You can get an incredible amount of information from looking at the
graph of a potential energy, *U(r).* This particular plot shows the typical potential energy
between two atoms. It is weakly attractive at large distances and strongly repulsive at short
distances [because the laws of quantum physics do not allow identical particles like electrons to occupy the same
region of space]. A bound system of two atoms has *E* less than zero. At any point,
*E = K + U,* so where *E* becomes equal to *U,* the classical kinetic energy
must be zero. These points are therefore called “classical turning points.”

In physics we always define the potential energy between a pair of objects,
due to a fundamental force acting between them, to be zero when the objects are an infinite distance apart.
That is, *U(r)* → 0 as *r* → ∞. There is only one case (forces between quarks) where this
is impossible!