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sammy
174.91.53.20

# Ball Roll and Grass...

July 14 2015, 3:23 AM
 Geoff .... could you expand on this statement of yours, and particularly comment on the "grass" component and the perception that some superior putter design can somehow overcome the chaotic nature of the grass surface as it affects ball roll?: "and 2) "true roll" is an illusional goal since the grass changes the worst putt into the best putt is about the first 2-3 inches of grass-ball friction no matter what stroke or putter design is THOUGHT to help the total putt for "better" "true roll"." Thanks....
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Geoff Mangum
Owner
71.54.49.31

# Ball-Green Physics and So-Called "True Roll" Strokes and Putters

July 21 2015, 12:26 PM

Sure sammy!

In almost all cases of putts of standard distances over greens of usual grass frictions, ANY stroke method and ANY putter design will "start" a ball off in a certain "state" of forward spin or back spin and launch angle level across or upward off or downward against the surface.

But after that, the GRASS takes over. Once a ball "engages" the grass friction in the level-moving sense, a the lateral direction across the surface, after any hopping in the air is over, the grass FRICTION opposes the area of the bottom of the ball in contact with the surface grass. This opposition results in SOME SKIDDING of the bottom of the ball across the surface and then SOME ROLLING.

If you watch the exact spot on the bottom of the ball at the start of engaging the grass, this spot slides or skids and does not get PUSHED BACK by the friction, at the beginning. But then the friction shoves this spot back and then a second spot on the ball becomes the bottom, and then a a third and so on as the ball starts to "roll" a new bottom spot over the surface.

So the question is whether the rolling speed of the ball's circumference matches the sideways "translational" velocity of the center of the ball across the surface in such a way that a NEW spot of the ball's circumference is presenting itself at the bottom, without any spot getting stuck there and SKIDDING across the surface.

The friction adds more and more rolling of the circumference until the skidding completely dies out. That state is called "true roll" in goofy golfspeak, and simply "rolling" in physics. (There was no "false roll" before then in the mix of some skidding and some rolling.)

So the FRICTION converts skidding balls into rolling balls. In physics this conversion is ALWAYS complete whenever the initial velocity of the center of the ball across the surface (e.g., 100 inches per second or 6 feet per second or something like that) has been reduced by the friction to 5/7th the initial velocity -- or about 70% velocity compared to the starting velocity. A ball that comes off the face and engages the grass at a starting velocity of 100 inches/second will ALWAYS be "true-rolling" without any more skidding once the friction slows that ball down to 70 inches/second. Every time, no matter what. So thicker grass with more friction finishes off the skidding sooner in a shorter span than does a faster, slicker green.

None of this post-impact physics has anything whatsoever to do with the "special true-roll stroke" or the "special true-roll putter design".

But the bogus idea remains that starting a ball in a state of MORE FORWARD SPIN will reduce the total distance the ball is NOT true-rolling COMPARED to what happens when a ball starts off with LESS forward spin or even with back spin.

Well, sort of. In physics what happens is always the same and is available in all the high-speed filming of strokes and putters that teachers and putter sellers use to "sell" the true-roll stroke or putter, but they do not ever look for it (because ignorant of the physics), or see it (because ignorant about how to view data), or wonder about what is the DIFFERENCE between stroke A and B or putter A and B as shown by the data in the film.

First, you have to see and understand any DIFFERENCE that a special stroke or putter makes in the actual rolling of the ball and then ask whether that difference is worth the trouble.

The DIFFERENCE in almost all cases is yes, the forward-spin does make the skidding phase of the putt shorter, but only by 2-3 inches, and specifically the only difference between stroke A and B or putter A and B is in the FIRST 2-3 inches that the ball engages the grass friction, when the ball has maximum momentum.

(When putter manufacturers show high-speed film to CLAIM that their putter shortens the "skid phase" in comparison to the rival putter by 8 inches or more, they are always IGNORANTLY and INCORRECTLY counting that part of the beginning of the putt when the ball is in the air as launched or as hopping or bounding. Any time the ball is off the surface is NOT legitimately included in any comparison of "skidding". If you subtract out the comparison distance the span of distance that the ball is in the air, the real DIFFERENCE is always only 2-3 inches.)

If a "good" true-roll stroke or putt starts the ball off with 45 degrees forward spin off the face of the putter, and a "bad" true-roll stroke or putter starts the ball of with 45 degrees of backspin off the face of the putter, once the "bad" putt engages with the grass friction, the friction converts that 45 degrees backspin into 45 degrees forward spin in about 2-3 inches. At that point, the two putts have an identical engagement of the ball and grass, the backspin putt perhaps with a trivial amount less of velocity after getting converted. But otherwise, after 2-3 inches at the beginning IN PHYSICS there is no significant DIFFERENCE remaining.

That's the actual effect, so is this DIFFERENCE worth the trouble -- is a forward spin ball going to have better line and distance for the putt than a backspin ball? No. The first two inches don't affect line unless there is something hard in the grass that the ball hits at a momentum insufficient to knock the obstacle out of the way. In 99%+ of the times, the only thing the ball "hits" when it starts on its way over the first 2-3 inches is tender, flexible, short grass blades that pliably fold out of the way of "Steamship Golf Ball". So line very very rarely is adversely affected by backspin and there is no worthwhile improvement in line with going for "true roll" forward spin.

As to distance, the question is not more skid or less skid, but consistency of whatever skid used to go all the way.So 2-3 inches of DIFFERENT skidding hardly matters, and consistency matters.

The confusion arises because golfers and golf teachers and golf club manufacturers are apparently uniformly uninterested in and ignorant of physics and reality, and because some of the efforts to get better rolling action cause OTHER improvements in stroke for better line and/or distance while the golfer delusional thought he was going for and getting better "true roll". So a forward press might tighten up the wrist and make for a straighter hand action thru impact and a more solid, straighter stroke -- nothing to do with "true roll", but golfers think that is what the forward press was FOR. It's just the general disorder and confusion of where golfers start out for skill that allows them to vest in these goofy, delusional notions of what is causing what for better-than-usual results for line and distance.

Cheers!

Geoff Mangum
Putting Coach and Theorist

PuttingZone.com -- over 200 Certified PuttingZone Coaches teaching in 21 Countries Worldwide and growing strong!
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 This message has been edited by aceputt from IP address 71.54.49.31 on Jul 21, 2015 12:34 PM

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Frans