The simulation below shows an interaction between two "particles." The interaction force is a 1/r2 force; that is, the interaction force varies as 1/r2 where r is the distance between them. However, it only "exists" when the particles are less than 0.2 m apart. This collision is similar to the type of collisions that occurred between alpha particles and gold nuclei in the famous Rutherford experiment. This experiment showed that the very massive nuclei is very small compared to the size of the atom.