Air is a fluid, so a person immersed in air has a buoyant force upon them. Estimate this force on a 100-kg person.

A) 0.01 N

B) 0.1 N

C) 1 N

D) 10 N

Solution: C. The formula for buoyant force is F_b = ρ_fV_dg. We know the density of air and g, so all we need to know is the volume displaced by a human. People are roughly the density of water, so we can use the definition of density and the mass to determine volume displaced: V = m/ρ = 100/1000 = 0.1 m^3

Plug this into the formula for buoyant force (rounding density and g):

F_b = ρ_fV_dg = 1*0.1*10 = 1 N

This is approximately 0.1% of the weight force of 980 N, so it is typically ignored. But a sensitive enough scale (which measures normal or tension force) would be able to tell the difference.