3.3 Performance Appraisal Fluid Mechanics 1 - Hydrostatics

Prior Learning: Biomechanics The science that examines the internal and external forces acting on a human body and the effects produced by these forces. Balance, stability and centre of mass Levers and projectile motion Inertia Force and force summation Momentum Functional Anatomy Bones Muscles Movement at joints

1.Why does a plank always adjust it’s position in water to float horizontally, not vertically? 2.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? 4.Why is it easier to pick up and hold an object in water than doing it on land?Why is it easier to pick up and hold an object in water than doing it on land? Fluid Mechanics Hydrostatics refer to forces which operate in water and air. This unit will only look at the forces operating in water. Fluids at rest and under pressure Consider these aquatic-questions…

 An individual who floats is said to display positive buoyancy  Others tend to sink in the water and display negative buoyancy Introduction to why things float...  A person’s ability to float may influence performance at both the beginner and championship levels  Beginners who have positive buoyancy are likely to learn to swim faster than those who float with difficulty, or not at all Buoyancy & Floatation

The 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. 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

Implications 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 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 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 Additional Movie

Factors affecting floatation 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 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

Factors affecting floatation Sex Females tend to have greater proportions of fat which aids their floatation because it lowers their specific gravities (mushroom test) (Controllable Factors) 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

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 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 This is why a person with a COG higher up in their body has dragging legs, creating a lot of drag (See notes for more detailed explanations) DVD on Fluid Mechanics?

Student Task Explain, in detail, how the Archimedes Principle affected your original performance in swimming.