Figure 6 shows wind-tunnel data [3] for CL as well as trajectory analysis results [1] for
CL for a 32-panel soccer ball. Though we distinguish between backspin (CL > 0) and topspin
(CL < 0) in our computer codes, the physical manifestation of the air force’s component
perpendicular to drag is independent of whether the ball has backspin or topspin. We are thus
interested only in the absolute value of CL.