Fluid mechanics

Drag, lift and buoyancy: how air and water slow down or hold up the athlete.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Drag: what is it?
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A fluid's resistance force: Fd = ½ × ρ × Cd × A × v².
It depends on frontal area and the square of speed.
In a peloton, why do riders take turns at the front?
- The leader faces the air, the followers shelter
- The front is the place most sheltered from the wind
- Riding at the front takes less power
- The rules make each rider lead in turn
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The leader faces the air, the followers shelter
Drafting greatly reduces the followers' drag: taking turns shares the leader's effort, the most costly one.
Frontal area: what is it?
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The area the body presents to the fluid.
Crouching on the bike reduces it.
Why does water drag slow far more than air drag?
- Water is about 800 times more viscous than air
- Water is about 800 times denser than air
- Buoyancy slows the swimmer down
- The swimmer moves faster than a runner
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Water is about 800 times denser than air
Fd = ½ × ρ × Cd × A × v²: drag is proportional to the density ρ, about 1,000 kg/m³ for water against 1.2 for air. Water is only about 55 times more viscous than air.