Gravity, including centripetal force of a
spinning sphere, pulls on air molecules to stay on the surface. As the
Earth spins, the gas molecules response by staying close to the ground.
Imagine you are at the amusement park and you enter into a ride. It is
circular and they strap you in a cell. The ride begins by spinning and
spinning, then the floor starts to fall. You feel like you will fall
with the floor, but you donít. You are still "stuck" to the
sides by centripetal force.
Movement of the air molecules is also caused by
the density of each of the gases. For example, helium is lighter than
oxygen so it tends to "escape" into the upper reaches of the
atmosphere. Movement of air is dependant on temperature, rotation, and
density. Air pressure also changes with height of the atmosphere. The
interplay of air pressure can create "fun" activities. Once
you do the following demonstrations in lab, you can create similar
experiments to amaze your friends.