The Influence of Shape on the Inherent Rolling Potential of Loose Rocks

The likelihood that rolling of a rock will be initiated and/or sustained on a slope depends on many factors related to the characteristics of the block and the slope. However, all other things being equal, some solid shapes have a greater potential to roll on a slope than others. This paper describes the results of a systematic laboratory study to determine how shape affects the ease with which rolling of ball-like blocks can be initiated, and its likelihood of being sustained. A simple scheme is presented to group basic shapes with similar rolling tendencies. Through systematic tests with polyhedral blocks on a frictional ramp with a range of inclinations, different basic ball-shaped forms are compared in terms of the ease with which rolling can be initiated, and the likelihood that it will be sustained. The results show that "ball" shapes (with principal dimensions of roughly similar size) are more prone to rolling but that even between shapes within this group, such as cubes and octahedra, the tendency to roll is strongly influenced by other factors including the number of faces. The importance of the starting position on the initiation of rolling is also demonstrated and quantified.