The method for calculating the coefficient of friction is as follows. Friction reverses the rotation direction so that the linear motion is forward and the rotational motion is backward at the contact point. Purpose. In each experiment, we will determine the coefficient of friction (COF) of each string. The friction force is greater perpendicular to the string than it is parallel for the main reason that the string sinks into the ball and has to climb out of the groove in order to slide, whereas for parallel movement, the string slides in the groove. And the difference between the spiniest polyester and stickiest nylon is almost 50%. There is no such thing as "THE" COF. As we can see, it is not perfectly round, as the ball slid a little bit while it was in contact with the ground, causing friction. As so formulated, both of these assumptions have proven incorrect. Movie Screen 2 — Tennis Cloth-String Interaction. As soon as you push harder than the maximum static friction force, the string will move on the other. How String-to-Ball Friction Affects Spin. Without the string moving, energy that doesn't spin the ball remains unavailable or is "burned up." The role of ball-string friction in this process is that it influences both the amount of lateral string movement and the torque the snap-back exerts on the ball (though the magnitude of its contribution is yet to be ascertained by experiment). This occurs when the ball transitions from sliding mode to biting mode (where static friction takes over). It used to be assumed that the main string movement was undesirable because the strings did not snap back and would get stuck out of position. The friction between the crosses and the ball (video 1) is not enough to reverse the ball's spin. Minimizing friction between strings is essential to this process. There are three different types of friction; Fluid, Rolling, and Sliding Friction. Notice that in the cross string video, there is not enough friction to reverse the backspin into topspin. Both the string load and string speed will be greater on an actual tennis hit, but it is difficult to replicate those forces and speeds in a mass produceable way in the lab. It is very interesting that the hybrid with poly crosses and gut mains is the combination with the lowest static COF. That means the snap-back must occur before the center of the contact area has slid too far past the stretched string or before the ball has progressed too far into the decompression phase of the bounce. Figure 3 — Nylon Comparison. Static Friction and Spin. If the main is softer than the cross, it will deform over the cross such that it forms a slot or groove to move over the cross. So, to gain maximum spin, the goal was to use a string with both a high ball-string friction and a high string-string friction to create a rigid surface parallel to the stringbed. The theory is that greater friction between the string and the ball will cause more spin. This is a necessary condition of the experiment. This is true for both string-to-ball and for string-to-string friction. To measure the string-to-ball COF parallel to the string, a string incline was created by hanging a 20 lb weight from two parallel strings spaced about 30 mm apart. Tennis strings are often marketed according to their ability to grip the ball via sticky coatings, rough/grabby surfaces, or "biting" shapes. The videos are stunning displays of how different strings interact differently with the ball but are not meant to be used for comparative purposes. When air passes over a surface a frictional force called air resistance is produced, this is particularly important at high speed. The incline was gradually raised using the crank on the tripod base until the string slid off the ball. But, string-to-ball friction is important here also. Sliding Friction and Spin. Each of these variables can have a larger or smaller effect depending on whether movement is being inititated or maintained. The equation for friction is F = μN, where F is the friction force, N is the normal force pushing the two surfaces together and acting perpendicular to those surfaces, and μ is the coefficient of friction (COF), which is the constant of proportionality between the friction force and the normal force. Thus, the coefficient of friction is calculated by dividing the friction force by the normal force. An oblique impact also causes a tangential force component parallel to the strings. The spin ratio (S) is the ratio of the rotational speed at the contact point to the linear speed. Gravity continuously accelerates the ball faster than friction can slow and spin it (especially because the only force pushing the string and ball together is the weight of the ball), so biting and rolling never occurs, and the ball slides all the way down the incline. If there is a lot of friction on a Tennis court, the tennis ball slows down and creates a higher bounce. However it is possible to compare strings for a given setup and, for most practical purposes, assume that the same relative values of string COFs will hold for a similar, non-extreme setup. Tests were done with different weights on the sled and with different initial sliding speeds. Movie Screen 4 — Mains, crosses and ball interaction. This highlights those strings that behave radically different in sliding compared to static situations. A nylon multifilament string, Wilson NXT Tour 17, was tested at multiple sled speeds at two different sled weights. If this were the end of the story, string-to-ball friction would not matter for most tennis shots. In a tennis hit, if the string moves sideways, say, 5 mm in 2 ms, then its speed would be 2.5 m/s (5.6 mph). This steep angle was chosen to make sure that the ball would slide all the way down the incline. The string was placed at the apex of the ball's curvature on each trial. d is the stopping distance For example, if the COF parallel to the string were 0.3, rolling would occur even at 40 degrees. It is the same formula for static, sliding, or rolling friction, except that in each case the COF will be different. Crawford Lindsey, Tennis Warehouse, San Luis Obispo, CA, 93401. That used to be considered the end of the story. To move the strings sideways, this has to be great enough to overcome the friction between the strings. There are several types of friction that influence spin of the ball in tennis. Figures 3, 4, and 5 show various comparisons of string materials tested at a sled weight of 6.4 lbs. Why Is String Stiffness So Important To Power? The biting process is as follows: at impact the ball slides across the strings with backspin and friction slows and rotates the ball. The string may stay stuck and held by the ball until the ball rebounds enough to lessen the normal force between strings, allowing the elastic energy to break the static friction and begin its slide back into position. The data above demonstrates that this is indeed possible. This force progressivly slows the sled until it stops. If the cross is softer than the main, the main will sink into the cross. But such a situation rarely occurs except in extreme topspin lobs. Friction affects Tennis in multiple ways. In both instances the ball ends up with the same amount of spin that can be attributable to ball-string friction. Gut mains seem to be the secret to achieving the slipperiest combination. Figure 2 is a graph showing a typical result. Movie Screen 6 — Videos of ball bounce from crosses only and mains only. Yes. These movies were filmed at 1,200 fps. How long it takes to get the string moving, how far it moves, and when, how far and with how much ball contact it snaps back are all important to the final spin effect. Selection and peer-review under responsibility of the Centre for Sports Engineering Research, Sheffield Hallam University. Polyesters tend to have the lowest sliding COF. 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. For this reason, we know that our court falls into the category of ‘typical medium-pace hard court’ in the International Tennis Federation’s Court Pace Rating list (see. The COF is the ratio of proportionality of the friction force between strings to the force pushing them perpendicularly together (the "normal" force). String Stiffness: The Alpha and Omega of String Performance, How To Measure Your Racquet's Swingweight, Maximum Shot Speed — Where To Hit on the Racquet, Weight, Balance and Swingweight Explained, Tennis Ball Trajectories: Aerodynamic Drag and Lift in Tennis Shots, Tennis Shoe Cushioning: Impact Testing To Compare Tennis Shoes, Tennis Shoe Traction: Developing A Test Rig To Measure Shoe Traction On Tennis Courts, Foam Cushioning Properties: Choosing Material for Tennis Shoe Soles, Hockey Stick Contribution to Puck Speed in Slap Shots, Introduction to the Physics of Hockey Sticks, Babolat VS Touch 16 M / Wilson NXT Tour 17 X, Baboat VS Touch M / Topspin Cyber-Blue 1.25 X, Babolat VS Touch 16 M / Tecnifibre TGV 16 X, Babolat VS Touch 16 M / Prince Synthetic Gut Original 17 X, Babolat VS Touch 16 M / Weiss Cannon Silverstring 1.25, Prince Tournament Poly 16 M / Babolat VS Team 17 X, SuperString Viper V2 Rough 16L M / Babolat VS Team 17 X, Klip Legend (uncoated) Gut 16 M / Tecnifibre Black Code 17 X, Babolat VS Touch 16 M / Gamma Live Wire 16 X, Head FXP Power 16 M / Babolat VS Team 17 X, Signum Pro Poly Plasma Pure 17 M / Babolat VS Team 17 X, Babolat VS Touch 16 M / MSV Focus Hex 1.18 X, Gamma Live Wire 16 M / Topspin Cyber-Blue 1.25 X, Wilson NXT Tour 17 M / Topspin Cyber-Blue 1.25 X, Baboat VS Touch M / Ashaway Dynamite 17 X, Babolat VS Touch 16 M / MSV Focus Hex 1.18, Baboat VS Touch M / Asnaway Dynamite 17 X.

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