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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?

You are asking two separate questions:

1. How can gases exist at all? Why do not all the molecules just fall down to the surface of the earth under the force of gravity? and

2. How can light gases, such as helium in balloons, rise, apparently against the force of gravity? I will address the two questions in turn.

1. Gravity acts on all masses, but this is counteracted by the constant thermal motion of all molecules. The temperature of a substance determines the average kinetic energy of its molecules. This means that light molecules will move a lot faster, on average, than heavy molecules at the same temperature. If the molecules do not tend to stick to each other particularly strongly, then they will tend to fly apart. If they are moving fast enough, gravity is not strong enough to keep them stuck to the surface of the earth. They do not necessarily have enough energy to fly off into space, but they will not hold still either.

You can compare gas molecules to bouncing balls. A bouncing ball is attracted to the earth by gravity, but it does not stay at the surface because it has kinetic energy. Gas molecules work exactly the same way, except that they don't lose their kinetic energy with each bounce. So they keep moving. The atmosphere stays near the surface of the earth because of gravity, but it does not collapse because of kinetic energy.

2. Gravity acts on all objects with mass. Why, then, should anything rise, against the direction of the gravitational pull? The reason is that something light will rise if in exchange something heavy can fall. It is exactly the same thing as having a light and a heavy person at opposite ends of a playground seesaw - the light person will go up, seemingly against the force of gravity! There is nothing mysterious about this, of course, because the heavy person moves downward at the same time.

When a helium balloon rises in air, what is effectively happening is that the air above the balloon and the balloon are switching places. When the light balloon moves upward, the heavier air moves downward. More mass goes down than up. Exactly the same thing happens when bubbles rise in water.

On a related topic, when you look at it this way, you realize that the common expression "heat rises" is not strictly true. What actually happens is that "cold sinks".

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois

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