If the buoyant force on an object is equal to the weight of the water displaced by the object, why do people have an easier time floating on their back than vertically with their head sticking out of the water?
[Physics] Why do people only float in certain orientations
buoyancy
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Best Answer
The chest region provides the least dense region in the body since it traps a large airspace. And for either scenario (back floating or vertical) the chest is submerged.
So the only difference you need to consider is that in the vertical position, the head (at least above the nostrils) needs to be above the water line so that the person can breathe. But in the back floating position the nostrils can be positioned at the highest position, and thus more of the head , relative to the vertical position can be submerged - taking advantage of Archimedes's principle.
An adult human head weighs about 15 pounds so submerging as much of it as possible can eliminate the need to dog-paddle and thus passively maintain access for breathing.
Survival experts and SCUBA instructors will always train their pupils to float on their backs to conserve energy.