The Magnus Effect and flying cups

Some of experiments with flying balls and cylinders give strange, like question-mark trajectories. For example please watch one of students' experiments focused on the Magnus effect: Initially the force causes curving of the track but later, for some time, the cups move along straight line with constant velocity. According to Newton's first law, at this stage of motion there are balanced forces. As a result we can determine quantitatively the Magnus force acting on the tested object. Use a slider to change a value of "curving" coefficient Cs, drag the cups along trajectories and fit a starting point S by moving the centre of the circle. Finally analyse the force components on the straight-line part of trajectory.
by Ball Group - iStage 3 "Football in Science Teaching"