Sun Yingsha and Liang Jingkun won the championship. Does the quality of playing table tennis have anything to do with the racket?

Table tennis, as my country's national sport, is not only a leisure sport loved by the masses, but also a traditional advantage for my country in the international sports arena.

On March 31, the WTT World Table Tennis Federation Incheon Championships completed its entire schedule in Incheon, South Korea. Chinese player Sun Yingsha defeated her teammate Wang Manyu 4-0 to win the women's singles championship; Chinese player Liang Jingkun defeated Brazilian player Hugo Calderano 4-1 to win the men's singles championship.

Liang Jingkun at the award ceremony on March 31. Photo by Xinhua News Agency (Photo by Quan Xiu)

But while you are proud of your achievements, have you ever paid attention to the details of table tennis games? For example: How does a small table tennis ball have such great power? Why does the referee have to check the rackets of both teams before the game? Why do people bring their own rackets? Are everyone's rackets different?

Sun Yingsha and Ito Mima check each other's rackets before the game (Photo source: Screenshot from CCTV)

Part 1

Lightning Room

Impact dynamics in Ping Pong!

First of all, we need to know that the "ping-pong" and reciprocating leaps on the court are mainly due to the collision dynamics process between the ball and the racket. During the game, the collision contact time between the ball and the racket is only one thousandth of a second, and the flight speed of the table tennis ball can reach up to 47 meters per second! In a flash, the rubber and sponge on the surface of the racket will deform due to the rapid squeezing of the ball, and absorb kinetic energy and store deformation energy (of course, the ball will also deform and store energy, and the bottom plate of the racket will also deform slightly); when the total kinetic energy reaches a certain minimum value and the deformation reaches the maximum value, the sponge and rubber layers will recover, and the deformation energy will be quickly released and converted into the kinetic energy of the table tennis ball, so the table tennis ball will "leave the board" and fly, and after reaching the opponent's racket, it will start the next collision and leap.

The collision between the ball and the racket (Image source: Reference [1])

If the force direction of the table tennis ball does not pass through the center of the ball at the moment of hitting the ball, the tangential friction will cause the ball to rotate. In addition to the relative motion of the ball and the racket, the main reason affecting the tangential friction is the material and craftsmanship of the ball and the racket. Therefore, through optimization technology, more advanced balls and rackets can be manufactured, making the hit more "lethal".

Part 2

Why should the racket be carefully inspected?

You will understand after reading this!

In current international competitions, table tennis balls are provided uniformly at the venue, but players have to carry their own rackets. Why is this?

Because the use of the racket fits the athlete's own characteristics and habits, it often takes a long period of running-in to improve the skill level and reach the state of "oneness of man and racket", while table tennis itself does not have such a big impact.

But on the other hand, the racket's self-carrying mechanism also brings great uncertainty to the game from a hardware perspective!

It can be imagined that in order to win greater victories, countries will invest a lot of research and development work to improve the performance of rackets, and the structure of rackets is becoming more and more complex; looking at the development of table tennis, the innovation of technical playing methods is closely related to the change of rackets.

At present, most rackets are composed of three parts: base, sponge and rubber. With the continuous innovation of the structure of these three parts, technical playing methods are increasingly pursuing speed and strong rotation, and emphasizing "quick-effect" playing methods that "kill with a few moves" and reduce rounds. But at the same time, this will significantly reduce the viewing and fun of table tennis games, and turn "competing in sports technology" into "competing in industrial technology", which is inconsistent with the competitive spirit of the Olympics.

Table tennis racket (Photo source: Veer Gallery)

Fortunately, the ITTF has given a series of specifications on important parameters that directly affect the collision process between the racket and the ball. Although there is no unified regulation on the shape and weight of the racket, there are mandatory requirements for the characteristics of the racket, including: the racket should be sufficiently flat and hard; the natural wood layer in the racket bottom plate should account for no less than 85%, the thickness of auxiliary materials such as the bonding layer between the wood should not exceed 7.5% of the total thickness and should not be thicker than 0.35mm, the thickness of the sponge layer should not exceed 0.4mm; and the rubber layer is not allowed to use long rubber, etc.

Therefore, in order to further ensure the technical fairness of the competition, a special testing center will test the rackets before the game. The test content mainly includes the flatness of the racket surface, the glossiness of the racket surface, the legality of the water-soluble glue and rubber sponge, etc.

Now, do you understand why athletes' rackets are checked one by one?

On August 1, 2021, Xu Xin of the Chinese men's doubles table tennis team hit the ball (see watermark for image source)

There is a small episode in the development history of rackets - early athletes used quick glue made of organic solvents to dissolve rubber essence to achieve variable impact to control the speed and rotation of the ball. Because it can form a special distributed air bag structure in a short time when bonding the racket bottom plate and sponge layer; however, this quick glue is very harmful to the human body, so it is gradually banned, and whether the racket contains quick glue has become an important detection indicator.

Part 3

Clothes make the man, saddles make the horse

Table tennis player looking at the bottom board

Although the racket bottom plate does not directly touch the ball, it is the load-bearing body of the entire racket. The parameters related to the bottom plate are: hardness, elasticity and bottom strength. What is "bottom strength"? From a simple mechanical point of view, it can be understood as a "force" that prevents the energy dissipation of the collision between the ball and the racket. The ultimate ideal situation with the most "bottom strength" is "how much you store, how much you can take out". Generally speaking, the wood of the racket bottom plate is no more than seven layers, because the more layers, the harder it is, so fast attack players often choose seven-layer hardwood boards, and arc players often choose five-layer sandwich soft bottom plates. It is true that "clothes make the man, saddles make the horse, and the bottom plate makes the table tennis player".

With the support of new materials and new technologies, especially the development and support of fiber-reinforced composite materials, the mechanical properties of racket bottom plates have been improved rapidly. There are many types of fibers, and the selection is very particular: typical ones are carbon fiber, aramid fiber, glass fiber and their blends.

Among them, the high-strength glass fiber base is most suitable for modern whip-style hitting. It feels closest to a pure wood base, but its deformation recovery speed far exceeds that of a solid wood base. The carbon fiber base with light specific gravity and high rigidity has a short impact deformation recovery time and fast return ball acceleration, which can help athletes create high-speed low-arc balls. The aramid fiber base has sufficient bottom strength and can reduce impact vibration, and long-term use is more conducive to protecting the athlete's wrists. It is not difficult to imagine that a mixed base of aramid fiber and carbon fiber may have the advantages of both, with a fast enough board-off speed and a soft enough feel.

In short, the materials for the base plate are rich and varied, and can be tailored to suit different people's needs.

Part 4

Rubber, sponge:

Soft material that complements the base

The sponge used in table tennis rackets is a kind of foamed rubber, which is pasted between the bottom plate and the rubber. The main function of the sponge interlayer is to prolong the contact time between the ball and the racket, improve the overall elasticity of the racket and its friction and ball cutting ability.

Roughly divided from the perspective of mechanical properties, sponges can be divided into two types: high-elasticity and low-elasticity. High-elasticity sponges have large rebound and high compression-load ratio, and rackets using high-elasticity sponges can produce higher rotation and speed. Low-elasticity sponges have small rebound, so they are also called inert sponges. Racquets with low-elasticity sponges have stable and easy ball control, and the batting athlete needs to exert more of his own strength when creating rotation.

As the rubber that directly contacts the ball, its craftsmanship is changing with each passing day. Simply put, there are two main types of rubber used in table tennis rackets, namely granular rubber and reverse rubber. Among them, the granular rubber has the particles facing outward. When the particle diameter is equal to, greater than, or less than the particle length, it is called positive rubber, raw rubber, and long rubber respectively. The reverse rubber has the particles facing inward, and usually needs to be matched with a thick hard sponge. The reverse rubber that is thick and hard is called anti-arc rubber, which requires a relatively soft sponge. It is not difficult to understand that different rubbers have different friction capabilities on table tennis balls, which can be matched with different technical playing methods.

The optimized combination of rubber and sponge complements and brings out the best in the performance of the baseboard.

Ultimately, different combinations of rubber, sponge and base allow rackets to demonstrate their own unique performance.

Part.5

Changes of the ball:

ABS and the Big Ball Era

After talking about the racket, let’s talk about the ball.

In order to increase the viewing value of table tennis, in 2000, the International Table Tennis Federation increased its diameter from 38 mm to 40 mm, ushering in the era of large table tennis balls.

A large ball has greater flight resistance, greater air friction torque when rotating, and greater rotational inertia, so the flight speed and rotation speed of a large ball are also lower. In order to make the ball spin quickly, athletes often need to make greater efforts.

Similar to the evolution of rackets, the material of table tennis balls has also been replaced several times. In 2014, the celluloid balls that had been in service for more than a hundred years were replaced with safe and environmentally friendly seamless plastic balls, because celluloid is toxic and flammable, which brings great difficulties to air transportation. However, in a dry and cold environment, due to mechanical effects such as cold shrinkage, dry shrinkage and cold brittleness, seamless plastic balls are prone to cracking. Data show that their loss rate is at least 20% higher than that of celluloid balls. However, in a dry and cold environment, seamless plastic balls are prone to bursting, and the loss rate is 20~30% higher than that of celluloid balls. Therefore, in 2017, the World Table Tennis Championships began to widely use ABS materials. As a terpolymer, ABS has better performance and lower cost.

Table tennis (Photo credit: Veer Gallery)

"If you want to do your work well, you must first sharpen your tools." The ball and racket are the weapons and equipment in the table tennis arena. For athletes, good equipment can maximize their high level of performance and bring more dazzling performances to the audience. Throughout the history of table tennis development, the technological progress of rackets and balls has challenged each other and complemented each other: rackets have been developing in the direction of higher elasticity and stronger friction control, with the aim of making the ball faster and the trajectory more unpredictable; table tennis balls have been developing in the direction of larger size and greater inertia, with the aim of making their flight more stable and exciting and lasting.

Table tennis is changing, and the rackets are changing, but the spirit of hard work and struggle of Chinese table tennis remains unchanged, and the Olympic spirit of world peace remains unchanged. Let us wish all the athletes to achieve greater success!

References:

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4. Wang Jisheng. How to choose and match table tennis baseboard and sponge rubber[J]. Table Tennis World, 1999(4).

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6. Su Qingqiang, Wang Haofeng. Analysis of materials and performance of table tennis racket bottom plate[J]. Journal of Xinxiang University: Natural Science Edition, 2010.

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Produced by: Science Popularization China

Author: Liu Cong, Wu Chenwu (Institute of Mechanics, Chinese Academy of Sciences)

Producer: China Science Expo

This article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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