BIOMECHANICAL PRINCIPLES CHAPTER 5 BIOMECHANICAL PRINCIPLES OF MOTION THROUGH AIR AND WATER

KEY KNOWLEDGE Projectile motion (including the human body as a projectile), Magnus effect, the spin (back spin, top spin, side spin), height release, angle release, velocity of release, angle of projection

Projectile Motion Anything that is launched into the air and affected only by the forces of gravity and air resistance can be considered a projectile. Examples in athletics, diving and gymnastics, Other forms of projectile motion in sport include balls, shuttlecocks, arrows, javelins and discuses Vertical component The vertical component of projectile motion is influenced by gravity and the vertical component of the initial projection velocity. The vertical component of motion relates specifically to the height reached by the projectile. Horizontal component The horizontal component of projectile motion is affected by air resistance and relates to the horizontal distance covered by the projectile.

Factors affecting the path of a projectile The path of a projectile depends on three factors: • angle of projection (or release) • speed of release (or projection) • height of release (or projection). Table on next slide

Angle of projection The angle of projection is the angle at which an object is released into the air. This angle will determine the flight path of the projectile. There are three shapes that a flight path can form, depending on the angle of release. A is the first is a purely vertical shape where the body or object goes straight up and comes straight back down again. b is the second flight path is parabolic and occurs when the angle of projection is between 0° and 90°. c is the final shape determined by the angle of release is half a parabola. An object projected at 0° (perfectly horizontal) will follow this path.

Human movement through air and water Laminar and turbulent flow An object moving through a fluid medium such as air or water with a relatively low velocity will not disturb the flow of the fluid very much. The air or water will flow in smooth, parallel layers around the object. This is called laminar flow. When an object moves with a significantly higher velocity through the fluid, the layers of fluid near the surface get mixed together. This is called turbulent flow. Consider a swimmer moving through the water. Their hands move slowly through the water and don’t disturb the layers of the fluid very much. However, when the swimmer begins to kick, the layers of water are disturbed and greater turbulence is created. In swimming, flow is not completely laminar or turbulent but midway between the two. The type of flow of a fluid around an object, whether it be a ball moving through the air or a body moving through water, affects the forces acting on the object.

Buoyancy Buoyancy is a force that acts vertically upwards on a body that is immersed in water. The buoyant force is equal to the weight of the fluid that has been displaced (Archimedes’ principle). Buoyancy is affected by the density of the fluid. Salt water is denser than fresh water so buoyancy is greater in seawater. This means it is easier to float in the ocean than in a freshwater lake. An object’s ability to float is linked to its buoyancy and its weight. When buoyancy and weight are equal, the object will float. Once the weight of the object is greater than the buoyancy force, the object will sink. Drag force Drag force is generated when a fluid flows around a stationary object or when an object moves through a fluid. It is a resistance force. This means that it slows the object down as it moves through the air or water. In air, this force is called air resistance and in water it is called hydrodynamic resistance. The drag force occurs because areas of different pressure develop in front and behind an object moving through air. As the velocity of the fluid increases, the pressure decreases. This is known as Bernoulli’s principle. This causes an area of turbulence behind the object where the pressure is less than in front of the object. This difference in pressure causes a force to act from the area of high pressure to the area of low pressure. This force is the drag force. Streamlining decreases the turbulence created at the back of an object in air and therefore reduces the effect of drag.

Lift force Another force is generated when a body or object moves through air or water. Lift force acts perpendicular to the flow of the fluid. The factors affecting lift force are similar to those affecting drag: • The velocity of the fluid • The density of the fluid • The size, shape and position of the object or body Lift can occur due to: • the foil shape • the angle of the object relative to the direction of the flow • the Magnus effect • the unevenness of surface on one side of a ball compared to the other. Foil shape A foil shape is simply a shape that can generate lift when in air or water.

Magnus effect Lift can also be generated by spinning objects. A spinning object increases the speed of the fluid on one side and decreases it on the other. According to Bernoulli’s principle, this will create regions of high and low pressure on either side of the object, generating lift. This pressure difference creates a Magnus force, which is a lift force that will act from the area of high pressure to the area of low pressure, causing the object to deviate in the direction of the spin. This deviation is known as the Magnus effect. Spin is important in many sports. Tennis, golf, table tennis, volleyball, baseball, soccer and cricket all use the Magnus effect to curve the flight path of the ball. There are three types of spin that can be applied to a ball or object and each affects the flight path in a different way. They are: • top spin • back spin • side spin. Elite soccer players can ‘bend’ the ball by applying side spin. Golfers can ‘stop’ a ball on the green by applying back spin and tennis players can get ground strokes to ‘dip’ over the net by applying top spin. Top spin causes a ball to drop more quickly that it would without spin and the ball will have a lower and faster rebound velocity. Back spin causes a ball to hold up in the air, lengthening its flight time and distance. Side spin causes a ball to follow a curved path, due to the Magnus effect in the direction of the spin.

Surface unevenness Have you ever wondered how a cricket ball is made to ‘swing’, or why cricketers polish the ball on one side and why it is illegal to ‘roughen up’ the ball? It is thought that the differences in texture on either side of an object can cause lift due to the differences in pressure. This Principle can be used to explain swing. The cricket ball is made up of two halves, joined together by a stitched seam. A new ball released with the seam at an angle to the direction of travel will swing due to the changes to air flow over the surface of the ball.

THE END BIBLIOGRAPHY: PHYSICAL EDUCATION VCE UNITS 1&2