A common example is the pressure of the Earth's atmosphere: atmospheric pressure is highest at the surface of the Earth, and decreases as you go up in altitude. We'll call the pressure on the top surf...A common example is the pressure of the Earth's atmosphere: atmospheric pressure is highest at the surface of the Earth, and decreases as you go up in altitude. We'll call the pressure on the top surface P1, and the pressure on the bottom surface P2. For a gas like the Earth's atmosphere, we typically have the density proportional to the pressure, so let's let the density ρ=KP, where K is a constant with units of s2m−2.
