  Lessons and homework problems involving a pendulum are often a big part of
introductory physics classes and laboratory courses from high school to
undergraduate levels. Although laboratory equipment for pendulum experiments is
commercially available, it is often expensive and may not be affordable for
teachers on fixed budgets, particularly in developing countries. We present a
low-cost, easy-to-build rotary sensor pendulum using the existing hardware in a
ball-type computer mouse. We demonstrate how this apparatus may be used to
measure both the frequency and coefficient of damping of a simple physical
pendulum. This easily constructed laboratory equipment makes it possible for
all students to have hands-on experience with one of the most important simple
physical systems.
