adjectives - "-ic" vs "-ical", which to use

Is there a rule about when an adjective should end with "ic" or "ical"?

Geometr-ic

Analyt-ical
Theoret-ical
Kinet-ic

Answer

The NOAD says that the -ical suffix is used to form adjectives:

• Corresponding to nouns or adjectives usually ending in -ic (such as comical corresponding to comic)


• Corresponding to nouns ending in -y (such as pathological corresponding to pathology)

As for the words you list, the NOAD says:

• Geometric and geometrical are both adjectives, with the latter being a derivative of the first


• Analytic is another term for analytical


• Theoretic is another term for theoretical


• Kinetical is not a word listed by the NOAD

In some cases, a suffix is preferred over the other one, but there isn't a rule that explains in every case when to use one or the other.