There is also an article by my long-ago Singapore colleague, Jack C. Cooke. The title is something like The boundary layer and seam bowling. It was published in the Math Gaz approx 40 years ago (pause while I make a brief search) -- Ah!!! Guy's Corollary to Hofstadter's Law is that that everything took place longer ago than you think, even when you allow for Guy's Corollary. The above title is correct, except that ``seam'' should be in quotes. Vol.39(1955) 196--199. The quantitative result is ``about 1 inch in 11 feet'' It's not clear why he used 11 feet. A cricket pitch is just 6 times as long, though the relevant flight is somewhat shorter. From umpiring at cricket I can vouch for some bowlers being able to ``move'' the ball several inches. I used to bowl ``outswingers'' myself. The trick was to place index and middle fingers on either side of the seam, the equatorial plane being vertical and containing the batsman. You dragged on the ball with the two fingers as you released it, imparting an angular velocity about a horizontal axis (i.e., through the `poles'). The gyrostatic action kept the ball in that orientation for most of its flight, but it slowly precessed, and the plane of the seam became diagonal to the direction of travel. The boundary layer effect then moved the ball slightly in a direction perpendicular to its flight. The idea of keeping the seam in the plane of the bowler and batsman at first was so that the swing came late, giving the batsman less chance to make allowance for it. I wonder if Barton cites Cooke? R. On Mon, 21 Jul 2003, Helaman R.P. Ferguson wrote:
Since my hobby is juggling, I try to collect three of any sort of balls, including cricket balls. Even the cast cricket balls have an elaborate equator seam, and yes, the aerodynamics of the seam are important. See the seminal paper below for details of theory and experiments on transverse aerodynamic forces, including wind tunnel photographs:
Noel G. Barton "On the swing of a cricket ball in flight" Proc. R. Soc. Lond. A 379, 109-131 (1982) _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun