##### Note: to run Physlets you need a browser that supports Java, JavaScript and Java to JavaScript communication. Most current browsers for the Windows and Linux OS support this, however at the moment, only the Safari browser for the Macintosh OS X.3 and  OS X.4 do.

What are Physlets?

Physlets are Java applets written by Dr. Wolfgang Christian and his students here at the Davidson College physics department. These physics applets, are interactive animations or animations representing physical situations, like Brownian motion. This particular applet was written by Dr. Wolfgang Christian and Jim Nolan ('00) and the script was written by Dr. Mario Belloni.

(For examples see Physlets: Teaching Physics with Interactive Curricular Material  and/or Physlet Physics: Interactive Illustrations, Explorations, and Problems for Introductory Physics  both by Christian and Belloni.)

Our professors use Physlets in all of our courses in one way or another. Why? Understanding physics relies on conceptual understanding and model building based on "seeing" and conceptualizing phenomena. It is often said that Feynman when asked to calculate the motion of an electron would ask himself, "if I was an electron, what would I do?" Many Physlet exercises are created to help build this type of modeling and visualization skill.

Consider the Brownian motion animation appearing on this page. Click either link below to start the animation.

 Brown Einstein

In 1827 botanist Robert Brown noticed that grains of pollen suspended in a liquid moved erratically. This motion is called Brownian motion. In 1905 Einstein explained this motion by creating a mental picture or thought experiment (also called a Gedanken experiment) that involves the invisible molecules in the liquid crashing into the larger pollen molecules.

But how can you visualize this erratic behavior and understand its cause? Since we cannot see the individual molecules that make up a liquid or air, it is hard to do an experiment or to draw a satisfying picture. We can, however, simulate the effect of smaller particles on a larger particle by using a Physlet. The  two animations differ in that for Brown's discovery the particles are invisible and for Einstein's explanation the particles are visible. The Einstein animation allows us to see exactly what Einstein "saw" in his mental model and what Brown was missing.

We can also represent the motion of objects  by graphing. An example of this visualization is shown below. The graphs depict the position of the particle (x,y) as a function of time and show exactly how erratic the motion of the large particle really is.

 Brown graph Einstein graph

Web Sites: the main Physlets website and the Physlets book Online CD.