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  1. BIOMECHANICS The Fluid Mechanics of Swimming

  2. HydrostaticsBuoyancy and Floatation • Why does a plank always adjust it’s position in water to float horizontally, not vertically? • Why, if you had an aluminium dinghy and a block of aluminium the same weight, the dinghy will float and the block sink?

  3. When they try to float on the surface without moving some people manage it easily while others always sink. Why? • Why is it easier to pick up and hold an object in water than doing it on land?

  4. An individual who floats is said to display positive buoyancy • Others tend to sink in the water and display negative buoyancy (OHT #1) • A person’s ability to float may influence performance at both the beginner and championship levels

  5. Beginners who have positive buoyancy are likely to learn to swim faster than those who float with difficulty, or not at all

  6. Archimedes Principle • The reason why one object floats better in the water than another is determined by Archimedes Principle of Forces which states that: • ‘A body submerged in a liquid is buoyed up by a force equal to the weight of the fluid displaced’ • This means that an object will float in a fluid if its density is less than the density of that fluid

  7. The density of fresh water is 1.00, and if an object has a density of less than this it will float • If the object’s density is greater than 1.00 it will sink • Also known as the ‘specific gravity’ of an object • The effective specific gravity of the human body is 0.95

  8. This means that in general the human body has the ability to float • A lump of steel will sink because it is unable to displace water that equals its weight (OHT #2) • But steel of the same weight but shaped as a bowl, will float • This is because the weight gets distributed over a larger area and the steel displaces water equal to its weight

  9. A heavily laden ship floats because its total weight is exactly equal to the weight of the water it displaces • It is this weight that exerts the buoyant force supporting the ship • DVD 02:10 – 04:21

  10. Factors Affecting the Ability to Float • Lung Capacity • The volume of air in the lungs has a pronounced effect on an individuals ability to float • Deep inhalation = considerable volume of air added to lungs = specific gravity reduced substantially • The majority of males and females will float if they have taken a full inhalation of air • But the majority of males will sink unless they have more than residual air in the lungs

  11. Body Build • Individuals with a high fat content will tend to be good floaters when compared with those who are heavily boned and well muscled • This is because of the specific gravity of body tissue: • Bone = 1.5 – 2.0 • Muscle = 1.0 • Fat = 0.8 • Endomorphs – overweight = floater • Mesomorphs – little fat, high proportion muscle, bone = sinker • Ectomorphs – slim, slight = floater

  12. Sex • Females tend to have greater proportions of fat which aids their floatation because it lowers their specific gravities • Muscular Tension • Can disallow the lungs to fill with air adequately, having a negative effect on floatation • Reduction in Buoyancy Force • The upward buoyancy force is reduced whenever a part of the body comes out of the water, therefore swimmers should keep as many parts in the water as possible

  13. Centre of Buoyancy / Centre of Gravity • Many individuals have the ability to float but cannot assume a horizontal body position as the legs tend to sink • This is because in air, the body rotates around the COG, while in water the axis of movement is the COB

  14. The actual floating position for a human body is when the COG is vertically aligned with the COB • The legs sink until the COG (hip region) and the COB (lung region) come into vertical alignment (OHT #3) • This is why a person with a COG higher up in their body has dragging legs, creating a lot of drag • DVD 00:35 – 02.10

  15. HydrodynamicsThe Study of Propulsion and Resistance in Water • Propulsion is the force which drives an object through the water • Human propulsion in water is generated by the use of….. • Arms, hands, legs and feet

  16. The structure of the shoulder, elbow and wrist joints allow a wide range of movement • These joints can be rotated to produce the required forces more functionally than the hips, knees and ankles

  17. Basic Forms of Propulsion • Paddling • Pulling and pushing action of hands and arms in water • Sculling • Movement of hands through the water at approximately right angles to the direction of intended travel • Finning • Leg kicking action of freestyle, backstroke and butterfly

  18. Newton’s 3rd Law • For every action there is an equal and opposite reaction • Swimmers must try to push the water backwards rather than downwards and upwards • When a swimmer pushes the water downwards this results in the body being forced upwards • As the arm pushes backwards the body is propelled forwards (OHT #4)

  19. While the hand presses upward the body is forced down • The consequence of these actions is that the body bobs up and down as the swimmer moves through the water • At the advanced swimmer stage resistance must be kept to a minimum

  20. Bernoulli’s Principle • Greater propulsion in water is obtained by moving a large amount of water a short distance than by moving a small amount a great distance • If a swimmer pulls their hand in a straight line it is pushing a small volume of water a long way • Once the water has started moving backwards the swimmer cannot apply as much force on the water as his hand meets less resistance

  21. To obtain maximum propulsion the hand must move faster than the water • The best method is to seek stationary or ‘still’ water by using a curved pathway for the hands (OHT #5)

  22. Bernoulli Effect(OHT #6) • When fluid particles travel over an object shaped like a wing where there is a convex shape on one of the sides, the particles that travel over the larger area meet up at the back of the wing at the same time as the particles that travelled over the flatter surface on the other side

  23. Therefore, the particles that went over the convex side must have moved faster • Bernoulli found that when particles move fast they create low pressure, and a higher pressure on the other side – and lift can occur • So when we move our cupped hand through the water, water travels faster over the knuckles than over the palm

  24. This creates a low pressure by our knuckles and a higher pressure under our palms • We can use this lift force that occurs to propel us through the water • In breaststroke, bringing cupped hands in towards the chest creates lift that enables the breathing subroutine of the stroke

  25. Propulsive Lift Force on the Hand • Lift always acts in a direction perpendicular to the flow • Lift force is felt as pressure on the palms of the hands when the slightly pitched or tilted hand moves through the water • E.g. sculling action; As the hand moves through the water at a slight angle, a pressure differential is created on alternate sides of the hand (OHT #7)

  26. Since motion occurs from high pressure to low pressure, the propulsive lift is perpendicular to the direction of the path of the hand • E.g. treading water, sculling with horizontal hand movements. Lift force is produced on the hands and maintains the head above the surface

  27. Propulsive Drag Force on the Hand • Propulsive drag is created by the backward movement of the hand through the water • As the hand is pulled or pushed against the water, a high pressure zone is created on the palm of the hand and a low pressure zone on the back (OHT #8) • The difference in pressure creates a force on the swimmer’s hand which moves the swimmer forward

  28. Drag can definitely hinder the progress of a swimmer – as you will see when we talk about resistance • But without it, a swimmer will not be able to move in water • Think of a sprinter on land – to gain greater speed they wear spikes to get more friction on the track to aid their propulsion

  29. Swimmers must do the same, but must be careful about not creating too much drag to slow them down • Drag can aid propulsion by the hand ‘grabbing’ the water • Newton’s 3rd law – action of grabbing the water, reaction of the body going forward

  30. The hand pulling backwards produces a high pressure in the palm and a low pressure at the back of the hand • Scientists have concluded that the swimmer gains propulsion by both drag and lift and by changing sequences of the hand during the stroke to get a ‘resultant force’

  31. Angle of Attack • The swimmer needs to continually change the pitch of the hand as it travels it’s curved path so that it is using both drag and lift forces to maximum effect (OHT #9) • The angle should be about 45 degrees so that the resultant force is an equal contribution of both lift and drag – so that the body moves forward • DVD 06:42 - end

  32. Resistance • Water offers a far higher resistance to objects moving through it than does air • Because of this, it is very important for a swimmer to obtain and maintain a streamlined position when performing a stroke • Resistance or the slowing down effect of the water is also known as ‘drag’

  33. Resistance / drag is the force exerted by the fluid against the body which reduces its speed • Skin Friction Resistance (or Surface Drag)(OHT #10) When swimming, the water must move around your body and limbs A thin layer of water next to the body actually sticks to it, and moves with it

  34. The overall effect of this is a considerable drag on the forward progress of the swimmer ‘Drag free’ swimming costumes and shaving down • Tail Suction Resistance (or Eddy Drag) Depends on the size, shape and speed of the swimmer

  35. When the irregular shaped human body is propelled through the water, the flow lines don’t remain smooth Instead they are deflected and break up into a number of whirls creating a great deal of turbulence This type of resistance is very costly in terms of energy output The greater the frontal area hitting the water, the greater the eddy resistance

  36. Frontal Resistance Determined by the amount of surface area exposed to the direction of forward movement Swimmers must maintain a swimming position that is as streamlined as possible – i.e. present as small a surface area as possible to the water • DVD 04:21 – 06:42

  37. Newton’s 1st Law • A constant application of force must be applied to swim at a uniform speed • With swimming the resistance to movement is so great that the body almost immediately stops when propulsion stops

  38. Breaststroke, which involves a glide, uses momentum • But if the glide is held too long, resistance overcomes the moving body and it comes to rest • It then requires excess energy to regain the momentum

  39. Newton’s 2nd Law • A swimmer accelerates forward by increasing their stroke rate • An even application of propulsion is more efficient in propelling a body forward than a fluctuating force • If a swimmer accelerates and decelerates in a stop-and-go manner, much of the force that could be used to overcome water resistance will be lost in overcoming inertia