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Name: Steven T.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, November 28, 2002

I was wondering why the force or pull of gravity does not collapse our atmosphere? Why can elements lighter than air like hydrogen or nitrogen float and be resistant to gravity when gravity is supposed to pull everything down at the same rate?

There are a couple of ways to answer your question. First, because the atmosphere is a gas and because gases can be compressed, the atmosphere is denser at lower altitudes In a way the atmosphere is collapsed by gravity.

But more then that, because of the kinetic energy of the molecules of the atmosphere and because of their random notion, air molecules move in all directions including in opposition to gravity. When we observe this - and we can by watching larger particles in suspended in the air - we see what is called Brownian motion. You might find reading about this motion of interest in dealing with your question.

Larry Krengel

Nothing is "resistant" to gravity. Gravity pulls on all matter.

Hydrogen floats because there are other molecules in our atmosphere that are more dense (weigh more / volume of space occupied) than hydrogen. So those higher density molecules simply push the lighter H2 (hydrogen) out of the way. It does this much like water (density 1 g / cc) pushes oil ( ~ 0.90 g / cc ) out of the way and makes the oil "float". Other higher density molecules will displace those lighter molecules. You might get a better education in this matter if you go to the Internet and do a search on "Archimedes principle".

I do no understand what you mean by collapsing our atmosphere. But it is the gravity that the Earth generates that holds our atmosphere close to the surface of the planet.

As a very rough rule of thumb, and irrespective of compound type (N2 or O2 or H2...etc..., I do believe that the "amount" of molecules or to put it another way the pressure of the atmosphere vs. altitude is an exponential

P (atmosphere) = function ( altitude ) = Po * exp ( ALTITUDE / C )

Po = 1_atm = 14.69 psia (at sea level)


C is a mathematical constant that makes the equation true for our given atmosphere ... ie ... 21% oxygen and 79% nitrogen.

Darin Wagner

Gravity DOES pull the atmosphere down. That is why it stays around. The moon and celestial objects like the planet mercury, that are not massive enough have lost most of their atmosphere. Our atmosphere has a certain pressure because of the balance of the gravitational attractive force and the resistance of a gas to compression, as stated fairly accurately by the ideal gas law: PV= nRT where P is the pressure in atmospheres, V is the volume in liters, n is the number of moles, T is the absolute temperature in kelvins, and R is the gas constant, R=0.0825 liter-atmospheres/moles-kelvins. The force of gravity decreases with altitude, but there are other atmospheric factors that come into play. Search the term "standard atmosphere 1976" and you will find numerous calculators that give the pressure and other properties as a function of altitude. One such is:, The lighter gases tend to escape because of the smaller force of gravity on them F = G mM/r^2 the smaller the value of m ( the mass of the atom/molecule) the smaller the attractive force of gravity.

Vince Calder

Hydrogen and any other gaseous molecules indeed have mass and are being pulled to earth by earth's gravity. But temperature (kinetic energy) is keeping them from collapsing --like colliding ping-pong balls in a wind-chamber. Under temperature conditions at the equator/tropics some molecules of hydrogen can be raised to the upper atmosphere and a smaller subset achieve escape velocity and indeed leak-off into space. Its a small % - most that get to the upper atmosphere lose energy and are held by earth's gravity. Larger molecules like nitrogen and oxygen cannot escape at all. If earth was a larger planet, nothing would escape as is the case of Jupiter. Earth's early atmosphere 4 billion years ago is believed to be much like Jupiter is today allowed lots of hydrogen to drift into space. But earth changed because of its size (weaker gravitational forc) and higher average temp (closer distance from the sun) and its evolutionof photosynthetic organisms. Its not surprising that Jupiter's atmosphere s much thicker and hydrogen-rich due to lower average temp and much highergravity. At very low temperatures the atmosphere would collapse around 300 F below zero. And that would need to be the warmest temp on a planet if similar to earth's size.

The moon and asteroids have no atmosphere because their gravity is just too small to hold any gases. With no counter pressure, liquids boil away at very low temperature. As a gas they are lost to space. But could there be a balance where "body" size and temperature could be acheived - a body larger than our moon with less average temperature?

Lou Harnisch


Gravity is not the only force acting on the molecules of our atmosphere. Gravity is not the strongest force, either.

The molecules in the atmosphere are moving very fast. This is because of the temperature. Until the temperature gets down around -250 degrees Celsius, all molecules move around very quickly. When moving around, they bump into each other. All this bouncing is what supports the molecules at higher altitudes. The common word for this constant bouncing around is pressure. The closer molecules get to each other, the more often they bang into each other. Molecules that are closer together experience more pressure. When they get too close together, the pressure pushes them apart. When they get too far apart, gravity pulls them back together.

Hydrogen and nitrogen float in the air because oxygen is heavier. Imagine a large jar. In it are many little balls. Some are lighter, some are heavier. You mix all the balls up. If you then jiggle the jar for several hours, the heavier balls will work their way to the bottom. The light balls will end up near the top. All the rapid bouncing does the same thing with our atmosphere. The heavier molecules are pulled harder by gravity.

Gravity only pulls objects down at the same rate if NOTHING else is pushing or pulling on the objects. For heavy objects such as a steel bar, the force from the air molecules is nothing compared to the immense weight of the steel. A steel bar falls with an acceleration of 9.8 m/s^2. For light objects such as a ping-pong ball, the force from the air molecules is important. A ping-pong ball falls with a much smaller acceleration. Exactly what it is depends on how fast the ball is moving. This is exactly what air resistance is. For objects as light as individual molecules, the force from the other molecules is large enough to completely counter the effects of gravity.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Steven, Gravity is a force and the strength of the force due to gravity depends on the mass of the object it acts upon. A bubble of air is pulled down with less force than an equal volume of water above it so the water displaces the air and the bubble rises. This same buoyancy occurs on a molecular scale. A molecule of hydrogen is pulled down with less force than the nitrogen molecules around them (because the hydrogen molecules have less mass) and the hydrogen rises as a result. It does not all collapse to the surface of the earth because the molecules have kinetic energy and are in constant (rapid) motion in three dimensions. The gravitational force on individual molecules is very small and they have sufficient energy to collide and rebound to keep an average distance between molecules that fills the volume of our atmosphere. If enough molecules stick together, for example when water condenses into clouds, the weight of the individual water droplet becomes too heavy for the energy of the molecules around them to keep them suspended. The result is that the heavy droplets fall to the sky as rain. Fortunately for us, oxygen and nitrogen molecules don't do this under the conditions that exist in our atmosphere.

Greg Bradburn

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