The Physics of Swimming
Swimming is a negotiation with water's density. Your body is almost exactly as dense as water, so tiny adjustments decide whether you float or sink — and every meter forward is bought by shoving water backward.
You are your own ballast
The human body averages ~985 kg/m³ — just under water's 1,000. A lungful of air lowers your average density and you float; exhale and you sink. Salt water (~1,025 kg/m³) floats you noticeably higher.
Drag grows with v²
Water is ~800× denser than air, so drag dominates. Drag ≈ ½ρv²CdA: doubling speed quadruples resistance — which is why elite swimmers obsess over streamline position and shaving milliseconds of form.
Newton's third law propels you
Hands and feet push water backward; water pushes you forward with an equal, opposite force. The catch-and-pull of freestyle is engineered to maximize the backward push per stroke.
Waves waste energy
At the surface you make bow waves like a boat, radiating energy away. Swimming just below the surface (underwater dolphin kick) avoids wave drag — which is why rules limit how far you can stay under.
Try this yourself
- Float with full lungs, then exhale fully — feel yourself sink as your density crosses water's.
- Push off a pool wall streamlined vs spread-eagled and compare glide distances.