WATER, WIND, COLD

To float in water, a body must be less dense than water, i.e., weigh less than if it were entirely filled with water. It is this difference in density that allows the body to float, thanks to Archimedes’ buoyancy force: any object immersed in a liquid experiences an upward force equal to the weight of the displaced liquid. So, when an object is placed in water and floats, it displaces a volume of water equal to the volume of the submerged part of the object. At equilibrium, the total weight of the object is equal to the upward buoyant force, itself equal to the weight of the volume of water displaced by the submerged part. Given the density ratios of water and the human body (1000 kg/m3 for the former, 950 kg/m3, or 95%, for the latter), the human body floats. This is well known when wakeboarding. So, on average, 95% of a human body is immersed in water. Of course, this depends on the body — as muscle is heavier than fat, a very muscular athlete with no fat will float less than the average Joe with the same mass.