For a ball to roll without slip across the surface of a rotating platform, a frictional force of the formAωd×v is required, whereAis a known constant, ωdis the platform’s angular velocity, and v is the particle’s velocity relative toinertialspace. Since this force is of the same form as that experienced by a charged particle moving through a magnetic field, the ball executes only closed circular orbits. Other similarities to the charged particle problem are easily demonstrated: for example, drift velocities are produced normal to any external force lying in the plane of the disk, and an adiabatic invariant can be defined for the case when the disk changes its spin rate.