1. The problem statement, all variables and given/known data Consider a horizontal slab of air whose thickness(height) is dz. If this slab is at rest, the pressure holding it up from below must balance both the pressure from above and the weight of the slab. Use this fact to find an expression for Dp/dz, the variation of pressure with the altitudem in terms of the density of air 2. Relevant equations PV=NkT PV=nRT 3. The attempt at a solution I don't know how to begin this problem. I guess the first thing I should do is determined the forces acting on the slab, which I guess would be the weight of the slab and the pressure above. Fnet=Pressure-mg=0 => P=mg. The slab also has some potential energy since it moves at a height of dz. not sure what to do with the air density term but I thin Pf=P0+rho*g*h. Not sure what else to do.