When several forces act... the total force is the sum... each component of the total force is the sum of the corresponding components of the individu… - Steven Weinberg

To start, we will consider a single particle moving in three space dimensions under the influence of a general central potential. Later we will specialize to the case of a Coulomb potential, and work out the spectrum of hydrogen. One other classic problem, the harmonic oscillator, will be treated at the end of this chapter.

Almost all of modern cosmology is based on this Robert-Walker metric, at least as a first approximation.

The principles of quantum mechanics are so contrary to ordinary intuition that they can best be motivated by taking a look at their prehistory.