How do air and water resistance, drag, lift and spin affect the movement of athletes and projectiles?
Apply the principles of fluid mechanics - drag, lift, the Magnus effect and buoyancy - to explain and improve movement through air and water.
How drag, lift, the Magnus effect and buoyancy act on athletes and projectiles moving through air and water, and how technique and equipment manipulate these forces.
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What this dot point is asking
You must apply the principles of fluid mechanics to explain movement through air and water, and to suggest improvements to technique or equipment.
Drag (fluid resistance)
Drag is the force that opposes a body moving through a fluid (air or water). It increases with speed, surface area and the roughness of the surface, and with the density of the fluid (water creates far more drag than air).
- Form (pressure) drag depends on the frontal area and shape facing the flow. A cyclist crouching low presents a smaller area and reduces form drag.
- Surface (friction) drag depends on the texture of the surface. Smooth swimsuits and shaved skin reduce it.
- Wave drag affects swimmers at the water surface, which is why they push off and turn underwater where there is no wave drag.
Lift and the Bernoulli principle
When fluid moves faster over one surface of a shape than the other, the pressure difference creates lift, a force perpendicular to the flow. A discus or ski-jumper's body angled to the airflow generates lift that extends flight. The angle of attack (the angle the surface meets the flow) must be optimal: too steep and the flow separates, increasing drag instead of lift.
The Magnus effect (spin)
The Magnus effect explains why a spinning ball curves. Spin drags air around the ball, making air travel faster on one side and slower on the other; the pressure difference pushes the ball toward the low-pressure (faster) side.
- Topspin makes a ball dip and bounce low and fast (a tennis forehand, a football free kick that drops).
- Backspin keeps a ball in the air longer and makes it sit up (a golf shot, a tennis slice).
- Sidespin swerves the ball sideways (a curving soccer free kick).
Buoyancy
Buoyancy is the upward force a fluid exerts on a body. A swimmer floats when buoyancy equals their weight. Buoyancy depends on body composition (fat is more buoyant than muscle) and lung volume. A swimmer adjusts body position to keep the body horizontal and high in the water, reducing form drag.