Buoyancy - Buoyancy of Air

Buoyancy of Air

Similar to objects at the bottom of its ocean of water looking upward at objects floating above it, humans live at the bottom of an "ocean" of air and look upward at balloons drifting above us. A balloon is suspended in air, and a jellyfish is suspended in water for the same reason: each is buoyed upward by a displaced weight of fluid equal to its own weight. In one case, the displaced fluid is air; and in the other case, the fluid is water. Objects in water are buoyed up because the pressure acting up against the bottom of the object exceeds the pressure acting down against the top. Likewise, air pressure acting up against an object in air is greater than the pressure above pushing down. The buoyancy, in both cases, is equal to the weight of fluid displaced - Archimedes' principle holds for air just as it does for water.

A cubic meter of air at ordinary atmospheric pressure and room temperature has a mass of about 1.2 kilograms, so its weight is about 12 newtons. Therefore, any 1-cubic-meter object in air is buoyed up with a force of 12 newtons. If the mass of the 1-cubic-meter object is greater than 1.2 kilograms (so that its weight is greater than 12 newtons), it falls to the ground when released. If an object of this size has a mass less than 1.2 kilograms, it rises in the air. Any object that has a mass that is less than the mass of an equal volume of air will rise in air - in other words, any object less dense than air will rise. Gas-filled balloons that rise in air, thus, are less dense than air.

Greatest buoyancy would be achieved if the balloon were evacuated. The weight of a structure needed to keep an evacuated balloon from collapsing would more than offset the advantage of the extra buoyancy. Thus, hot air balloons are filled with gas less dense than ordinary air, which keeps the balloon from collapsing while keeping it light. In sport balloons, the gas is simply heated air. In balloons intended to reach high altitudes or stay up for extended periods of time, helium is generally used. The density of helium is small enough so that the combined weight of helium, balloon, and whatever the cargo happens to be is less than the weight of air it displaces. Hydrogen is the least dense gas, but it is highly flammable and thus seldom used. Low-density gas is used in a balloon for the same reason that cork or closed cell foam buoyancy material is used in a swimmer's life preserver. The cork or foam possesses no strange tendency to rise. Both are buoyed upward like anything else. They are simply light enough for the buoyancy to be significant.

Unlike water, the atmosphere has no discernible surface (there is no "top"). Furthermore, unlike water, the atmosphere becomes less dense with altitude. Whereas a cork will float to the surface of water, a released helium-filled balloon does not rise to any atmosphere surface. With regards to how high a balloon will rise, a balloon will rise only so long as it displaces a weight of air greater than its own weight. Air becomes less dense with altitude, so, when the weight of displaced air equals the total weight of the balloon, upward acceleration ends. We can also say that, when the buoyant force on the balloon equals its weight, the balloon will cease to rise. Equivalently, when the average density of the balloon (including its load) equals the density of the surrounding air, the balloon will cease rising. Helium-filled toy balloons usually break when released in the air because, as the balloon rises to regions of less pressure, the helium in the balloon expands, increasing the volume and stretching the foil until it breaks.