Archimedes wrote:Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
When I exhale a deep breath, I release about 5L (50 cm^3) of air. So the real question is, when I exhale that much, do I get 5L smaller? More than 5L smaller? Less?
If I get exactly 5L smaller: my weight doesn't change.
If I get 6L smaller: I become 6L "heavier," because I am displacing less air and the upward buoyant force is less, but I am 5L "lighter," because I've released that much air and the downward gravitational force is also less. So overall I am 1L "heavier."
If I get 4L smaller: I become 4L "heavier," due to reduced buoyancy, and 5L "lighter," due to reduced weight. So I am 1L "lighter."
My first instinct is to say that when I take 5L of air from the atmosphere my volume increases by less than 5L, because I can sustain an internal lung pressure greater than atmospheric pressure. Considering only this, the scale would say I weigh less when I exhale. But the real answer depends on the specific structure of the lungs/chest muscles/torso, and the detailed way they all work together to expand and contract as I breathe. So I'm not sure. One way to measure this might be to submerge someone in a full bathtub, have them breath out, and measure how much the water line drops.