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Gravity and Friction

Ristometer to measure green friction

In the "weightlessness" and "vacuum" of space, a putt will impart to a golf ball a force of acceleration that will change the ball's state from one of rest to one of constant velocity, and the velocity will stay the same thereafter until the Crack of Doom (unless the ball is affected by some other force). On the earth, there is always "some other force" hanging around: gravity. That's why a canon ball fired at 45 degrees inclination from the surface makes a parabolic rainbow trajectory as gravity pulls it back to earth. Gravity is not sudden or variable, but is always steady. That's why the parabola shape of the canon ball is smooth and predictable. That's very helpful in putting, since gravity is very predictable and putt paths never have kinks or angles in them.

But a putted ball never goes up into the air like a canon ball, so how can gravity affect it? If you place a ball on a flat and level marble table that extends for a mile and then putt the ball, the ball will roll straight but it will eventually slow down and stop. Why? Because of friction between the bottom of the ball and the table (and a little air resistance) -- in this case, not much at all, but some. The steady influence of this friction eventually wears down the forward motion of the ball. Now imagine the same marble table in space. The putt will roll straight on the table surface but will never stop. The lesson is that gravity is what makes friction.

The other lesson is that the surface also makes friction, or more correctly, the contact between the ball and the surface. (That's why tire treads have best contact with the road when they are not fully inflated.) So, in putting, since neither the ball nor gravity ever changes, the only thing that affects the extent of friction is the surface. Tightly mown, tournament-condition bent grass has little friction compared to shaggy, late-in-the-day bermuda. Wet grass is slower than dry grass, because it has less friction.

And, although it is a minor point, it is not really accurate to say that gravity is never different for a given putt, although gravity itself never changes. That's because it does matter in any given putt what sort of tilted surface the ball has to cross. For friction, not all gravity matters, and only that component of gravity that is directed flush into the surface creates the friction. So a ball rolling across a flat and level putting mat has Friction=2 but when you tilt the matt on its side and roll the ball across it, then the Friction=1 (less). Only some of the gravity is holding the ball against the surface in the second case. So, sidehill breaking putts can actually involve less friction than putts across an untilted surface and putts downhill have a little less friction, too (not much). Putts uphill have a little more friction, not from the tilt's reducing the component of gravity, but from the forward motion of the ball running it into the surface harder.

Grain in a green is an "opposing force" when the ball is rolling into the grain because the angled tips of the grass blades act like miniature pikes defending against the onslaught of the ball, and grain is very similar to friction, but only when the ball is going "into the grain." Like tilt, grain has its maximum effect only when the ball is going directly into the grain, and when the ball rolls athwart the grain or with the grain, there is less to none opposing force. (It's not really correct to think of grain as making putts slower OR FASTER, since grain only works to slow the ball's roll, and never really speeds it up. When the ball rolls with the grain, there is simply less opposing force, and there may even be less friction too, so that the same putt across a level no-grain surface rolls at Speed=1 whereas the same putt across a level with-the-grain surface will roll faster, at Speed=2.)

So Touch necessarily involves an appreciation for gravity's effect on a ball in general, how gravity creates friction between the ball and the green, and how this friction can be greater or lesser depending upon the contour and condition of the green (including grain).

Green Speed Factors

The main factors in green speed are type of grass and grass condition. Bent grass that has been recently mowed to under 1/8th an inch and rolled will be a lot faster (less friction) than day-old bermuda that has seen 50 foursomes tromping through. The other factors are relatively insignificant compared to these basics. Other factors include moisture, surface irregularities (footprints, debris, spike marks, ball pitch marks), maintenance and preparation practices, and invasive weeds or disease patches.

Mositure is not simply dew or rain or recent sprinkling for irrigation: it also includes the water content of the grass blades and turf as well as the humidity of the air. Sometimes winter greens can be very fast because the growth of the grass is curtailed and the air is dry (although frigid). These greens are faster than the same grass height on a hot humid summer day in the South. The humidity tends to prevent the water content of the grass blades from evaporating, so the leaves have more of a lush character that retards the roll of the ball more. So watch out on a day when the air is dry, the temperature high, and the wind on the fly -- the green will get blow-dried to a crisp and be faster than a cat on fire. By the same token, watch out for greens where half the green is in the shade of a tree -- the shade will keep the grass cool and moist and will be a little slower than the sunny part of the green. And, by the same reasoning, greens that sheltered down in the terrain are apt to be slower than greens perched on a hilltop exposed to the wind. And, same thing, greens that are down low in the surrounding terrain in the pathway of the drainage of higher terrain will be slower than greens on an elevation in the local terrain, with no "upstream" drainage coming its way.

You can often tell mositure content of a green from looking at its color (moist makes it darker), from the sponginess of the turf under your feet, and from taking a peek into the hole at the exposed turf to see if it looks moist or dry. (PGA Tour greens have this exposed turf covered up with white paint for television.) A greenkeepr has a tool made from a iron with the shaft cut off near the head, and he sticks the hollow shaft into the green 3 to 6 inches and removes a sample plug to inspect by sight, taste, smell or feel for its moisture content. The golfer can use sight, feel of the feet (not the hands), and smell, without violating the Rule against "testing" the surface.

Greens are typically watered overnight and then perhaps again at the start of the day, so morning greens have the most moisture on a rainless day. As the sun warms up, the greens dry out and get faster. Later in the day, as the grass grows, there is a mixture of influences -- some slowing the green (growth, humdity, and rain) and some speeding it up (dry air, heat, sunlight, wind). The change in green speed from recently mown morning greens to dried mid-morning / early afternoon greens to cool and humid early evening greens can be very considerable. It is always a good idea to listen to the green speed through your feet. At Augusta in the 1950s and 1960s, the greens were bermuda and to make them fast, the greenkeeper

Surface Condition Factors

Spatial Perception

Distance Perception


Space and the Body

Cerebellum Role

Calibration to a Green




Relation to Targeting and Tempo


Updated Monday, July 7, 2008 6:14 AM

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