Another way to view how they relate is in terms of what parts of the universe they study within the great hierarchy of the structure of reality.  As one moves from a micro scale perspective to a macro scale perspective we find physics dealing with the two greatest extremes: nuclear physics at the sub-atomic particle level, and astrophysics at the cosmic and universal level, with string theory arguably attempting to deal with both simultaneously.  As one moves up the hierarchy from nuclear physics through greater levels of aggregation one moves through realms studied respectively by chemistry, molecular biology, organic biology, psychology, economics, political science and sociology, ecology, climatology and geology, and finally to astrophysics as one moves beyond the scale of the planet earth.  In this hierarchy we find the social sciences occupying an intermediate scale, with economics providing a crucial link from the behavior of individual people to the behavior of groups of people