The tendency to treat force as partially or completely “used
up” in rotational motion dominated responses to almost all of
the problems we administered. This tendency appeared in contexts
in which students’ interpretation of the situation, and our
interpretation of their responses, hinges on their understanding
of technical terms. It also appeared in contexts in which we
made efforts to minimize these issues. An informal survey of
introductory physics textbooks reveals that little attention is
given in these texts to asserting that Newton’s second law
holds even for rotating rigid bodies. In most cases, Newton’s
second law is said to have a “rotational analogue,” which may imply to students that Newton’s second law itself is not appropriate
for rotating bodies. Furthermore, most textbook explanations
of rotational motion emphasize energy relations,
demonstrating that the greater the rotational energy of an
object rolling without slipping down an incline, the less its
translational energy. The Newtonian approach is implied, for
example, when determining the force of constraint on a falling
yo-yo or a wheel rolling down an incline. However, we have
not seen any textbook make the point explicit that the reader
might be surprised to learn that Newton’s second law is still
true, and that this might have some surprising consequences.
Students may be understandably confused as to which aspects
of one-dimensional dynamics, they carry forward to more
complicated motion and which aspects were specific to the
simpler case