Buoyancy fundamentals

A body floats if its specific mass is LESS than the water's. If equal, it hovers. If greater, it sinks. Salt water has higher specific mass than fresh water → easier to float in the sea than a pool.

Body composition matters

• More fat → lower specific mass → floats more easily
• More bone/muscle (lean, athletic, very muscular) → harder to float
• Holding a deep breath lowers specific mass — helps float

Why legs sink

In most swimmers the upper body has lower specific mass than the legs, so the buoyancy force acts above the gravity centre. The legs drop. Stretching arms above head or bending knees reduces this gap. Women tend to find horizontal float easier than men because the two centres are closer.

Drag forces during swimming

• Vortex drag (water swirling around the body)
• Frontal drag — depends on body cross-section and velocity squared
• Wave drag — created when swimming on the surface; absent when fully underwater (so underwater is faster)