Chapter overview What is biomechanics? page 73 Motion page 74 Balance and stability page 78 Force page 85 Projectile motion page 103 Fluid mechanics page 109 Now that you’ve finished … answers
Back to chapter overview What is biomechanics? Page 73
An overview • Biomechanics is the study of the body as a machine. • Biomechanics can be used to: • Refine technique • Prevent injury • Develop equipment • Correct errors • Whilst many biomechanical principles are inherent in each sport skill, it is important to discern the most influential principles in your sport in order to look to optimising technique.
Back to chapter overview Motion Page 74
Motion • To initiate motion, one must overcome inertia. • Once in motion, factors such as velocity and acceleration become influential. • Motion can be curvilinear or rectilinear Curvilinear motion Rectilinear motion
Motion • Factors such as velocity and acceleration are variable amongst athletes and can determine the effectiveness of one’s performance.
Newton’s laws of motion • Newton’s first law of motion states that a still object continues to be still, and a moving object continues to move at its current velocity, unless an external force acts on the object. • His second law states that the sum of the force that moves an object is equal to the object’s mass multiplied by the acceleration. This law can be expressed as the following equation: Force = mass × acceleration (F = ma) • Newton’s law of action reaction states that for every action there is an equal and opposite reaction.
Back to chapter overview Balance and stability Page 78
Balance and stability • Every athlete must be able to maintain stability and balance. • Failure to do so can result in loss of force and or accuracy. • Balancecan be dynamic or static. Personal reflection Do you find balance easier when you are moving or stationary? How does this affect your performance in your current physical activity?
Factors affecting balance include base of support, centre of gravity and height of gravity. Personal reflection Have you ever done the stork stand balance test? Did you find it challenging?
Centre of gravity Personal reflection Think of a time when you were performing a dynamic movement. When your centre of gravity falls outside of your body, what is your natural movement to try to correct it?
The height of the centre of gravity can have a significant affect on your stability. Personal reflection Do you think your height has an effect on your stability?
Back to chapter overview Force Page 85
Force • Force is anything that causes or has the potential to cause the movement, diversion or slowing of the object on which it acts. • All forces have four common properties. They have: • magnitude (an amount, or how much is applied) • direction (the angle at which the force is applied) • a point of application (the specific point at which the force is applied) • a line of action (represented by a straight line through the point of application in the direction that the force is acting).
Integration Consider the sport you are currently studying. Describe the forces required in terms of the four properties listed on the previous slide.
Force production: Most sports require an athlete to be able to both generate and control forces. Integration In your sport, can you differentiate between the skills that require maximum force and those that require control and accuracy?
Summation of forces • To obtain maximum force, it is necessary to combine or add up the forces applied by different body parts. This concept is known as the summation of forces. • The summation of force is influenced by the: • number of body parts used in the movement • order and timing of their involvement • force and velocity generated • way in which the body and body parts are stabilised and balanced.
Summation of forces The long jumper combines the forces in his legs, abdominals and arms to propel himself further into the pit.
Personal reflection Choose one physical skill from your current sport. Which body parts do you use to produce maximum force? In what order?
Momentum • Whether it is a sprinter running along a track or a bowling ball rolling down an alley, any object or person has a certain mass and a certain velocity. The product of these is known as the momentum. Personal reflection When you sprint, how many steps does it take you to come to a complete stop? How does this compare with your classmates or competitors? Why?
In most sports, mass is constant, so velocity becomes the main factor influencing momentum. So, to increase momentum, simply increase velocity.
In many sports, it is necessary for momentum to be transferred to another object or body part. The greater the momentum an object has, the greater its effect on other objects it collides with. Personal reflection Have you ever played pool? Why is that when the white ball strikes another ball straight on, the white ball stops moving while the ball that was struck moves forward?
Stabilisation must occur for effective transfer of gained momentum • For example, in tennis it is common for athletes to brace (tense up) their muscles just before the impact of a forehand. Stopping the rotation of the body by bracing causes a whip-like effect on the arm
Accuracy • It is important in many sports to be accurate in the production of sport in both direction and magnitude. Application In the following sporting situations, consider ways in which the performer can increase their directional accuracy. 1. Batting into space in softball 2. Digging to a setter in volleyball 3. Kicking a penalty goal in soccer
Rotary forces • Not all forces are produced in straight lines. • Concentric forces are applied through the centre of an object and cause motion in a straight line • Eccentric forces are applied off-centre and cause rotation in the object to which the force is applied. Practical example Place your textbook on the desk in front of you. Push the book away from you with two fingers, applying the force through the centre of the book. Now repeat this, applying the force to one side of centre. What do you notice?
Rotation about an axis (torque) • More torque, and a greater rotation or twisting movement, will be generated when more force is applied further from the centre of an object.
Rotational momentum • A spinning object will have momentum that will allow it to continue to spin until an external force is applied. Practical example Sit on a swivel chair and clear a space around the chair. Ask a friend to gently spin the chair in any direction. You can control the speed at which you spin by tucking your legs into your body to speed up and extending them out to slow down. The velocity of the spin is affected by the distribution of the weight from the axis of the object.
When spinning on the ice, skaters can control the rate of spin by moving their limbs closer to the axis of rotation (to rotate faster) or extending their arms and legs (to rotate more slowly).
Centrifugal and centripetal forces • Centripetal force is the force that causes rotating objects to move towards the centre, or axis, of rotation. • Centrifugal force is the force that causes rotating objects to move away from the centre, or axis, of rotation.
Levers • Three types: • First class - fulcrum between the force and the resistance e.g. oar on row boat. • Second class - resistance between the force and the fulcrum • Third class - force between the resistance and the fulcrum e.g. baseball batter.
Third class levers are the most common in sport. • Sports using bats or racquets all use third class levers. • The velocity of a lever is fastest at the point furthest from the fulcrum. • By increasing the length of the bat, the velocity is increased.
It is important to consider strength and accuracy when looking at levers. • It is not ideal to use a 4m long bat/racquet even if a greater velocity could to produced.
Maximising leverage • Due to inertia, a shorter lever is easier to move than a longer one. • To maximise the efficiency of a lever, it is often better to begin a lever’s motion by shortening the lever. • This can be achieved by bending the relevant joint. • E.g. a soccer playing kicking a ball starts with a bent knee, a javelin thrower bends the elbow to bring the javelin through and a tennis server bends their elbow before extending to hit the ball.
The ‘sweet spot’ • The sweet spot is the point that, when struck, causes no backwards nor forwards rotation of the bat. Personal reflection Have you ever felt the effects of hitting a ball outside the sweet spot?
Back to chapter overview Projectile motion Page 103
Projectile motion • Factors that influence flight • Height of release • Angle of release • Speed of release • Gravity and air resistance
Angle of release • The optimal trajectory of a projectile is just less than 45 degrees.
Height of release A projectile thrown from a height will travel further. Personal reflection Do you think you have a height advantage when throwing?
The effects of spinning • The Magnus effect explains how a spinning projectile, such as a ball, curves in flight.
Back to chapter overview Fluid mechanics Page 109
Fluid mechanics • Fluid mechanics refers to forces that operate in both the water and in air.
Buoyancy • An object’s buoyancy determines whether or not it will float. • Buoyancy is influenced by density. • Density is a measure of mass in relation to volume. Personal Reflection Have you ever tried floating on your back in water? What happens when you allow your body to relax? The high density muscle and bone in our legs cause them to sink when in the water whilst the low density air and fat in our chests keep them afloat.
Centre of buoyancy • The centre of buoyancy of a floating object is different to its centre of gravity. • In swimmers, the centre of buoyancy can change depending on the position and movement of a swimmer’s body, particularly the legs, and how much of the body is submerged. • A swimmer’s centre of buoyancy is closer to their heads due to the lower density of the chest and core compared to the legs.
Propulsion • Swimmers are required to both push and pull themselves through the water. • In this pushing and pulling action the forces of lift and drag are at work.
Lift • Lift force occurs due to the factors involved in Bernoulli’s Principle. • The difference in velocity of the water travelling over the surface of the hands creates a pressure difference that, when applied correctly, creates lift.
Drag • Drag force opposes the forward movement of a swimmer, providing resistance. • Swimmers can minimise this effect of drag by streamlining their bodies and pitching the entry of their hands to ensure the water surface is cut cleanly. • Drag can also assist a swimmer. Lift and drag can work together to create propulsion.
Water resistance • Skin resistance is the resistance created by water ‘gripping’ the skin as is passes through. • Turbulence resistance is created due to the high pressure created at the front of the swimmer interacting with the low pressure formed at the rear of the swimmer, creating a suction effect. To minimise turbulence and resistance, good swimmers streamline their bodies in the water.