  The concept of "force" is abstract and challenging for many physics students,
and many studies have revealed misconceptions that hinder students'
understanding and learning in classical physics. One reason for this may be
that physics textbooks do not define the concept of force in concise and
consistent ways. In the present study, we have investigated how "force" is
defined in physics textbooks used in upper secondary schools in Norway, and
present a framework of eight categories for how the concept is defined. The
framework is developed inductively from textbooks, and motivated and discussed
in light of how the concept has developed historically in physics. Examples
from one of the textbooks that constituted the empirical basis for the
framework are given. These reveal that textbooks may present students for a
multiplicity of definitions of "force" and hence contribute to students'
challenges in learning and using the concept.
