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Core 2: Body in Motion

Bold heading 3: How do biom

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2Core 2: Body in Motion Bold heading 3: How do biomechanical principles influence movement?

Introduction to BiomechanicsBiomechanics is a science concerned with forces and the effect of these forces on and within the human body. A thorough understanding of the biological and mechanical aspects of human movement can facilitate better coaching, learning, movement and exercise therapy for humans. Knowledge of biomechanics helps us to:choose the best technique to achieve our best performance. For instance, an understanding of the biomechanical principles that affect athletic movements, such as the high jump, discus throw, golf swing and netball shot, improve the efficiency with which these movements are made. This improves how well we perform the skill.e.g. what technique will enable a hockey player to shoot a ball at maximum velocity and how do we learn to produce that technique?

reduce the risk of injury by improving the way we move.e.g. how might catching a cricket ball cause injury to the hands and how do we prevent this?design and use equipment that contributes to improved performance.

e.g. graphite shaft golf sticks, swim suit designs, increased size of head of tennis racquet…..MotionDefinition of Motion: is the movement of a body from one position to another.The movement or motion of a human body, a human limb or objects, e.g. a ball, propelled by a human body can be described in terms of either a line or a circular pathway. Movement along a line is called linear motion.

Linear Motiontakes place when a body and all parts connected to it travel the same distance in the same direction and at the same speed. There are 2 types of linear motion, one that takes place in a straight line, rectilinear, and the other motion that follows a curved pathway called curvilinear. However, we are going to concentrate on rectilinear motion only.

This skier is experiencing linear motion.

What are some other sporting examples where linear motion is experienced?

Syllabus:Students learn about: Students learn to: • motion • apply principles of motion to enhance the application of linear motion, velocity, performance through participation in speed, acceleration, momentum in practical workshops. movement and performance contexts.


The easiest way to determine if a body is experiencing linear motion is to draw a line connecting two parts of the body; for example, the neck and hips. If the line remains in the same position when the body moves from one position to another, the motion is linear. This is shown in figure 6.2.

Improving performance in activities that encompass linear motion usually focuses on modifying or eliminating technique faults that contribute to any non-linear movements. Excessive up and down, rotational and lateral movements are examples of faults that erode performance directed towards achieving the shortest, most efficient pathway. Sprinters who rotate their arms across their bodies and swimmers who use an irregular arm pull that results in a zigzag movement pattern along the pool surface are examples of poor application of linear motion.

Application: Linear motion and swimming performance

The 50 metre sprint in swimming is an example of the application of predominately linear motion in a sporting event. Poor swimming technique can result in wasted energy and poor performance. The aim in sprinting is to direct all body action directly up the pool by eliminating excessive rotational and lateral movements. Use the Efficient Swimming weblinks and the Hackett swimming PDF file included to answer the following questions.

List 10 points that you consider to be fundamental to better freestyle technique.


1. Discuss how the application of linear motion principles can enhance swimming performance.


Speed

Definition: is equal to the distance covered divided by the time taken to cover the distance

Speed can be calculated using the equation: speed (m/sec) = distance (m) time (secs)

e.g. Eamon Sullivan set a freestyle world record at the Beijing Olympics with a time of 47.05 seconds for 100 metres. Use the formula to work out Eamon’s speed for this race.

Speed is important in most sports and team games. The player who can move quickly has a distinct advantage in games such as touch football, rugby and soccer because not only is that player difficult to catch, but he/she can use their speed to gather opponents quickly in defence.

Much of our potential for speed is genetic and relates to the type of muscle fibre in our bodies. However, individuals can develop their speed as a result of training and technique improvements, the basis of which is the developmentof power and efficiency of movement.

Application: Improving speed

This application requires students to perform the 30 metre flying speed test.1. Use the 30 metre flying speed test weblink below to establish how the test is conducted.

http://www.brianmac.co.uk/flying30.htm

2. Record your times for the first 30 metres and the entire 60 metre sprint. Use the inbuilt calculator to predict your 100 metre time.

3. Use the momentum sports and perfect condition weblinks below to establish the characteristics of good technique in running.

http://www.momentumsports.co.uk/TtRunTechnique.asp

http://www.perfectcondition.ltd.uk/Articles/Improving%20running%20technique.htm

4. Work in pairs with one person observing while the other performs short sprints for analysis. You could develop a rating scale using the points listed in table below to show progress in acquiring better technique through feedback from practice.

Technical Point Excellent Good Needs ImprovementAdvice from observer

Toe up

Heel up

Knee up

Reach out

Claw back

Core stability

http://www.perfectcondition.ltd.uk/Articles/Improving%20running%20technique.htm

http://www.momentumsports.co.uk/TtRunTechnique.asp

http://www.brianmac.co.uk/flying30.htm


When, in the opinion of the observer, technique has improved sufficiently, re-run the test and compare times with the previous effort.

Comment on the effect that better running technique made to speed improvement over a short distance.

Discuss the importance of good technique in other sporting activities such as surfing, downhill skiing and speed skating where speed with stability is crucial to success.

VelocityDefinition: is equal to displacement divided by time.

Velocity = displacementTimeDisplacement is the movement of a body from one location to another in a particular direction, or an ‘as the crow flies’ measurement.


Activity: Calculate the speed and velocity of a swimmer in a 100m race in a 50m length pool who completes the race in 80secs.

Acceleration

Definition: is the rate at which velocity changes in a given amount of time.

Acceleration (m/sec2) = (final velocity – initial velocity) Time

Athletes need to be able to increase and decrease velocity rapidly. A rugby league player carrying the ball needs to build up as much velocity as possible to make it difficult to be tackled. A softballer stealing a base needs to be able to build up velocity before the fielders can react. The softballer needs to sprint to the base, but then slow down in order to avoid over-running the base. These are examples of linear acceleration and linear deceleration, which are a requirement for most team sports and short-distance sprints.

The length and frequency of an individual’s stride is also believed to affect acceleration. The legs of a faster runner will have greater angular velocity. In general, a combination of long stride and high frequency indicates a fast runner. A stride is the distance between one foot striking the ground and when it next strikes the ground; that is, two steps. There are also some slight differences in posture and movement patterns of athletes competing in events of various lengths, as highlighted in Figure 6.5. The trunk angle is almost the same, with the knee angle of the lead leg higher and more vigorous arm movement observed over shorter distances.


Practical Application: Velocity and acceleration

In this workshop, measure the velocity and acceleration of a person sprinting 100 metres.

Equipment• Measured 100-metre straight line with a cone at each 10-metre interval (see Figure 6.4)• 10 stopwatches• Starting whistle Procedure

1. Place a person with a stopwatch at each cone (10-metre interval).

2. On ‘Go’, everyone starts the stopwatches, and stops them at the moment that the sprinter runs past their cone.

3. Copy and complete the table opposite to record your results.

Tasks1. Graph the results for velocity and acceleration.2. Identify the point at which the sprinter had the:

a. greatest velocity? Why do you think this occurred?

b. least velocity? Why do you think this occurred?c. greatest acceleration? Why do you think this

occurred?d. greatest deceleration? Why do you think this

occurred?

3. Discuss the variations in the sprinter’s velocity and acceleration over the 100 metres.

4. Explain the effect these variations could have on the sprinter’s overall performance in a 100-metre sprint race.


Application: Speed, acceleration and performance

Use the run faster weblink, and watch the video How to run faster: Speed and acceleration specifics (5 minutes). As you view the video, note the five laws that relate to improved acceleration.

All the laws mentioned relate to the development of power through better technique.Inquiry: Speed, acceleration and performance

1. List and explain six principles that assist in improving acceleration.

2. Discuss the relationship between better technique and improved acceleration


Momentum

Definition: refers to the quantity of motion that a body possesses.

When a body (or object) is in motion, whether it is a sprinter running down a track or a bowling ball down an alley, it has a certain mass and a certain velocity. The product of these two is known as the momentum. Once a body is in motion, it will tend to stay in motion (unless acted upon by another force).

Momentum is a product of mass and velocity. It is expressed as follows:

momentum = mass × velocity

M = mv

The application of the principle of momentum is most significant in impact or collision situations. For instance, a truck travelling at 50 kilometres per hour that collides with an oncoming car going at the same speed would have a devastating effect on the car because the mass of the truck is much greater than that of the car. The car would be taken in the direction that the truck was moving. The same principle can be applied to certain sporting games such as rugby league and rugby union, where collisions in the form of tackles are part of the game.

The greater the momentum of the body, the greater effect it has on bodies that it collides with. For example, if two people who are tenpin bowling have exactly the same technique, and release the ball with the same velocity, the one bowling the heavier ball (that is, with greater momentum) is more likely to get a better result. This is because the heavier ball will cause the pins to fly around more, knocking down other pins

Velocity also affects momentum. If a softball batter wants to hit a home run, he or she will swing the bat faster, with a higher velocity, when hitting the ball. If the batter wanted to bunt, he or she would swing the bat with reduced velocity so that the ball won’t go very far.

Differences in momentum are therefore affected by variations in mass and velocity. In most sports, mass is constant, so velocity becomes the main influencing factor in momentum. That is, to increase momentum, simply increase velocity. By increasing the velocity of the bat, a cricketer can hit a ball further. When an ice skater brings his or her arms in closer to the body during a spin, he or she will spin faster. To slow down, the skater moves the arms away.

Practical Application: Momentum

Participate in any of the activities below to experience the effect of momentum.

1 a Perform and measure the distance travelled when completing a standing long jump and a long jump with a measured run-up.

b Explain the reasons for the differences in distances reached.

2 a Using a T-Ball stand hit a ball off the stand with and without force. b Compare the differences in the force applied and the distance the ball travels.

3 a Complete a 50-metre sprint at a sub-maximal pace, pulling up as quickly to the finish line as possible. Repeat but this time run at your maximum capacity.

b Compare your ability to stop in both cases and suggest reasons for the differences based on your understanding of momentum.

4 Analyse video footage of a recent contact football game. a Describe what happens to the velocity of a player when tackled. b Compare the mass of the players running the ball and executing the tackles and suggest why some players are more

successful than others in the game.


Balance and stability

The concepts of stability and balance are closely related to equilibrium. Stability is concerned with the resistance of a body to changes in its equilibrium; that is, changes in its linear or angular acceleration. When an individual can assume a stable position and then control that position, he or she is said to be in a state of balance. There are two types of balance:

• If the body is at rest (not moving) it has static balance.

• If the body is moving, it has dynamic balance.

Centre of gravity

Definition: is the point at which all the weight is evenly distributed and about which the object is balanced.

Location of the centre of gravity

Students learn about: Students learn to:


All masses within the gravitational field of the earth experience an attraction towards the centre of the earth. The greater the mass, the stronger the attraction. The gravitational force that the earth exerts on objects is their weight, which is the product of the mass of the object, the mass of the earth, and acceleration due to gravity (9.8 m/s2). If the whole force were to be concentrated at a single point, this would be the centre of gravity. The centre of gravity (CG) is also the balance point of the system or the point at which all mass seems to be concentrated.Remember:• The CG need not lie within the physical limits of an object or person. For example, during high jump, diving or gymnastics events, the CG changes rapidly and lies outside the body of the athlete.

• The CG is constantly changing position; for example, as we sit, stand, run, jump, eat and breathe.

• The height of the CG is relative to the base of support. An object with a low CG will tend to be more stable than one with a higher CG. For example, during contact a rugby player tries to lower his or her CG to maintain force, and raise the CG of the opposition player to put him or her off balance.

Line of GravityA straight line drawn from the CG to the ground is called the line of gravity. Individual body segments (limbs) also have their own CG. An object is most stable when the line of gravity falls through the centre of the base of support. Increasing the area of the base of support will generally increase stability. Remember: a body may be stable in one direction but not in another.

Movement occurs when the line of gravity changes relative to the base of support. Movement results in a momentary state of imbalance being created, causing the body to move in the direction of the imbalance. In specialised sporting movements, such as the start in athletics, diving and rhythmic gymnastics, the precision with which the line of gravity moves in relation to the base of support directly affects the quantity and quality of movement.

During practice of specialised skills, athletes progressively develop a feel for the line of gravity relative to the base of support, enabling the controlled instability required for movement. This means that less force is required to initiate the desired movement. For example, swimmers on the blocks bend forward, moving the line of gravity to the edge of the base of support so that less force is required to execute the dive. Springboard divers do likewise by moving the line of gravity to the front edge of their base of support, enabling frontward movement with


Base of support

Definition: is the region bounded by the body parts in contact with a surface that is applying a reactive force against the applied force of the body.

For example, when you stand up, the area covered by your feet is your base of support. If you hang from a parallel bar, your base of support is the area between the outer limits of your hands.

Any other objects or surfaces that exert a force against your body in some direction other than vertical also form part of your base of support.The greater the mass of an object, the greater its stability. It takes a greater force to move a heavier object. Wrestling and boxing competitions, in which stability is an important component, enforce weight divisions to make competition fairer.


Application: Base of support and different positions

Have five students assume a range of positions with varying bases of support; for example, a pirouette, headstand, crouch balance start and boxing stance. Try to move each student from their position. Record the results.

1. Draw the shape and size of the base of support for each activity above. Compare the amount of effort required to displace each person from their position.

2. Discuss the degree to which the mass of the student affected your ability to dislodge each from their position.


Fluid Mechanics

Definition: refers to forces that operate in water and air environments. These forces will affect how well we can move through the water (either in a vessel or as a swimmer) or how we can move ourselves or projectiles through the air.

The type of fluid environment we experience impacts on performance. For example, when we throw a javelin, hit a golf ball or swim in a pool, forces are exerted on the body or object and the body or object exerts forces on the surrounding fluid. Knowledge about how to equip ourselves and better execute movements in specific fluid environments improves safety and can significantly enhance performance.

FloatationTwo forces operate on a body in a fluid environment to determine its buoyancy (ability to float). These forces are the buoyant force that pushes the body up and the weight force that pulls the body down (gravity). Archimedes’ Principle states that a body that is partially or totally immersed in a fluid will experience buoyancy that is equal to the weight of the volume of fluid displaced by that body. So, if the buoyant force is greater than the weight force, the body will float. Conversely, if the buoyant force is less than the weight force, the body will sink.

. Centre of buoyancy

The centre of buoyancy is at the centre of gravity of the water that the swimmer displaces. When the body is fully submerged, the centre of buoyancy of the swimmer will fall directly above the swimmer’s centre of gravity. The centre of buoyancy and centre of mass will also change as a result of the movement changes, particularly the legs.

A person with a higher fatto muscle ratio will floatmore easily than a heavilymuscled individual.Taking a deep breathbefore attempting to floatcan also assist buoyancy



Question: How has the swimmer become more streamlined in Figure 6.21?

Fluid Resistance

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.

Forces act on us when we attempt to propel ourselves through a fluid environment. These forces include drag force and lift force. Elite athletes understand and use these forces in a way that will benefit the efficiency of their performance, as indicated in the diagrams below.

o Drag Force is the force that opposes the forward motion of a body or object, reducing its speed or velocity.

It is a resisting force because it acts in opposition to whatever is moving through it. Drag forces run parallel to flow direction (airflow, water), exerting a force on the body in the direction of the stream.

An example of where we find drag forces in sport is to watch a swimmer push off the pool wall following a turn. The swimmer’s forward motion gradually decreases due to resisting forces applied by the water, which makes the swimmer stop unless arm or leg action begins. A body that is streamlined (contoured to reduce resistance) and technically efficient moves through the medium, creating less drag than a body that is not as streamlined. This difference in the amount of drag created by non-streamlined and streamlined bodies is illustrated in figures 6.20 and 6.21.


The amount of drag experienced depends on a number of factors, including:

• fluid density. Because water is denser than air, forward motion in this fluid is more difficult.• shape. If a body or object is streamlined at the front and tapered towards the tail, the fluid through which it is moving experiences less turbulence and this results in less resistance.• surface. A smooth surface causes less turbulence, resulting in less drag.• size of frontal area. If the front of a person or object (area making initial contact with the fluid) is large, resistance to forward motion is increased.

There are two types of drag forces — surface drag and profile drag.

Surface Drag (or Skin Friction) Definition: refers to a thin film of the fluid medium sticking to the surface area of the body or object through which it is moving, as shown in Figure 6.20 & 6.21 on page 17.

The effect of surface drag is:

the courser or less streamlined the body or object moving through the fluid, then the greater the surface friction on the body or object which then causes it slow down.

Profile Drag (called form or pressure drag)

Figure 6.22 The blocking effect of bigger objects causes greater turbulence.

Definition: refers to drag created by the shape and size of a body or object. When we swim, for example, fluid pressure at the front of our body is greater than fluid pressure behind our feet. The effect of profile drag is:

objects with bigger cross-sectional areas produce more profile drag in comparison to streamlined objects which, because of their shape and smoothness, cause less drag. The turbulence that results at the back of the moving body or object is call the wake.

Cyclists try to reduce form drag by reducing the size of their frontal area (bending forward) and by ‘drafting’ orfollowing closely behind other cyclists to reap the benefits of being in the low pressure area or wake, which actually creates an upstream direction.

What advantage do cyclists have when they draft?

How are surface drag and profile drag reduced?

By the body or object moving through the fluid being as streamlined as possible, to create the least amount of surface friction between the object and the fluid or the less turbulence (wake) behind the movement.


Application: The effects of drag on performance

Examine the illustrations in the left column of table 6.3. Use the right column to identify types of drag and the effect of this on performance. Discuss your findings with the class.

Body or object Types of drag and effect on flight/performance


Much has been done to try to minimise resistance forces that oppose movement in fluid mediums. Most developments have taken place in regard to technique, tactics, clothing and equipment design. For example:

technique. Cyclists, speed skaters and downhill skiers all bend forward at the trunk. tactics. Distance runners and cyclists follow one another closely where possible. clothing. Tight bodysuits made of special friction-reducing fabrics are worn by runners, cyclists and swimmers. equipment design. Designs of equipment such as golf balls, golf clubs, cricket bats, bicycle helmets, footballs and

surfboards are continually being modified to make them more aerodynamically efficient.

Application: Using principles of fluid mechanics to improve performance

In table 6.4, compare performance A with performance B for the three activities. In the space under performance A, identify relevant types of drag and elaborate on the effect this has on performance. Next to performance B, identify any technique,equipment, design and other modifications that have influenced performance/design.

Performance 1a Performance 1b Performance 2a Performance 2b

Types of drag and impact on performance:

Technique/equipment modifications to reduce drag:


Technique/equipment modifications to reduce drag:

Performance 3a Performance 3b


Technique/equipment modifications toreduce drag:


o Lift Force

Definition: is the force that operates at right angles to the drag.

Lift force is much greater than that lift created in air, as water is denser. This force occurs perpendicular to the flow of the water over the body when swimming. When performing an eggbeater kick in water polo (see Figure 6.14), hydrodynamic lift force is created as the legs alternately circle under the water creating pressure differences between the top and bottom of the leg and foot. The lift force acts to push the athlete upwards. Swimmers experience a lift force as they stroke the water, as the flow of water over the hands creates a forward lift force that is equal to the force exerted bythe swimmer, thus pushing the athlete to the surface.

o Magnus effect

The Magnus effect (see Figure 6.15) occurs when a spinning object createsa whirlpool of rotating air or liquid around it. Velocity increases on one sideof the object, where the fluid travels in the same direction as the whirlpool.As the velocity of a fluid increases, the pressure exerted by the fluid willdecrease. The opposite side of the object experiences decreased velocity asthe motion of the whirlpool is reversed. This creates spin and makes it difficultfor an opponent to read the direction of flight and respond accordingly.

What sports use the Magnus effect to surprise their opponents?


Force (Biomechanics)

Definition: is the push or pull acting on a body.

A force causes or has the potential to cause, divert or slow the movement of an object upon which it acts.When a body is at rest or in motion, forces are acting upon it. Whether you are sitting at a desk, running around a track or jumping out of an aeroplane, forces are acting on your body. Force is measured in a unit called a Newton (N). A force can be described as internal or external relative to the system that is being examined. For instance, if we consider the system as the whole human body, the muscles that contract to exert a force on bones, cartilage or ligaments around a joint are considered inside the system and are therefore internal forces. Any forces exerted outside the body (such as gravity, friction, contact with the ground or another body, air resistance and fluid resistance) are considered external forces.


All forces have four common properties. They have:

• magnitude (an amount; how much is applied)• direction (the angle at which the force is applied)

• point of application (the specific point at which he force is applied to a body)• line of action (represents a straight line through the point of application in the direction that the force is acting).The directions in which forces act are drawn as an arrow called a vector.


How the body applies force

The scientist Sir Isaac Newton devised three theories to describe the relationship between force and motion.An understanding of the applications of these three theories is essential in developing an appreciation of how biomechanical principles influence the way we move.

Newton’s First Law of Motion

Every body continues in its state of rest or motion in a straight line unless compelled to change that state by external forces exerted upon it.

Put simply, no force equals no movement. This seems to be a basic, commonsense theory, but it becomes more important when we examine how forces change the state of motion of a body, and the resistance of a body to a change in its state of motion (inertia). 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.

Newton’s Second Law of Motion

The rate of change in motion of a body is proportional to the force causing it, and the change takes place in the direction in which the force acts.

This law means that a body will experience a change in its motion in proportion to the force applied to it, and in the direction of the force. For example, a golf ball putted on a green moves in the direction in which it is hit and according to how hard it is hit. Another, not so obvious interpretation is that when force is applied to a moving body, such as an outstretched hand to a basketball, the motion of the ball is adjusted according to the force. That is, it may slow down or be deflected but it may not be in the exact direction of the force.

Describe another sporting example where Newton’s second law is evident.

This law generates the equation of: Force = mass × acceleration (F = ma)

which relates force, mass and acceleration. The greater the force that acts upon the body, the greater its resultant force. Therefore, a putt in golf will not travel as far as a drive. As the mass of the body increases, a greater force is required to produce the same acceleration.

To throw a 4-kilogram discus as far as a 3-kilogram discus, the force applied must be greater. This law also relates to momentum. If you wish to apply large forces while kicking a ball, increasing the momentum of the leg during the swing will increase force of impact because of the greater momentum of the leg.

Why does hitting a hockey ball with the hockey stick cause the ball to go further than when it is pushed?

Newton’s Third Law of Motion

For every force that is exerted by one body on another, there is an equal and opposite force exerted by the second body on the first.

You may have heard the saying ‘For every action there is an equal and opposite reaction’. This is another way of saying Newton’s Third Law. This law illustrates that forces act in pairs, and are equal and in opposite directions. However, the result is not always the same. For instance, when you land after performing a long jump, you apply a force to the ground and it applies one back to you. The effect on you is much greater than your effect on the ground, however, because the earth is much bigger and heavier. In this way, Newton’s Third Law relates to the Second Law.


An example of how Newton’s third law 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.

Give another sporting example where Newton’s third law is evident.

Friction is another force that can be applied by the body that also relates to Newton’s laws of motion. The amount of friction is determined by the amount of force holding the two objects together and by the nature of the two surfaces in contact. Friction can be both detrimental and beneficial to sporting performances.

What are 2 examples where friction can be detrimental to sporting performances?

What are 2 examples where friction can be beneficial to sporting performances?


How the body absorbs force

Forces exerted on the body are absorbed through the joints, which bend or flex in response to the impact. We see evidence of the body absorbing forces in activities such as rebounding in basketball, landing in high jump and stopping the bounce while on a trampoline (see figure 6.30).

When the body lands on a floor or similar surface, it exerts a force on the surface. In response, the surface exerts a force on the body. If we did not bend the knees and allow a slow, controlled dissipation of the forces by the muscles, the risk of injury to the joint would be increased. In an activity such as the landing phase of a long jump, the muscles in the front of the thigh (quadriceps) lengthen while absorbing the force (see figure 6.31). Joint flexion helps prevent injury to surrounding tissue.

The body also absorbs forces while catching balls or similar objects. In the process of catching, a force is exerted by the ball on the hand and a force is exerted by the hand on the ball. Catching a ball can sting if the force of the ball is not absorbed effectively.Since the force of the ball remains constant, the only variable that can be changed is the distance through which the hands move when catching the ball. To increase the catching distance and thereby absorb the force more effectively, we can use a number of techniques, including:

• the catching arm can be outstretched. When the ball meets the hand, the arm can be drawn quickly to the body.• smothering the ball with the other hand• catching with an outstretched arm and moving it past and behind the body to increase the distance over which the ball is caught• pivoting the body during the catching action.

While some of these principles may help to reduce the impact from objects such as cricket balls, an overemphasis on reducing pain from impact may result in a dropped catch. Correct technique and practice is essential.

What other methods can be used to absorb force in sport?


Application: Observation of forces being absorbed while catching

Have two students throw an egg underarm to one another. Gradually increase the distance between the two. Closely observe the action of the hands as they receive the egg. It would take little force to break the egg.

From your observations, discuss how the students avoided breaking the egg for a period of time (if they did).

Applying a force to an object

When applying force to objects, such as to a barbell, cricket bat or netball, there are a number of considerations. First, the quantity of force applied to the object is important. The greater the force, the greater is the acceleration of the object. A small soccer player whose mass and technique allows only small effort production provides little force to the ball in comparison to the same ball being kicked by a bigger player (other factors being equal) (see figure 6.32).

Second, if the mass of an object is increased, more force is needed to move the object the same distance. For example, if a football becomes heavier as a result of wet conditions, more force is required to pass or kick it (see figure 6.33).


Third, objects of greater mass require more force to move them than objects of smaller mass. The size of the discus, javelin and shot-put is smaller for younger students than older students. This assumes that older students have greater mass and are thereby able to deliver more force than younger students because of their increased size (mass) and (possibly) strength.

In many sports and activities, the body rotates about an axis. When this happens, centripetal force and centrifugal force are experienced. Centripetal forces are forces directed towards the centre of a rotating body and centrifugal forces are directed outwards. Two examples are the golf swing and hammer throw. Here a body rotates, generating powerful forces on objects (in this case, a golf club/ball and hammer ball), allowing them to be propelled distances far greater than would be possible without body rotation.


We experience these forces often in our lives. Passengers in cars experience centripetal and centrifugal forces each time a car goes around a bend. The centrifugal forces cause the passenger to slide towards the outside of the bend. Similar forces operate in the spin-dry cycle of a washing machine, removing water from the clothes. The greater the speed about the axis, the greater the force produced.

Another example is that of ice skaters who link arms to form a ‘chain’. When the chain rotates about an ‘axis’ (the person closest to the centre), considerable speed is experienced by the person at the end of the chain. To counteract the centrifugal force, the skaters need to lean towards the centre and push outwards against the ice to maintain balance.

To manage centripetal and centrifugal forces in sporting situations it is important to:

begin carefully so that you learn to feel the forces as they develop respond gradually, trying to match the force exactly work on your balance so that you become comfortable leaning beyond where you would normally be balanced ensure you have a firm handgrip if holding an object such as a bat or high bar bend your knees and ensure you have good traction if working on a track field or circuit.

Application: Applying force to objects

Use the tennis serves weblink

http://learnhowto.tv/how-to-play-tennis-developing-correct-serving-technique/

to view the short video on developing correct serving technique in tennis. Notice how more force is gradually added to the serve once control is established.

1. Discuss what a player can do to gradually increase the amount of force applied to the ball while maintaining control of the serve.

2. Why is the development of force an advantage in most sporting activities?

http://learnhowto.tv/how-to-play-tennis-developing-correct-serving-technique/

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