How Does Buoyancy Affect a Swimmer or Athlete in Water? (HL)

Buoyancy provides an upward force that opposes an athlete’s weight. It affects whether the athlete floats or sinks, their body orientation, and the resistance they experience while moving through water.

The buoyant force follows Archimedes’ principle: it equals the weight of fluid displaced by the submerged body:

$$F_B = \rho_{\text{fluid}} gV_{\text{displaced}}$$

Here, $\rho_{\text{fluid}}$ is fluid density, $g$ is gravitational field strength, and $V_{\text{displaced}}$ is displaced volume. The athlete’s weight, , acts downward through the , while buoyancy acts upward through the .

For example, an athlete displacing $0.070\,\text{m}^3$ of fresh water experiences:

$$F_B=(1000)(9.81)(0.070)=686.7\,\text{N}$$

This approximately balances the weight of a $70\,\text{kg}$ athlete, since $W=(70)(9.81)=686.7\,\text{N}$. Greater fluid density, as in salt water, produces a greater buoyant force for the same displaced volume.

An athlete’s average body density also matters. Higher proportions of adipose tissue and greater lung volume generally increase flotation, while denser bone and muscle tend to reduce it. If the centre of buoyancy near the chest is not vertically aligned with the centre of mass, a turning moment can rotate the body and cause the legs to sink.

A horizontal, streamlined position reduces frontal area and therefore pressure drag, improving swimming efficiency. The common misconception is that buoyancy propels a swimmer forward; buoyancy acts mainly upward, while forward motion comes from propulsive forces.

Exam technique: Define buoyancy, state Archimedes’ principle, compare $F_B$ with weight, and link centre alignment to body rotation and drag. Do not confuse buoyancy with hydrodynamic lift.