort and where air resistance plays a role (sprint, long jumping), but not for endurance effort (running, cycling, etc.)
Bouncing balls
Bouncing. Ball bounce is important in many sports including soccer. The ideal height of bounce varies for different sports but should be constant from one part of the field to the other.
• Measurement. The Coefficient Of Restitution (COR) is a measure of the elasticity of the collision between the ball and the ground. Elasticity is a measure of how much bounce there is, or in other words, how much of the kinetic energy of the colliding objects before the collision remains as kinetic energy of the objects after the collision.
• Elastic vs. plastic bounce. A perfectly elastic collision has a coefficient of restitution of 1. A perfectly plastic, or inelastic, collision has COR=0. Example: two lumps of clay that don’t bounce at all, but stick together. So the coefficient of restitution will always be between zero and one.
• BBR. Another measure of the bouncing quality of a ball is given by the Ball Bounce Resilience (BBR). It is equal to the COR squared, and then expressed as a percentage. For example, if a ball is dropped from 3 meters, hits the ground, and bounces up 1 meter, the BBR is 33.3%.
COR 2 = Height of bounce = BBR Height of drop 100
Coefficient of restitution (COR)
Influent factors. For a given ball, the COR varies with • The surface characteristics (material, thickness, moisture, etc.) • The temperature, which influences the internal pressure of the ball (e.g. squash) • The atmospheric pressure Type of ball COR Basketball 0.75 Golfball 0.60 Tennis ball 0.70 Soccer ball 0.75 Baseball 0.57 Superball 0.90 Tennis balls. This is why tennis balls are less inflated in altitude to compensate for the lower atmospheric pressure and thus ensure a constant COR If your ball has a whole, even the tiniest, than it COR drops because the inner and outer pressures become the same (atmospheric pressure)
Aerodynamic lift
The Bernouilli effect for fluid dynamics means that: if a fluid (gaz or liquid) flows around an object at different speed (due to assymetry of object), the slower moving fluid will exert more pressure than the faster moving fluid on the object, which forces the object towards the faster moving fluid. This expresses nothing else but the conservation of energy. Bernouilli’s equation along a streamline of constant elevation writes:
Where p is the local pressure, r is the fluid density and v is the fluid speed.
The lift is what makes any flying object fly. The flying condition is that the aerodynmic lift is equal (or greater at take-off) than the object’s weight.
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