Surface drag results from friction between a fluid and an athlete's surface, form drag results mainly from pressure differences around the athlete, and wave drag results from energy transferred into waves at a fluid's surface. Together, they resist movement through a fluid.
In IB SEHS HL fluid mechanics, total drag can be represented as:
| Type of drag | Mechanism and important factors | Sporting example and reduction |
|---|---|---|
| Surface drag | Fluid viscosity creates friction as fluid moves across the body's wetted surface. It increases with surface area, speed, viscosity, and surface roughness. | A swimmer experiences friction between water and the skin or swimsuit. Streamlined suits, smooth equipment, and reducing unnecessary wetted area can limit it. |
| Form drag | Fluid pressure is higher in front of the athlete than behind. A large frontal area and flow separation produce a low-pressure turbulent wake, increasing the pressure difference. | A cyclist sitting upright experiences more form drag than one using a streamlined position. Reducing frontal area and improving body or equipment shape reduces it. |
| Wave drag | An athlete moving at or near the water's surface creates waves and transfers kinetic energy to them. It generally becomes more significant as speed increases. | A surface swimmer creates bow waves. Maintaining a streamlined body position and, where rules allow, travelling sufficiently underwater reduce wave formation. |
A common misconception is that all drag is simply surface friction. This is incorrect: surface drag depends on friction at the boundary, form drag depends on pressure differences and wake formation, and wave drag depends on creating surface waves. Wave drag is especially relevant to aquatic movement, whereas form and surface drag occur in both air and water.
Exam technique: If asked to distinguish these forces, identify the mechanism behind each one, then link it to a sporting example and a method of reduction. Do not merely state that all three forces “slow the athlete down.”