Williamson and Willoughby (2012) found that while students may hold a combination of these alternative
models, they may also simultaneously hold other misconceptions as well as correct conceptions related to the
scientific model. Students interviewed in this study who had even a cursory knowledge of Newtonian gravity
typically understood that mass and distance are important factors in determining gravitational force, but they
have trouble applying these ideas in a consistent and coherent manner. For instance, some students believed that
only very massive objects can exert a force on other objects, or they confused the ideas of mass and density. The
most common errors associated with distance were in measurement, with students measuring the distance as the
radius of the planet or the distance from the surface. Additionally, it was clear that many students do not think of
gravity as a mutual force of attraction, but rather as a property that emanates from a material or object.