Gravity on a half shell

Although much attention has been given recently to treating the troublesome ‘‘preconceptions’’ that beginning students have acquired concerning elementary kinematics and dynamics, higher‐order misconceptions should not be ignored by instructors. In this article, the problems students have with Newtonian gravitation and extended bodies are examined. A three‐step program involving a classroom exercise (the mass dipole), a homework problem (thin, massive rod), and an extra‐credit assignment (thin, hemispherical shell—‘‘gravity on a half shell’’) is suggested in order to increase student understanding of this important law of nature.