chapter 3 movement? Linear and projectile motion Speed and velocity

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1 chapter 3 How do biomechanical principles influence movement? Activity1 Figure 3.1 Swimmer gliding in a straight line, body extended Biomechanics explores the science of internal and external forces acting on the human body and the effects produced by these forces. In sport, where technique plays a major role, careful attention to detail by the coach can improve an athlete s performance. Knowledge of biomechanics is certainly regarded as an essential part of planning skill development sessions. motion conduct a series of simple movement workshops to analyse the effect technique has on linear motion, speed, velocity, acceleration and momentum the application of linear motion, velocity, speed, acceleration, momentum in movement and performance contexts Linear and projectile motion Linear motion is motion that takes place in a straight line. For example, a glide off the wall in swimming, where there is no change in travel direction. Projectile motion is the path of any object moving through the air under its own influence and does not have any kind of propulsion system. Some examples of this motion include a figure skater or aerial skier jumping and a thrown object such as a javelin or ball. Speed and velocity Speed and velocity describe the rate at which a body moves from one location to another. These two terms are often thought, incorrectly, to be the same. To determine the average speed of a body the distance is divided by the time taken. The average velocity is calculated by dividing the displacement by the time taken: speed and velocity = distance travelled time taken For example, a swimmer in a 100-metre race in a 50-metre length pool completes the race in 80 seconds: the distance is 100 metres and displacement is 0 metre, because the swimmer is back where they started. Therefore: speed is calculated as: 100 m 80 sec = 1.25 m/s and their velocity is 0 m 80 sec = 0 m/s 124

2 the body in motion chapter 3 Acceleration and inertia Acceleration is defined as the rate at which velocity changes with respect to time: Average acceleration = (final velocity initial velocity) elapsed time From Newton s second law: Force = mass acceleration Acceleration = force mass If the mass of a sprinter is 70 kg and the force exerted on the starting blocks is 700 N, then acceleration = = 10 m/sec² Acceleration due to gravity: while a body is in the air it is subject to a downward acceleration, due to gravity, of approximately 9.81m/s². Inertia is the resistance to acceleration or the unwillingness of a body to change whatever it is doing. Momentum Momentum is the product of mass and velocity: momentum = mass (kg) velocity (m/s) Therefore, a person can increase momentum by increasing either of these elements. In sport, examples include using a heavier bat or racquet, increasing running or hand speed. In golf, momentum applies through the transfer of momentum from the golf club to the golf ball. Momentum is the product of the mass and its acceleration, hence the heavier the golf club and the faster it is swung will result in greater velocity of the golf ball when hit. Of course, there is no guarantee of the ball travelling straight and this presents another consideration for the golfer of speed versus accuracy. Angular momentum is determined by the angular velocity moment of inertia. The angular momentum of a system remains constant throughout a movement provided nothing outside the system acts with a turning moment on it. A good example of angular momentum in action is figure skating. A figure skater starts a spin by pulling in their arms to reduce their moment of inertia. By the conservation of momentum principle, the angular speed must then increase. To come out of the spin, a skater simply extends their arms to increase angular momentum and decrease angular velocity. In a closed system, such as when two objects collide, the total momentum remains the same, though some may transfer from one object to the other. Momentum is always conserved in a closed system, but most sporting situations are not a closed system. For example, when a tennis racquet hits the ball, the ball will be squashed to a certain degree. After a few milliseconds, the ball rebounds back. This contraction and rebound action causes the release of heat energy and some momentum is lost, or transferred elsewhere. Performance in running technique Improvements in running form may not reduce energy expenditure, but will make better use of the runner s energy. The expected benefit is an improvement in performance time by expanding on the abilities that the runner already possesses. The technique for running focuses on two areas: the arm and leg movement. Arm movement is the forward movement of one arm and the backward movement of the other, which creates a force that rotates the upper body around a vertical axis in a given direction. The legs move in to create a force on the lower body around a vertical axis in the opposite direction of the upper body, thus creating angular momentum. Runners who have poor running form may swing their arms in to create more upper body angular momentum than the lower body produces. This results in the body rotating and being turned in between each stride. The runner must then spend unnecessary energy to keep facing forward. Activity 2 F i g u r e 3. 2 Golfer driving the ball Activity 3 125

3 PDHPE in focus preliminary course sample student answer Figure 3.3 Australia s Melissa Wu, left, and Lindsay Croak, in pike position, on their way to winning a silver medal at the 2006 Commonwealth Games Activity 4 One of the runner s objectives is to move the arms in such a manner as to produce enough angular momentum in the upper body to cancel out the angular momentum of the lower body, such that the total body angular momentum is zero. balance and stability apply principles of balance and stability to enhance performance through participation in practical workshops There are two types of balance. Static balance is when a person remains over a relatively fixed base and dynamic balance is when a performer is in motion. Stability relates to the degree to which a body resists being upset or moved. The major factors that affect a person s stability are the: area of the base of support relation of the line of gravity to the edge of the base height of the centre of gravity mass of the person. base of support centre of gravity The centre of gravity is the point where all the body weight is concentrated or the point about which the body weight is evenly distributed. The body s centre of gravity shifts with each body movement. External loads may alter the position of the centre of gravity. The greater the external load, the farther the combined centre of gravity will be from the person s centre of gravity. The person s weight may be distributed in such a way that the centre of gravity falls outside the body. An athlete who performs a pike position in diving would have their centre of gravity falling outside the body. line of gravity The line of gravity represents the direction gravity acts perpendicular to the earth s surface through the body at the centre of gravity. For an object to be balanced, the line of gravity must pass within the base of support. For example, a rugby player who knows they will be pushed from the front should shift their line of gravity forward (that is, lean forward), so that they reduce the chance of losing their balance. A person competing in a tug-o-war will lean backward (move the line of gravity back) in order to absorb a strong forward pull from their opponent. A tennis player will keep their line of gravity centred, so that the centre of gravity can be shifted quickly in any direction without loss of balance. The larger the base of support, the greater stability an individual has. A person is more stable when standing on two feet than when standing on one. Stability can be improved by: widening the base of support lowering the centre of mass increasing the mass of the body. 126

4 the body in motion chapter 3 Another factor associated with stability is that the greater the friction between the playing surface and the body, the more stable the body will be. For example, softball players wear cleats to aid in running and to provide stability for actions that require quick and forceful movements. However, there is a trade off between stability and movement time. In other words, the more stable a person is, the less agile they will be. For example, in a sport such as basketball, which requires speed, the player may need to give up some level of stability, otherwise they may be too slow to respond to the opposition s movements. fluid mechanics apply principles of fluid mechanics to enhance performance through participation in practical workshops A fluid is a substance which moves and changes continuously as a result of an applied pressure. Fluid mechanics involve the study of fluid statics, fluids in motion and the effects of fluids on boundaries. flotation, centre of buoyancy How is a person with their stretched body out flat in a pool able to float, yet if they wrap their arms around their legs and curl up into a ball they sink? This has to do with how much water is pushing against the body a principle known as buoyancy or floatation. When the body is stretched out flat, more water pushes against the body due to it being laid out flatter than if curled up in a ball. The person s body must make room for its own volume by pushing aside, or displacing, an equal volume of liquid. The body is applying a downward force on the water and the water is therefore applying an upward force on the body. The floating object s weight comes into play also. The solid body floats when it has displaced just enough water to equal its own original weight. This principle is called buoyancy. Buoyancy is the loss in weight an object seems to undergo when placed in a liquid, as compared to its weight in air. Archimedes principle states that an object fully or partly immersed in a liquid is buoyed upward by a force equal to the weight of the liquid displaced by that object. From this principle, Archimedes concluded that a floating object displaces an amount of liquid equal to its own weight. It is also important to know that any change in the density of the surrounding water affects the level at which an object floats. Fresh water is less dense than salt water; and so a boat will float lower in fresh water than it does in salt water. Warm water is less dense than cold water, so a boat will float lower in the water if the temperature rises. The centre of buoyancy is the centre of the gravity of the volume of water which the person or object displaces. When upright, the centre of gravity and centre of buoyancy is on the same vertical line and would be considered as stable. F i g u r e 3. 4 Basketballer in defensive position 127

5 PDHPE in focus preliminary course Figure 3.5 Centre of buoyancy Centre of gravity Centre of buoyancy righting moment Activity 5 When the person or object tilts, the centre of gravity remains in the same position, however the centre of buoyancy moves to fit a new centre of gravity of the volume of water replaced. If the body or object is tilted too much, the centre of buoyancy moves to a position where the person or object may roll over. In the human body there is variation from person to person, due to the amount of air in a person s lungs and the percentage of bone, muscle and fat, which all vary among masses of individuals. Both bone and muscle are heavier than fat. Therefore, a lean and muscular body or one with a heavy bone structure would not float very well at all. explore how principles of fluid mechanics have influenced changes in movement and performance, eg technique modification, clothing/suits, equipment/apparatus fluid resistance (drag, lift, The Magnus Effect) Fluid resistance is defined as the force applied by a gas or liquid resisting the motion of a body through it. Applying fluid resistance to freestyle swimming Natural forces affect the movement of swimmers in water. Having an understanding of these forces assists coaches to analyse swimming skills. There are a range of forces acting on a swimmer. In the vertical plane, the weight of the swimmer is counteracted by their buoyancy. However, since people have varying natural ability to float, another means of overcoming the swimmers weight must occur. This is accomplished from the arm stroke and Figure 3.6 Diagram of swimmer Buoyancy Thrust Drag Weight 128

6 the body in motion chapter 3 (a) (b) Figure 3.7 (a) Swimmer producing a lot of drag (b) A streamlined swimmer kick. By pressing down on the water, an equal and opposite reaction occurs which lifts the swimmer higher in the water. The other primary forces existing are the thrust force and drag. Drag Drag can be divided into two components: pressure drag and skin friction drag. Pressure drag comes from the frontal area exposed to the water and the separation that occurs behind the swimmer. A swimmer must streamline their body to reduce the amount of separation as shown in Figure 3.7. Skin or frictional drag is when in freestyle swimming, resistance occurs due to new water rubbing against the swimmer s body. One way this resistance could be reduced would be if the swimmer s body could carry on its surface a very thin layer of surface water. This naturally occurs with underwater animals like dolphins and sharks. To improve the resistance in the freestyle action, swimmers can try to make their bodies as smooth as possible. Some examples of this are seen in close-fitting swimwear or shaving the body and head. In freestyle swimming, the amount of surface friction is determined by the: speed of water relative to the swimmers speed amount of surface area of the body smoothness of the body qualities of the water. Lift When swimming freestyle, the arm stroke produces the majority of the drive required in lifting the body. The best swimmers not only achieve drive by pushing back on the water, but also by moving their hands and arms like an airplane s propeller blade. Figure 3.8 Moving through water for lift 129

7 PDHPE in focus preliminary course Figure 3.9 Kicking for stabilisation Drag Resistance Lift Optimum efficiency in the water is achieved by pushing a large amount of water a short distance rather than by pushing a small amount of water a large distance. Looking at the most efficient arm stroke, that is, moving the arm along a curvilinear path, allows the swimmer to be always pushing back on still water. The advantage is that the still water offers more resistance than the water that is already moving back. It is also important that the hand must continue to move in the directions perpendicular to the direction of forward movement in order for this lift force to be produced. This same lift force is generated while treading water. While treading water, a person does not push down on the water. Instead, they scull their hands back and forth; this results in producing lift which in turn keeps the swimmer s head above water. Stabilising The kick provides a stabilising effect in addition to the propulsive force. Most swimmers only get a small amount of propulsive force from their kick. The first way to improve the kick is to keep the feet in the water. When a swimmer s feet enter the water, a significant amount of air enters too. The air increases the drag and reduces the propulsive effect. The best swimmers go further by moving their feet during the kick to produce the same lift force achieved by their hands. An effective kick helps to keep the body streamlined and thereby reducing the drag. The Magnus Effect The Magnus Effect describes the action of a spinning ball through the air. As a ball spins, it creates an outer layer of air that sticks to the ball and rotates with it. As a result, on the side of the ball that Figure 3.10 The Magnus Effect Air flow Force 130

8 the body in motion chapter 3 this outer layer of air collides with the air flowing past the ball, it decelerates the ball producing a high-pressure zone. While on the opposite side, the spinning ball moves in the same direction as the air flowing past it, which then accelerates the ball and creates a low pressure area. As a result, the ball curves in the direction of the low pressure. Applying the Magnus Effect to tennis To generate lift, a player creates spin on the tennis ball. When the ball rotates, the fluid (air) that is in contact with the surface rotates with the ball. When the ball is in top-spin mode, the top of the ball spins forward (top to bottom) into the oncoming air. There is greater movement of the air towards the bottom surface. More fluid needs to pass through the same space on the underside of the ball, which means that the flow of air is compressed on the lower side of the ball. On the top-side of the ball a lower velocity creates a higher pressure and at the bottom the higher velocity creates a lower pressure (known as the Bernoulli principle ). This in turn leads to an imbalance in the forces on the ball. Therefore, when playing a top-spin shot the higher pressure on the top curves the ball downward from its straight line path. force apply principles of force to enhance performance through participation in practical workshops how the body applies force Force can be defined as a pushing or pulling action that causes a change of state of a body. In biomechanics, any force exerted by one part of the body on another is known as an internal force, whereas all other forces are external. Force is defined by the equation: Force = mass acceleration. Therefore, a person wishing to increase their degree of force will need to increase the weight of the person or object applying the force, increase the speed of the person or object, or perform a combination of both. Getting the balance is very important, as the increase of one factor may be to the detriment of the other. For example, a person increasing their mass may then decrease their ability for speed. Newton s laws of motion Sir Isaac Newton created three biomechanical theories that are used to describe the relationship between force and motion. First law: Every body continues in its state of rest (inertia) or motion in a straight line unless compelled to change that state by external forces exerted upon it. This applies to any sport, for example, where a ball is at rest and does not move until an external force such as a bat or club impacts on it. Second law: The rate of change of momentum of a body is proportional to the force causing it and the change takes place in the direction in which the force acts. An example would be in rugby when a player is running with momentum, until a defender tackles him, which changes the direction and motion of the attacker. Third law: To every action there is an equal and opposite reaction. An example of how this applies in sport would be bouncing a basketball. The force applied onto the ball when it hits the court surface is equal to the force the court applies back onto the ball. Activity 6 Applying force during take-off in long jump The long jumper prepares for take-off by initially sinking their hips and then raising their hips into the take-off phase. This usually results in the second last stride being longer than normal and the final 131

9 PDHPE in focus preliminary course Figure 3.11 Long jumper taking off board mid-line Activity 7 Figure 3.12 Gymnast performing landing stride being up to 20 cm shorter than their normal running stride. At take-off, the athlete would look to ensure that their hips are slightly forward of their shoulders. When the take-off foot is placed on the board, it is slightly in advance of the jumper s hips and should strike the board on the mid line. The final two-foot supports in the take-off should be flat contacts. Vertical height is achieved by the upward explosive acceleration of the arms and the non take-off leg. how the body absorbs force Following are a few important points to remember when learning how to land on the feet in sport: The initial contact with the floor should be on the balls of the feet, followed by bending of the ankles, knees and hips. The feet should be about shoulder width apart when making contact with the floor. The heels should not spring back up but should remain on the floor and the arms are held horizontally out in front of the body for balance. Absorbing force in gymnastics Landing forces in gymnastics are created when completing a jump or some form of aerial skill. Gymnastics landings are usually performed from either a height above the floor, as in the dismount from equipment such as the beam or bars, or as the closing part of a floor routine. The ability of the athlete to efficiently control the landing forces will translate directly into both improved scoring and physical safety. Controlling the landing depends on the athlete restricting their body s momentum at landing and their ability to spread the forces impacting on their body. Many landings in gymnastics also include some form of angular movement, such as a twist. Using the abovementioned method of landing will allow the gymnast to achieve immediate spread of the force at landing starting from the feet, through the flexed legs, into the hips and through the upper body. Absorbing force when catching a ball An athlete should apply the following points when learning how to catch a ball. Knees should be flexed (bent) with both feet roughly shoulder width apart. Weight should be evenly distributed. Hands should be together with fingers pointing down. Keep the head steady with eyes level. Watch the ball into the hands. When the ball is caught, the hands should give or be brought into the body for cushioning. 132

10 the body in motion chapter 3 Catching with soft hands means that when a person catches the ball they should allow their hands to draw back towards their body to ensure a nice, smooth and secure catch. This is a safer way to catch a hard ball and a much less painful than catching the ball with no give. applying force to an object Applying force using a tennis serve Force generation is initiated with the extension of the legs and the downward acceleration of the tossing arm. Using Newton s third law, to get into the air a player must apply an explosive thrust with the legs against the ground to exert a force, which exceeds their own body weight. The ground pushes back up against the athlete and the athlete becomes airborne. Dropping the tossing arm serves to initiate a force and increases the angular momentum of the racquet arm. The power that a tennis player is able to achieve in their serve is largely a result of their flexibility and range of motion in their hips, torso, shoulder and back. This flexibility ability to combine to form a whip action, will in turn increase the speed of the racquet head and apply a great degree of force to the ball. The ball is contacted though the centre of the racquet and the tennis player may create top-spin by flexing their wrist over the ball; this produces a low pressure below the ball, resulting in a ball of high velocity which still drops down and into the service box. A tennis player may also choose to cut the ball to the side. This action creates spin on the ball creating the Magnus Effect, which causes the ball to arc well outside the service box after it bounces. Activity 8 Activities Activity 1 (Page 124) Compare stride lengths by getting students to line up and take 10 normal steps. Discuss the differences and what impact these may have on an event such as the marathon. Explore what a person with shorter strides could do to ensure they are running at the same speed. Activity 2 (Page 125) Use a range of golf clubs (such as a driver, 4 iron, 7 iron, or wedge) to determine how momentum can apply to hitting a golf ball. Fast and slow swings should be applied to each club. Compare how the different club sizes may impact on the actions and final result. Activity 3 (Page 125) Analyse a student s running technique to determine their level of efficiency and effectiveness in terms of angular velocity and momentum. Activity 4 (Page 126) The baseball catching position requires a combination of balance and stability. The catcher needs to be quick on their feet, have extremely quick hands, good arm strength, and good balance and stability. The catcher spends most of the time on the field in the crouch position, and their weight will be on the heels and the knees will be bent. In this position, the rear end is near the back of the heels. The catcher s head is held high and the gloved hand held out. This crouch position (and the ability to catch pitches) relies heavily on the catcher s mastery of balance and stability. In pairs, wear the appropriate safety equipment to practise the technique of softball/baseball catcher and determine the level of balance and stability required to catch effectively. 133

11 PDHPE in focus preliminary course Activities cont. Activity 5 (Page 128) Research on the internet and explore how technique modification, clothing and equipment have changed over time in order to improve movement and performance. Activity 6 (Page 131) Practise long jumping, with one person observing (or videoing) and making recommendations as to how to improve technique and apply optimum force. Activity 7 (Page 132) Using hard and soft hands technique, practise catching a hard ball using a cricket ball, and then with a softer ball like a sofcrosse ball, Compare results and discuss how this information may be transferred to effectively absorbing force in other sports. Activity 8 (Page 133) Practise tennis serving and top-spin forehands. Once the serve has been sufficiently mastered, attempt to create top and side spin serves. A partner may be used to assist in techniques correction and analysis. Partners then practise ground strokes, with a particular focus on creating top spin forehand shots. Discuss as a class the difficulties that arose, and how a tennis player may overcome these issues and what the opposing player would be looking for in a game to assist them in reading a serve or top spin shot. chapter summary Review Questions 1. Outline the key points associated with the biomechanical principles influencing movement. 2. Describe the difference between speed and velocity. 3. Explain the application of acceleration and momentum in relation to swinging a baseball bat. 4. Examine the use of angular momentum in relation to figure skating. 5. Identify the major factors affecting stability. 6. Describe how the centre and line of gravity can impact on performance. 7. What are fluid mechanics and explain how flotation and buoyancy are linked? 8. Analyse the impact drag and lift have on performance. 9. Apply the Magnus Effect to the sport of table tennis. 10. Investigate how the body applies and absorbs force. 134