Williamson andWilloughby (2012) describe three main alternative models Astro 101 students implement when
thinking about gravity—The Boundary Model, the Orbital Indicator Model, and the Mixing of Forces Model. The
Boundary Model encapsulates student misconceptions related to a sudden change in the gravitational force for
objects leaving a celestial body. For example, the majority of students believe that one simply must leave the Earth
(and its atmosphere) to feel zero gravitational force from Earth. These students likely view the apparent
weightlessness of astronauts as zero gravitational force and associate weightlessness with the absence of an
atmosphere. Therefore, the atmosphere could be viewed as either a boundary that contains Earth’s gravity or simply
an indicator of the presence of gravity. The Orbital Indicator Model is a related idea in which the objects that orbit
a planet indicate the strength of gravity on the planet’s surface. Students apply this in two opposing ways: (1) A
planet must exert a strong gravitational force on objects if it can hold far away massive objects in orbit, or (2) A
planet must exert a strong gravitational force on objects if they are orbiting close by because it essentially “reigns”
them in. The third alternative model, the Mixing of Forces Model, explains the well-documented (Smith and
Treagust 1988; Sharma 2004; Feeley 2007; Asghar and Libarkin 2010) student misconception that gravity is
confounded with other forces associated with magnetism, rotation, and atmospheric pressure.