Produced in recent experiments, such movies have advanced physicists' understanding of what determines the path of a sheet of paper as it falls to the ground. Yet it may surprise you to know that these common occurences still present a scientific challenge. The complex downward motion of paper can provide insights into such things as how to prevent aircrafts from stalling and how insects can fly with a minimum of effort. How paper falls to the ground is a process impossible to describe exactly with the laws of physics. This is because paper falls through air .
Air is a fluid, the term in physics to describe any gas or liquid.
And the behavior of any fluid can be incredibly complex. The equations describing the motion of a fluid can be written out on a single line, yet their properties are a topic for entire books, a subject for specialists, and an open question in physics. Understanding fluid dynamics is one of the last great classical problems in physics. The equations for the motion of a fluid, known as the Navier-Stokes equations, relate the velocity and pressure at each point in the fluid for each instant in time. The equations also depend on the given values of density and viscosity of the fluid.