Gravity and the Atmosphere
Name: Steven T.
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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