Ask a Physicist Answers

If air is heated and its starts to go towards the space, what is the reason for this? Do you think the air molecules will get accelerated and because of this they will resist Earth’s gravitational force?

-Asked by Mikko Artsola from Finland

A principle of physics called Archimedes’ principle states that when a body is submerged in a fluid, there will be an upward “buoyant force” acting on the body, equal to the weight of the surrounding fluid that has been displaced. There will also be a downward gravitational force on the body (called the “weight” of the body). If the density of the body is less than the density of the surrounding fluid, then the upward buoyant force will overcome the downward weight. Consequently, the net force will be upward and the body will rise upward through the fluid.

Hot Air Balloons

Hot air balloons rise over California.
Image Credit: Willy Volk

Now suppose the “body” is a parcel of air (such as the air inside a hot-air balloon) that has been heated. Each of the air molecules will gain energy and will move faster than before the air was heated. This causes the air’s volume to increase; since we still have the same number of air molecules but in a larger volume, the density (mass of air per unit volume) will therefore decrease, leading to the hot air becoming less dense than the surrounding air. If the average density of the balloon is less than the density of the surrounding air, then, by Archimedes’ principle, the balloon will rise upward in the air.

Of course, it’s not just the air: in computing the density of the balloon, we must divide the total mass of the balloon (including the mass of the hot air, the balloon, basket, equipment, and passengers) by its total volume. All of these components must also be included in computing the weight. The balloon will rise upward only if the total upward buoyant force is greater than the total downward weight.

Answered by:

David G. Simpson, PhD
Research Physicist
NASA Goddard Space Flight Center