Skiing: It’s All About Friction

glide2It’s all about friction. Really. Friction from the snow, friction from the air, friction from the surface of the ski or the clothing you wear.  The physics of skiing is all about how to overcome drag and resistance and allow a skier to slice his/her way down the mountain.  And if Newton’s laws have anything to do with it, a skier who controls friction best has the best chance of winning.

Find out the basics of friction and skiing.

Articles by Marcia Howell

Skiing: It’s All About Friction (Basic)

glide3aWhen Bioengineer, Parker Tyler, goes skiing, she probably isn’t thinking about the biology or the physics behind the activity.  Rather, he is enjoying the crisp, cool, mountain air, the clear view of the slope, and the anticipated exhilaration he will feel as she maneuvers to the bottom. She likely doesn’t consider earth’s gravitational force (9.81 m/s), or potential energy, or kinetic energy, or her own mass, or any of those other factors that will contribute to his acceleration.  However, scientists do think about these things and their thinking has affected many facets of the industry from clothing to equipment to style.

It’s all about friction. Really. Friction from the snow, friction from the air, friction from the surface of the ski or the clothing you wear.  The physics of skiing is all about how to overcome drag and resistance and allow a skier to slice his/her way down the mountain.  And if Newton’s laws have anything to do with it, a skier who controls friction best has the best chance of winning.

Back to Parker, her potential energy is greatest at the top of the hill where she perches until the start of her run.  Her body is physically fit and adrenaline is taking over, sending added energy to her muscles, vision center, quick decision making regions of the brain, and the area that controls coordination.  Once she leaves starting position, gravity pushes down, mass pushes down, but the acceleration down the slope kicks in and changes how the forces affect the ride.  Potential energy turns into kinetic energy, or energy of motion, and everything he touches tries to resist and slow the movement.

Since Parker is a wise skier, she wears a GS suit (a sleek, form fitting suit with a minimum of abrasive surface area) and aerodynamic boots, hat (or helmet), gloves, etc.  As she accelerates, she assumes a crouching position to reduce air resistance and tighten the air current close to her body.  His skis are designed specifically for the type of skiing being done, the edges are sharp, and the bottoms are carefully waxed.  The wax waterproofs the skis, prevents them from drying out, and it reduces the wet drag of a kind of “suction” type friction from the snow.

When Parker comes to a curve, the skis will either be eased into the turn with the ski pointed in the same direction as her velocity, making a sharp cut in its wake, or she will choose a skidding type of maneuver where the skis will be forced in the direction she wishes to go, leaning away from the curve at a 45-90 degree optimal angle, and literally plowing snow away from her. Some skis have special designs that scientists have found will decrease the drag and increase the speed these curves can be safely made.  Surely, Parker will have researched and purchased those that fit his style and goals for skiing.

By the time she reaches the bottom, her potential energy is expended, the ensuing kinetic energy is maxed out, and now friction works against him to slow down her acceleration to a stop.  His adrenalin will return to normal levels, and her blood circulation and other systems will begin to function normally once again.  (At least until the next run.)

Research will continue to change the sport of skiing.  And no doubt, the savvy skier will keep tabs on the newest and best ways scientists will come up with to help us beat the forces working against us.

By: Marcia Howell, University of Utah

References:

Energy Transformation for Downhill Skiing. 2012. Retrieved from http://www.physicsclassroom.com/mmedia/energy/se.cfm

The Physics of Skiing. Real World Physics Problems. 2009. Retrieved from http://www.real-world-physics-problems.com/physics-of-skiing.html

Locke, B. 2012. The physics of skiing. Retrieved from http://ffden2.phys.uaf.edu/211_fall2002.web.dir/brandon_locke/Webpage/homepage.htm

Mears, A. 2002. Physics of Alpine Skiing. Retrieved from http://www.suberic.net/~avon/mxphysics/anne/Annie%20Mears.htm

From Tee to Fairway: How Physics Affects the Drive, the Club, and the Golf Ball

Golf Ball Velocity

Golf Ball Velocity

The average golfer drives the golf ball with an initial velocity of over 100 miles per hour.  If the player uses a club with a flexible shaft, the act of swinging adds an additional measure of torque as the head of the club also propels forward to connect with the ball.  The head of the club has grooves that increase the friction between the club and the ball, allowing the club to more effectively focus the area of contact.

The optimal angle to hit the ball ranges from about 12 to 20 degrees.  Putting a backspin on the ball increases lift and can add significant distance to the drive.  The dimples on the golf ball itself help reduce drag from the air stream by reducing turbulent air pressure around and behind the ball, shifting the wake further behind the ball, thus allowing for smoother, less resistant flight.   Any combination of these variables contributes to how well the ball overcomes the forces of gravity and air resistance.

Learn the basics of how physics affects golf or read the more technical details here.

Articles by Trevor Stoddard

From Tee to Fairway: The Basics of How Physics Affects the Drive, the Club, and the Golf Ball (Basic)

The motion of a golf ball can be thought of as a projectile, whose trajectory is parabolic and acted upon by gravity.  The initial velocity imparted to the ball by the club head can be broken down into both a horizontal and vertical component.  Numerous scientific studies have identified the optimum launch angle as 11-20° to achieve maximum distance (Erlichson, 1983). Though drivers are typically 8-10° in loft, the flexibility of the graphite driver shafts increases the launch angle through a split second whipping action (Zumerchik, 1997).  Golf clubs have grooves added to their faces to add some friction to the club head, so that momentum is transferred to the ball and backspin is created to generate lift. The difference between a ball with backspin and one without can add up to 100 yards after 2 or 3 seconds of additional flight time (Zumerchik, 1997).

There are several forces that act upon the aerodynamics of a golf ball in flight.  The most recognizable force acting upon a golf ball is gravity, which pulls the ball downward and creates the parabolic trajectory common of projectiles.  Another force on a golf ball is lift, the force that opposes gravity.  When backspin is transferred to the ball from the grooves on the clubhead, the velocity of air on top of the ball (which is moving in the direction of the backspin) is higher than the velocity of air on the bottom of the ball.  To counteract this, the Magnus effect generates lift on the ball and pushes it up.

In addition to gravity and lift, another force acting on a golf ball is drag, or air resistance.  As a golf ball is sent flying through the air, the molecules that come into contact with the front of the ball exert a large pressure force on the front of the ball (drag). Drag slows down the forward velocity of the ball. As the air comes into contact with the front of the golf ball, the fluid motion of the air becomes turbulent.  Turbulent flow can be thought of as smoke from a smoke stack- chaotic and wispy. As the turbulent air swirls around the golf ball, the dimples capture some of the swirls and keep them close to the surface of the golf ball. This means that the boundary layer of air stays close and hugs the ball longer, which means that there is a smaller pressure difference between the front of the ball and the back (when compared to a ball without dimples). The dimples thus allow the golf ball to travel farther than a smooth ball because the golf ball experiences less drag.

By: Trevor Stoddard, University of Utah

Learn more about the technical science behind golf.

References:

Benson, T. (2010) Drag of a Sphere.  National Aeronautics and Space Administration, Date Accessed:  8/10/2012 <http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html>

Bird, R. B., W.E. Stewart and E.N. Lightfoot. 2007. Transport Phenomena, 2nd edition, Wiley & Sons, New York.

Cochran, A. (ed.). 1990. Science and Golf.  New York: Chapman and Hall

Cochran, A. (ed.). 1992. Science and Golf II.  New York: Chapman and Hall

Davies, J.  1949. “The Aerodynamics of Golf Balls.”  Journal of Applied Physics 20: 821-828

Erlichson, H. 1983. “Maximum Projectile Range with Drag and Lift, with Particular Application to Golf.”  American Journal of Physics 51: 357-362.

Jorgensen, T. 1994. The Physics of Golf. New York:  American Institute of Physics

McDonald, W. 1991. “The Physics of the Drive in Golf.”  American  Journal of Physics 59: 213-218

Werner, F. and R. Greig. 2000. How Golf Clubs Work and How to Optimize Their Designs.  Jackson Hole, WY:  Origin Inc.

Wesson, J. 2009. Science of Golf.  New York, Oxford University Press

Williams, D. 1959. “Drag Forces on a Golf Ball in Flight and Its Practical Significance.”  Quarterly Journal of Mechanical Applications of Mathematics XII 3: 387-393

Zumerchik. J. (ed.). 1997. Encyclopedia of Sports Science.

Zumerchik J. 2002. Newton on the tee- a good walk through the science of Golf

History of the golf ball <http://www.golfeurope.com/almanac/history/golf_ball.htm> last accessed 8/10/12

From Tee to Fairway: How Physics Affects the Drive, the Club, and the Golf Ball (Technical)

Introduction

According to the PGA there are 27 million golfers in the United States (citation).  By understanding the science behind the game, golfers are more likely to improve their scores because they can better understand the errors that they are making.

From Tee to Fairway: The Physics Behind Golf (Projectile Motion)

Projectile motion is the motion of an object that is imparted with an initial velocity (such as hitting a golf ball with a golf club) that moves in a parabolic trajectory that is caused by the effect of gravity on the object.    Initial velocity can be broken into horizontal and vertical components as in Equation (1).

Where the subscript 0 denotes initial, x and y are the directions and i and j are the vector directions (i is in the x-direction and j is in the y-direction).  The initial horizontal and vertical velocities can be determined if the angle the ball is launched from the horizontal,α, is known, given by Equations (2) and (3).

Acceleration due to gravity pulls the golf ball down during flight and as time goes on actually forces the vertical velocity in the downward direction, the components of the golf ball velocity at any given time, t, during its flight are given by Equations (4) and (5).

Where g is the acceleration due to gravity, which is a constant assumed to be -9.8 m/s2 (or 32 ft/s2), and the time t is in seconds.  The magnitude of the velocity is given by Pythagorean’s theorem.

A diagram of these velocity vectors is given in Figure 1.

Figure 1.  Golf ball velocity

The average golfer drives the golf ball with an initial velocity of over 100 miles per hour (Zumerchik, 2002)! This means that the golf ball, if struck at an angle of 12°, will be initially traveling at 87 mph in the horizontal direction and 13 mph in the vertical direction.

Typically the projectile motion equations used to calculate range and height of a golf ball’s flight do not account for drag, and are therefore only estimates as to the true maximum height and range that the golf ball will travel.

Drag force (or air resistance) is the force that acts opposite to an object that is moving through it. When a golf ball is hit, the air molecules flow past the golf ball as the golf ball flies through the air, thus creating a retarding force on the forward motion of the ball known as drag.

In 1949, Davies conducted experiments to determine the magnitude of drag and lift forces that occur on a golf ball by dropping rotating golf balls into a wind tunnel.  Davies found that “drag increased nearly linearly from about 0.06 lb for no spin to about 0.1 lb at 8000 rpm” and that “lift varied with the rotation speed” (Davies, 1949).

In 1959, Williams used the previous findings of Davies and conducted an analysis on golf ball carry as a function of velocity.  Williams showed that the drag force varied linearly with the velocity.  The drag force on a golf ball can be calculated by Equation (7).

Where CD is the drag coefficient, A is the cross-sectional area of the ball, ρ is the air density, and v is the ball velocity (in ft/s).

Williams showed that the drag coefficient can roughly be estimated as 46/v, showing that at higher speeds the drag coefficient drops significantly.  Williams found that the drag force on the golf ball varied linearly with the speed, he found D to be as given in Equation (8).

Where D is in pounds, and 0.000783 represents the constant c, to be discussed.   One note is that Williams’ (1959) calculations used a British ball with a diameter of 1.62 inches compared to the American ball of 1.68 inches.  The easiest calculation is to consider the case of a nonspinning golf ball (no lift) with linear air resistance in calculations of range and height.  The reader is directed to the paper by Erlichson for a more in depth analysis of range calculations that incorporate lift forces into the derivations.

Erlichson (1983) gives the equations for range and height as in Equations (9) and (10).

Where c is 0.000783 lb/(ft/s) and m is in [lb] and g is 32 ft/s2 and t is time, initial velocities are given in ft/s.

Using these equations a golfer could estimate how long they will hit the ball and where it will end up.  Variables such as wind and weather can affect these numbers in reality so they are best used for only estimations on distance.

Several studies have been conducted on the optimum launch angle.  Scottish physicist Alastair Cochran calculated that the optimum launch angle of 20° achieves the longest carries (Cochran, 1990). One needs to take this finding with a grain of salt, however, because the ball will land at a high angle and will have less bounce and roll, unless the grass is wet.

Erlichson (1983) found that the optimum launch angle was around 12-13°, only slightly more lofted than the drivers available on the market, and with the additional loft generated from shaft flex. Zumerchik concludes that anywhere from 12°-20° will give the ball maximum range with only a few yards difference between the different angles. (Zumerchik, 2002)

Golf Clubs: Loft and Grooves

Several forces act on the golf club, such as torque (exerted by the golfer on the club), centrifugal acceleration, and gravity.  Typically golfers can average a whole 4-5 hP of power generated from their golf swings. (Wesson, 2009)

At the top of the backswing, the club head coils because the shaft is flexible and the center of mass is in the clubhead. As the swing moves forward, the shaft of the club coils, unloads, and recoils as the club head attempts to catch up to the wrists during the swing.  At impact between the clubhead and the ball there is a final forward oscillation that creates a snapping effect that increases the velocity of the club head through the ball, and by extension also increases the initial velocity imparted to the golf ball.  It has been found that a “forward shaft flex of about 3.3 degrees can add 8.7 percent to the velocity of the club head” (Jorgensen, 1994).  During the course of the swing, the club oscillates 1.5 times (Zumerchik, 1997).

Werner et al. (2000) conducted a full suite of simulations and experiments on optimizing club designs. Their findings showed that there is an optimum combination of loft angle and center of gravity location, and that an extra-large face provides advantages (Werner et al., 2000) But does the addition of more surface area on the club head create an increased drag on the club head, diminishing initial velocity?

It’s been found that air drag on the shaft and clubhead results in an energy loss of 10% of the energy of the club at impact with the ball.  This results in a reduction of roughly 15 yards in the range of the ball (Wesson, 2009).

When the club head strikes the ball, the grooves on the club head increase the amount of friction at the ball-club head interface, allowing for the ball to have backspin, which increases the lift force.  Without friction, the ball would merely glide up and off the club face.

When the club head comes into contact with the golf ball, some of the energy is lost and is not entirely transferred into velocity of the golf ball. The ratio of the velocity transfer between the club face and golf ball surface is given by the coefficient of restitution.

If a ball hits a surface at a right angle with a speed v and leaves the surface with a speed v’ then the coefficient of restitution is defined by

When bounced off a hard surface, a typical golf ball has a coefficient of restitution of around 0.7.  This means that if the golf ball were dropped from 100 feet above the ground, it would rebound and bounce back 70 feet. The coefficient of restitution is about 1.46 for a driver, 1.3 for a 5 iron, and 1.12 for a 9 iron.  (Zumerchik, 1997).

Why all the dimples: The Fluid Mechanics behind a golf ball in flight

The aerodynamic forces that act on a golf ball in flight are shown in Figure 1.

Figure 1. Forces that Act on a Golf Ball in Flight

Typically golf balls have between 330 and 500 dimples on their Surlyn covers.  Some dimples are round while most golf ball manufacturers have started making their dimples in a variety of hexagonal shapes.  As stated by Euler’s principle (Bird et al, 2007) separation of the boundary layer (the layer of air molecules next to the surface of the golf ball) is likely to occur in regions where the pressure increases in the direction of the flow. The following youtube video gives a good look at a simulation of the boundary layer.

The reason for the large number of dimples on a golf ball is to ensure that the boundary layer does not separate until the back part of the ball.  A smooth sphere will generate a large wake behind the ball, as there is much lower pressure behind the ball than in front.  The air will move to rush into the area of low pressure, exerting a pressure drag force on the ball.  However, as the turbulent air swirls around the golf ball, the dimples capture some of the swirls and keep them close to the surface of the golf ball. Dimples force a turbulent trip at the surface, ensuring a later separation of the boundary layer which decreases the amount of drag force on the ball. Dimples on the golf ball do not reduce the drag at the front of the ball, because the cross-sectional area of the ball is always the same. However, they do decrease the size of the low pressure wake behind the ball, lowering the overall drag force, and improving flight.  An example of this is shown in Figure 2, a simulation run in Fluent between a sphere and a dimpled golf ball.

Figure 2. Fluent simulation showing the pressure field around a smooth sphere and dimpled golf ball. Balls are traveling to the left.

As shown in both cases, the pressure contacting the front face of both balls is large. However, the magnitude of the pressure difference (front side versus back side of the ball) in the sphere case is much greater than in the golf ball case. The large amount of high pressure (red and yellow) pushing against the flight of the ball slows it down much faster than in the golf ball case, where there is less pressure behind the ball.  In addition to this pressure calculation it is of interest to look at the shear stress distribution on the face of both the sphere and modeled golf ball, as shown in Figure 3.

Figure 3. Wall shear stress of both a dimpled golf ball and a sphere modeled in Fluent

As shown in Figure 3, the shear stress on the surface of the sphere (red/orange) covers a greater surface area, showing where the boundary layer separates from the sphere.  The dimples on the golf ball keep the turbulent flow boundary layer close to the golf ball wall and greatly reduce the shear stresses over the golf ball’s surface area.  This means that the golf ball is slowed less.  The boundary layer separation in laminar and turbulent flow is shown in Figure 4, a COMSOL simulation of a sphere in flow.

Figure 4. Boundary layer separation

As shown in Figure 4, in the laminar case with a low Reynolds number (Re=0.1) (top) has no boundary layer separation as the streamlines hug the outside of the ball. This situation is unrealistic for a golf ball because of the high velocities that the golf ball travels at (~200 ft/s) once it is hit  (Zumerchik, 2002).

The second case (middle) shows a smooth ball at a high velocity. There is a large pressure difference between the front and back side of the ball, and the boundary layer separates much earlier than the dimpled sphere case (bottom).  The dimples allow for the boundary layer to hug the ball and boundary layer separation does not occur until the very tail end of the ball. The smaller the blue zone of low pressure behind the ball, the farther the golf ball will fly.

Similar to an air foil, the dimples on a golf ball also allow for a lift force to be exerted on the ball.  Backspin, as generated from the loft of the clubface that strikes the ball, deforms the airflow around the ball and creates a lift force due to the Magnus effect.  The Magnus effect is a phenomenon where a spinning object flying in a fluid creates a whirlpool around itself and experiences a force (in this case, lift) perpendicular to the line of motion.  Because the top of the ball is spinning with the direction of the air, the air on top of the ball moves more rapidly than the air at the bottom of the ball.  The air at the bottom of the ball moves against the wind, and this shifts the pressure behind the ball downward, in the direction of the backspin.  Because of the differing velocities on top and bottom of the ball, there is a resultant force upward known as lift.

The magnus effect can have a large impact in golf. If two golf balls are hit with the same velocity, a ball hit with backspin will stay in the air 2 or 3 seconds longer and may travel 18 to 30 meters farther (Zumerchik, 1997).

Despite the benefits of backspin, the use of too much spin can be a problem because some of the momentum is imparted into the high spin of the golf ball.  As a ball travels faster, it needs less backspin to generate lift. (Zumerchik, 2002).

Conclusions

Golf equipment has evolved over the centuries to increase the distance a ball can travel. By understanding the forces acting on the golf ball, a golfer can maximize his/her performance

By: Trevor Stoddard, University of Utah

 

References:

Benson, T. (2010) Drag of a Sphere.  National Aeronautics and Space Administration, Date Accessed:  8/10/2012 < http://www.grc.nasa.gov/WWW/k-12/airplane/dragsphere.html>

Bird, R. B., W.E. Stewart and E.N. Lightfoot. 2007. Transport Phenomena, 2nd edition, Wiley & Sons, New York.

Cochran, A. (ed.). 1990. Science and Golf.  New York: Chapman and Hall

Cochran, A. (ed.). 1992. Science and Golf II.  New York: Chapman and Hall

Davies, J.  1949. “The Aerodynamics of Golf Balls.”  Journal of Applied Physics 20: 821-828

Erlichson, H. 1983. “Maximum Projectile Range with Drag and Lift, with Particular Application to Golf.”  American Journal of Physics 51: 357-362.

Jorgensen, T. 1994. The Physics of Golf. New York:  American Institute of Physics

McDonald, W. 1991. “The Physics of the Drive in Golf.”  American  Journal of Physics 59: 213-218

Werner, F. and R. Greig. 2000. How Golf Clubs Work and How to Optimize Their Designs.  Jackson Hole, WY:  Origin Inc.

Wesson, J. 2009. Science of Golf.  New York, Oxford University Press

Williams, D. 1959. “Drag Forces on a Golf Ball in Flight and Its Practical Significance.”  Quarterly Journal of Mechanical Applications of Mathematics XII 3: 387-393

Zumerchik. J. (ed.). 1997. Encyclopedia of Sports Science.

Zumerchik J. 2002. Newton on the tee- a good walk through the science of Golf

History of the golf ball <http://www.golfeurope.com/almanac/history/golf_ball.htm> last accessed 8/10/12

 

For more information:

  1. DeNevers, Noel, Fluid Mechanics for Chemical Engineers, McGraw-Hill, New York, 2005.
  2. Libii, Josue Njock (2007)  “Dimples and Drag:  Experimental demonstration of the aerodynamics of golf balls.”  American Journal of Physics, 75, 764.
  3. Millne, R. and J. Davies (1992)  “The Role of the Shaft in the Golf Swing.”  Journal of Biomechanics 129:  975-983
  4. Zander, J. and A. Chou (February 1999)  “Max out your ball:  Increasing your launch angle and decreasing your spin rate will help you hit farther.”  Golf Digest 50: 76-80

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Tennis Courts and Equipment: How Physics Affects the Speed of Play (Technical)

Introduction

Technological advancements have played a key role in making power and spin prominent features in the game of tennis.  The transition from wood to composite graphite rackets has produced larger sized rackets (in terms of both head and shafts) with frames that are thicker and lighter (Brody 1997), allowing players to hit harder than ever before. Changes in racket construction have altered the tennis serve, to the point where serves can dominate many tennis matches.  In order to make match points last longer, scientists and athletes have explored ways to slow the serve and restore balance to the game. One approach has been to engineer new types of tennis balls with properties that can counteract the power and speed of the serve (Haake et al. 2000).  Changes in tennis ball construction and interactions between the ball and different types of court surfaces have become primary considerations.

Types of Tennis Court Surfaces

Tennis was first played on natural grass courts.  Modern day grass courts consist of a soil foundation with a seeded turf overlay (Miller 2006).  While grass is still used at Wimbledon, its use has diminished due to the cost associated with high maintenance.

Clay courts gained favorability in the 1950s and consist of a base layer of crushed stone covered with a layer of rough particle material such as crushed brick (Miller 2006). This produces high amounts of friction between the ball and surface, but low amounts of friction between the player and the surface. On a clay court, the player has a tendency to slide, particularly when slowing down or attempting to change their direction of movement (Miller 2006).  Lower injury rates have been associated with players that frequently use clay courts (Dragoo et al. 2010), possibly because of lower impact forces due to the sliding motion.  Currently, the French Open is the only major tennis tournament played on clay.

Acrylic hard courts have rapidly gained popularity since their introduction in the 1940s and are used in two major tennis tournaments, the US Open and the Australian Open.  These courts utilize either asphalt or concrete as the foundation layer, a rubber mid-layer, and a top coating made of an acrylic paint/sand mixture (Miller 2006).  These courts produce the highest amount of friction between the surface and player, and have been associated with the most player injuries when compared to other surfaces (Dragoo et al. 2010).

Tennis Court Surfaces Affect the Speed of the Game

One of the most important considerations in tennis is the influence of the court surface on the ball.  Aside from the force of gravity, a bouncing ball additionally experiences normal and frictional forces (Brody 2003).  The normal force acts perpendicularly to the surface and the frictional (or sliding) force will act parallel to the surface (horizontally). The combination of these forces impacts the bouncing movement of the ball. The amount of friction generated between the ball and court dictates if the court is considered to be “fast” or “slow.”  In particular, the amount of sliding friction that is present, dependent upon the surface type, is of interest.

A “slower” court is one where more friction is generated between the ball and the surface.  Clay, with its rough surface composition, has a high coefficient of friction.  When more frictional contact is produced, the horizontal speed of the ball is reduced.  This reduction in forward motion creates a high vertical bounce. The longer the ball is in the air, the more time a player has to move and react, making clay a “slow” paced court.

A “faster” court produces less friction between the ball and the surface.  Grass, with its firm and slippery (even more so when wet) surface composition has a low coefficient of friction making it a “fast” paced surface.  With less friction, the ball will slide more easily across the surface, and it will retain more of its horizontal speed. This produces a low vertical bounce.  For these reasons, points on “fast” surfaces are often much shorter, as a lower bounce means the player has less time to react and move towards the ball.  In an examination of rally lengths in men’s singles tennis, 66%  of rallies on clay lasted less than six seconds, but this figure increased to 88% on grass courts (Lees 2003).

Engineering of Tennis Balls

In order to engineer tennis balls with more desirable properties, understanding ball construction is important. The two main components of a tennis ball are the core and covering.  The core is typically made of natural rubber that is mixed with powder fillers to produce desirable properties, such as strength and color (Manufacture).  The outer surface is made of cloth material; either of a wool-based fabric (Melton) or less expensive cloth (Needle cloth) that contains more synthetic components (Manufacture).  In addition, most tennis balls are pressurized, and the amount of internal pressure (ranging from 0-15 psi) will be determined based upon the ball type (Miller 2006).

Tennis balls are manufactured through a series of processes. The first of these processes is an extrusion, where the rubber is forced into a cylindrical shape through an application of pressure (Manufacture). The resultant rubber rod is then sectioned into smaller segments.  Subsequent processes include forming the material into a spherical shape (by using a hydraulic press to form two individual hemispheres that are later joined), curing and pressurizing the ball, covering the ball with fabric and finally joining the core and covering together in a molding process that utilizes pressure and heat (Manufacture). The final step is to steam the ball, thus producing a more raised outer covering.  Once finished, balls must pass tests related to mass, size, compression and bounce (ITF 2012).

There are three major categories (types 1-3) of tennis balls, each designed for specific use on set court types to speed up or slow down play.  The weight and rebound of the tennis balls does not change across type.  The standard and most utilized ball type is type 2, as it is suitable for medium paced surfaces.  Type 1 balls are the same size as type 2 balls, but are harder, which is reflected by smaller amounts of forward and reverse deformation, when compared to the type 2 balls (Miller 2006).  Because type 1 balls are considered to be “fast” balls, they are suggested for use on slower surfaces such as clay.

Type 3 balls differ from type 2 balls only in size.  Type 3 balls are typically 6-8 percent larger than type 2 balls (Miller 2006).  As demonstrated by Andrew et al. (2003), Type 3 balls are “slow” and travel through the air more slowly than their standard tennis call counterparts. Since type 3 balls are larger in size, they encounter greater drag (resistance) when traveling through the air. Thus, type 3 balls are suggested for use on faster court surfaces, such as grass, to help slow down the pace of play and reduce the dominance of the serve.  Furthermore, the additional drag associated with type 3 balls also allows for a larger amount of spin to be generated (Blackwell 2007).  Type 3 balls may also be beneficial for new players, as a slower pace allows for more reaction time and increased spin can aid in accuracy.

Although less prevalent, there are also balls designed for use at high altitude.  By changing either the internal pressure of the ball or the elasticity of the core material, high altitude balls can be made to bounce lower than type 2 balls  (Miller 2006).  This is done so that at the lower air density at higher altitude the same bounce height (as that of a type 2 ball at sea level) can be achieved.

Other Tennis Ball Considerations

Despite careful efforts to engineer tennis balls and select a ball that complements court conditions, there are additional factors that impact the speed of the game. One of these is unavoidable ball wear-and-tear, which affects even the most well-engineered tennis balls.

Four distinct phases of tennis ball wear have been identified: new ball, loose fuzz, tufted fuzz, and finally the bald ball (Steele et al. 2006).  The ball covering itself is porous, which creates additional instances of drag (Mheta et al. 2001) when compared to a smooth covering.  As the surface material becomes worn, the “fuzz” on the ball is depleted, and both lift and drag forces are reduced (Goodwill et al. 2004).  The drag coefficient for new tennis balls was found on average to be higher than 0.6. The drag coefficient was reduced to values near 0.5 for worn balls (Mehta et al. 2001). This means that a worn ball will fly faster with less force to push it down when compared to a newer ball, increasing the likelihood that the ball will be hit out of play.

The impacts of a worn ball may be reduced if a player applies spin to the ball. Spin is possible due to the Magnus effect, or a nonsymmetrical distribution of air that flows across the ball surface while it is in flight (Mehta 1985, Miller 2006).  When topspin is applied, some of the air (that is flowing in the same direction as the spin of the ball) will interact with the ball surface longer, which has the effect of deflecting the wake of the ball upwards while other forces act in a downwards direction (Mheta et al. 2001, Miller, 2006). By applying topspin to the ball, a player can help a tennis ball fall to the ground and remain in-bounds.

What Players Should Know

Beginning players may be most successful on clay courts, since the speed of play is likely to be slower, and the player is better able to slide

  • A type 2 ball is most common, but a beginner may consider using a type 3 because of its slower air speeds
  • Old, worn balls do have a noticeable impact on play

 

By: Lindsay Sanford, University of Utah
Lindsay received her B.S. in Mechanical Engineering from Washington State University and is currently pursuing a PhD degree in Bioengineering. In her spare time, she likes to travel, hike, read, and play with her two year old son.  She is also an avid runner and tennis player.

 

References

Andrew, D., J. Chow, D. Knudson, and M. Tillman. 2003. Effect of ball size on player reaction and racket acceleration during the tennis volley. Journal of Science and Medicine in Sport. 6(1): 102-12.

Blackwell, J., E. Health, and C. Thompson. 2006. Effect of the Type 3 (oversize) tennis ball on physiological responses and play statistics during tennis play: Third world congress of science and racket sports.  Journal of Sports Sciences. 24(4): 333-53.

Brody, H. 1997. The physics of tennis III: The ball-racket interaction. American Journal of Physics. 65(10): 981-87.

Brody, H. 2003. Bounce of a tennis ball. Journal of Science and Medicine in Sport. 6(1):113-19.

Dragoo, J.L., and H.J. Braun. 2010. The effect of playing surface on injury rate. Sports Medicine. 40(1): 981-90.

Goodwill, S.R., S.B. Chin, and S.J. Haake. 2004. Wind tunnel testing of spinning and non-spinning tennis balls. Journal of Wind Engineering and Industrial Aerodynamics. 92:935-58.

Haake, S.J., S.G. Chadwick, R.J. Dignall, S. Goodwill, and P. Rose. 2000. Engineering tennis- slowing the game down. Sports Engineering. 3(2): 131-43.

IFT Tennis Technical Reference. (2012). Retrieved from http://www.itftennis.com/techical/equipment/balls/manufacture

Itf 2012 rules of tennis. (2012). Retrieved from http://www.itftennis.com/media/117960/117960.pdf

Lees, A. 2003. Science and the major rackets sports: a review. Journal of Sports Sciences. 21(9): 707-32.

Mehta, R.D. 1985. Aerodynamics of sports balls.  Annual Review of Fluid Mechanics. 17: 151-89.

Mehta, R.D., and J.M. Pallis. 2001. Sports ball aerodynamics: effects of velocity, spin and surface roughness. Structural Materials Division of the Minerals, Metals and Materials Society Symposium, Coronado, CA, April 22-25.

Miller, S. 2006. Modern tennis rackets, balls, and surfaces. British Journal of Sports Medicine. 40(5): 401-5.

Murias, J.M., D. Lanatta, C. R. Arcuri, and F.A. Laino. 2007. Metabolic and functional responses playing tennis on different surfaces. Journal of Strength and Conditioning Research. 21(1): 112-7.

Steele, C., R. Jones, and P.G. Leaney. 2006. Tennis ball fuzziness: assessing textile surface roughness using digital imaging. Measurement Science and Technology. 17:1446-55.

How Air Resistance Determines the Pitch (Basic)

To pitch a the highest level, a pitcher needs strength, flexibility, and intense focus.

When the ball is finally released, several forces act on it. First is gravity. The moment the ball leaves the pitchers hand, gravity begins to make the ball drop toward Earth. Gravity is basically the pull that an object of mass has on another object. Everything on Earth is affected by gravity. For example, a pencil you are using to take a math test has a pull on you and you have a pull on it! The only difference is that you have more mass than the pencil so you cannot feel the effects. The same is true for a baseball.Earth has an effect on the ball but also the ball has an effect on the Earth. However, the force of the ball on the Earth is not seen because of the tremendous difference in mass.

Another force that acts on the ball is air resistance. Air acts just like water! The only difference is that air is much less dense than water. This means that the microscopic air particles are much farther apart than the particles in water.  In a swimming pool, it is much harder to walk than when on land. This is because the water is providing a resistance against your body. Air provides a resistance as well but it is not felt as much because of the density differences. However, since a baseball is much lighter than you, air plays more of a role on a ball than on your body. The ball essentially must move air out of the way and this slows it down.  Imagine a skydiver opening up his parachute and falling slowly to Earth. This is exactly what happens to a baseball but on a much smaller scale.
Air resistance is also responsible for a pitcher being able to throw different kinds of pitches! When a pitcher throws a fastball, he throws it in such a manner that the spin is straight up. This will keep the ball going straight. When the pitcher throws a curve ball, he will tilt the spin so that the air resistance will push the ball in different directions (usually down or to the sides). In the case of a knuckleball, a pitcher will try to put zero spin on the ball. This will allow the air to push the ball in all sorts of directions and it appears to hitters that the ball is “dancing” through the air. This makes the pitch very deceptive and can lead to more strikeouts.

By: Kenny Morley, Ohio State University 

 

References:

Zarda, B. (2008, August 06). Science of a pitching freak. Retrieved from http://www.popsci.com/score/article/2008-08/science-pitching-freak

Free fall and air resistance. (2012). Retrieved from http://www.physicsclassroom.com/class/newtlaws/u2l3e.cfm