IB Sports, exercise and health science B.2.2.3 Buoyancy, Drag, and Lift Effects on Objects Moving Through Fluids Notes

Buoyancy: the force that keeps objects afloat

This force is governed by Archimedes' Principle, which states the buoyant force acting on an object is equal to the weight of the fluid displaced by the object.

Key factors affecting buoyancy

1. Density of the Fluid: Denser fluids exert a greater buoyant force. For example, saltwater provides more buoyancy than freshwater because it is denser.
2. Volume of Fluid Displaced: Larger objects displace more fluid, resulting in a greater buoyant force.

Drag: the force that slows motion through a fluid

1. It is a critical factor in sports like swimming, cycling, and skiing, where reducing drag can significantly enhance performance.
2. The drag force equation is F_drag = ½ × C_d × A × ρ × v².
3. C_d is the drag coefficient (shape and surface dependent), A is the frontal area, ρ is the fluid density, and v is the relative velocity.
4. Drag is proportional to velocity squared, so doubling speed quadruples drag.
5. Drag can be categorized into three types:

Surface drag

1. This occurs due to friction between the surface of the object and the fluid.
2. A smoother surface reduces this interaction, minimizing drag.

Form drag

1. Form drag arises from the shape of the object and how it pushes against the fluid.
2. Streamlined shapes reduce this type of drag by allowing fluid to flow more smoothly around the object.
3. Luge athletes lie feet-first on their back to present a small frontal area to the air.
4. Skeleton athletes lie head-first on their stomach, also with a small profile.
5. Small differences in body angle drive measurable differences in finishing time.

Wave drag

1. Wave drag occurs when an object moves at the interface between two fluids, such as air and water.
2. The creation of waves consumes energy and increases resistance.

Laminar vs turbulent boundary layers

1. A laminar boundary layer has fluid molecules moving in smooth, parallel layers.
2. A turbulent boundary layer has chaotic, mixed flow.
3. A turbulent boundary layer often produces less total drag because it stays attached to the surface for longer, delaying flow separation and shrinking the low-pressure wake.
4. This is why golf balls have dimples: dimples trip the boundary layer into turbulence and make the ball travel further.

Factors influencing drag

1. Fluid Viscosity: Thicker fluids create more drag.
2. Surface Area: Larger surface areas increase drag.
3. Shape and Texture: Streamlined and smooth surfaces reduce drag.
4. Relative Velocity: Drag increases with the square of velocity, meaning faster motion results in exponentially greater drag.

Drafting reduces drag on the trailing athlete

1. Drafting in cycling reduces drag on the trailing rider by up to 25% because the leading rider's wake provides a low-pressure pocket.
2. The same principle let Eliud Kipchoge break the 2-hour marathon barrier behind a phalanx of pacers.

Lift: the force that defies gravity

1. Lift is a force that acts perpendicular to the direction of motion. It is often associated with objects moving through air, such as aeroplanes, golf balls, or javelins.
2. Lift is generated by pressure differences on opposite sides of an object, as explained by Bernoulli's principle:
3. As the velocity of a fluid increases, its pressure decreases.