Analysis of Video showing Pull
Observations on Swing Machine Results
A). Energy Distribution
From the Howard Holdsclaw's graph (below) of a number of trials (a-h), the lengths of travel of the balls were measured (arbitrary units) and their ratio, scroqueted/sstriker,calculated. The ratios varied from 1:1.29 to 1:1.88 as shown in the following table.
If we were dealing with the classical ice pucks and striking exactly between the expected paths we would expect both pucks to travel the same distance. Here we find a ratio of ~1.6. It does not seem to be due to the coefficient of restitution, since when the balls are initially struck there is solid contact between the balls and they can be treated as a single entity. Being of equal mass they should share the total input energy equally. Perhaps the striker’s ball is making multiple transits between the croqueted ball and the mallet, or it is a solely mallet-ball interaction, e.g. possibly the mallet rubbing on the ball causing back spin, or rubbing the striker’s ball into the turf.
B). Reproducibility of Measurements
There was no trend in the ratio of distance travelled by both balls with the strength of the shot (see table above) so this suggests experimental scatter. Scatter could be due to the balls being not quite in contact, variations in the balls or the terrain conditions. This latter could be debris on the route, wind or wear. It has been noted previously when a ball is rolled down a ramp and its track measured, its track gets longer and longer with each successive trial as the grass gets progressively flattened.
C). Pull Demonstrated
The final positions of the balls deviate from the simple (school level physics) projection of their initial paths. They lie on the dashed lines not the solid lines in Howard's graph. They move, as folk-law on pull predicts, towards the aiming line.
D). The Croqueted Ball Pulls More than the Striker’s
For the same distance travelled, the croqueted ball ‘moves in’ around twice (1.92x) the distance of the striker’s ball from the geometrically predicted paths. Clearly the croqueted ball gets more of whatever is causing pull than the striker’s. If, as conventional wisdom suggests, it is spin about a vertical axis then one can imagine that the interaction of the striker’s ball with the mallet face could decrease the spin. It would be interesting to try the same stroke with the mallet faced with emery paper or Teflon film to change its grip on the striker's ball.
E). Pull Results in Curved Tracks?
(... for the striker’s ball at least!) I put plastic film over my monitor and plotted the ball positions of the striker’s ball with respect to the marking strings from the video. I moved the film to compensate for the frame jitter which shifts the image. The curved track is shown in the diagram (right). The green lines are just to guide the eye, the red spots are the data. Clearly there is a perspective effect due to the angle of view which affects the scaling of the image but the string (blue lines) shows that this is marginal. Further tests would be needed to be sure that this was repeatable and not say, due to bias on the ball.
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