The theory is that greater friction between the string and the ball will cause more spin. First, experiments show that for a tangentially noncompliant surface, the spin will be nearly the same for all magnitudes of string-ball friction for impact angles typical for tennis shots. Sliding Friction and Spin. The following table lists the static COF for a number of individual strings and hybrids at a normal force of about 13 pounds at each string intersection and sorted by COF (more will be listed as they are tested). Friction affects Tennis in multiple ways. Figure 3 — Nylon Comparison. Why Is String Stiffness So Important To Power? Spin is or is not induced depending on the friction between each string and the ball and which string releases its bite last. In video 3, the braided string in back produces higher friction, so in addition to topspin, a sidespin is generated. Friction Some examples … Friction & Air Resistance In Sport Read More » Though many combinations are possible a couple types of deformation seem likely as most important. The slipperier polyester in the second video achieves the same final spin but takes longer to do so. There is obviously a difference in COF between strings and combinations of string types, but most of the setups fall in a COF range between 0.045 and 0.065 over the range of relative string speeds tested. Furthermore, when one or more of the surfaces deforms, the surfaces fit together as hills and valleys. (Note: To see details of rapid movement, movies are best viewed frame by frame using keyboard arrow keys or movie controls.). There is both Static Friction and Kinetic Friction. String Incline to Measure String-to-Ball COF Parallel to the String. This is contrary to the case in static friction where we found the gut mains and poly crosses to be universally the slipperiest combination. The COF indicates how much the friction force increases as the normal force increases. A string that has a low coefficient of friction between strings will commence, sustain, and reverse its sideways slide more easily, optimizing the timing of the snap-back and losing less energy in the process. The second video shows polyester in front and braided nylon in back. Ball deformation becomes a much larger factor in the magnitude of friction in the direction perpendicular to the mains. During the sliding stage, the ball-string friction will help determine how far and fast the main string(s) will move. So, each times a tennis ball bounces, it loses energy. We use cookies to help provide and enhance our service and tailor content and ads. So it seems that the relative deformation between main and cross is what matters in the differing movement between the strings for a hybrid and reverse hybrid. The figure below shows the coefficient of static friction (COF) vs the force pressing the strings together for several popular "spin friendly" polyesters. As it ends up, this is simply the tangent of the measured incline angle. The angles of incidence of the ball in the figures are 60, 45, 30 and 15 degrees from top to bottom respectively. This range appears narrow, but low-to-high it is about a 45% difference. The polyesters have, on average, a lower sliding COF than do nylons. If this were the end of the story, string-to-ball friction would not matter for most tennis shots. In the past, two assumptions were the foundation for the conventional wisdom that (1) a rough string surface creates more spin by increasing the string's bite, grab and push on the ball, and (2) that inter-string motion would lessen that grabbing and thus should be minimized. Movie Screen 2 — Tennis Cloth-String Interaction. That may actually facilitate movement, especially if the deformation causes a leakage of some oil, lubricant, etc. But friction parallel to the cross strings may be different than friction perpendicular to the main strings. Each of the two strings could be the same make, material, shape, or texture, or they could be different. The force of friction from the court on the ball will always depend on the type of surface the ball is bouncing on. We tend to slide when friction is lower and this increases the time over which the force is produced, and the peak force is lower. The method for calculating the coefficient of friction is as follows. How String-to-Ball Friction Affects Spin. (Note: the String Friction Tool will eventually have data on all types of friction available.). Friction ceases as the ball goes into rolling mode and no more spin is imparted. Friction of all types depends on the perpendicular force ("normal" force) pushing the surfaces together and the coefficient of friction (COF). g = gravitational acceleration of 9.8 m/s2. (Note: To see details of rapid movement, movies are best viewed frame by frame using keyboard arrow keys or movie controls.). Large- scale parametric testing could then be carried out for a variety of surface types and components under a range of loading conditions, without the risk of injury to human participants. The braided nylon/zyex string (first video) achieves rolling sooner than the others. In the static COF tests it was about 13 lbs. Figure 4 — Polyester Comparison. There is no such thing as "THE" COF. These include a study that compared different artificial clay court designs; and a study that examined the effect of different acrylic hard court parameters on friction and the tribological mechanisms that explain the observed interaction. Once movement has been initiated, it becomes easier (less force required) to keep the string moving than it was to initiate the movement. Several qualitative experiments were performed to see friction in action between string and ball and to observe the effects on spin. Each of these variables can have a larger or smaller effect depending on whether movement is being inititated or maintained. Figure 2 — Test setup for measuring friction perpendicular to the main strings. All four types of friction are clearly displayed in the sport of table tennis. Crawford Lindsey, Tennis Warehouse, San Luis Obispo, CA, 93401. Excessive friction (or traction) acting between shoe and surface can lead to injury caused by overloading in the lower extremities (Wannop et al., 2010). These movies were filmed at 1,200 fps. Thus, it would seem that the ultimate spin string (i.e., maximum spin generation) involves maximizing string-to-ball friction while minimizing inter-string friction. It is interesting that there is no predictable correlation between the string surface condition and the COF. The reason is that both high- and low-friction string surfaces change the ball's forward motion and spin in ways as to cancel each other and make friction ('sliding friction') disappear. So the range of most sliding COFs is a narrow area between 0.04 and 0.07. Friction is the resistance that a surface or object comes across when moving over another. A variety of materials and surface conditions were chosen to get a good cross-section of strings on the market. This happens at the contact area when sliding friction has changed the ball's spin speed and direction and slowed its progress across the strings to the extent that the ball's periphery comes to spin at the same speed backwards as the entire ball is moving forward. The main strings simultaneously maintained their grip on the ball until one or both strings released, resulting in the expected spin. Sliding friction is almost always less than the maximum static friction. Strings vary dramatically in string-to-string friction, and hybrid combinations accentuate that difference for better or worse in unexpected and counter-intuitive ways. The amount of Friction may differ with every type of situation. All irregular seam orientations were avoided, as were tests on logo ink (which, in a small independent test, raised the COF slightly). String movement by itself does not necessarily equate with spin, but it is as good a predictor that we have. Frictional forces act against the movement of one surface over another, such as tennis shoes on a grass court. As shown elsewhere — Spin and String Material, Spin and String Movement, and Spin and String Pattern — main strings that move sideways in the stringbed may, depending on circumstances, create more spin. For strings tested in both Table 1 and Table 2, it is evident that the COF for movement perpendicular to the string is much greater than that parallel to the string. A very high speed impact may produce a normal force in the neighborhood of 250 pounds, but most topspin shots for most players will be around 150 pounds. This force progressivly slows the sled until it stops. Figure 1 — Apparatus to measure sliding friction between strings. The sliding COF is generally about 40-60% of the static COF. Friction plays a very important role in many sports, such as bowling and curling. The amount of Friction may differ with every type of situation. Most community tennis clubs are the same type of court; a medium-paced hard court with average bounce, since different surfaces that cause the ball to bounce lower or higher are more expensive.

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