Wang Chuqin topped three categories! How to play various loop balls well?

On March 19, 2024, Beijing time, the International Table Tennis Federation officially announced the world rankings for the 12th week, with Wang Chuqin topping three categories. Men's singles 8020 points (leading by 2232 points), men's doubles 4425 points (leading by 1117 points), mixed doubles 6780 points (leading by 2945 points). In the previously concluded Singapore Grand Slam, Wang Chuqin won the men's singles and mixed doubles championships!

When it comes to table tennis, no matter whether it is a young student who never finishes his homework in school, or a retired old man wearing a mandarin jacket and slippers in the community, as long as he picks up the racket, he can perform various professional technical moves such as cutting, slicing, rubbing, pulling, and pulling with ease, topspin, backspin, sidespin... every move is unexpected and winning.

Source: People's Daily

Why does a ball that was originally moving in a straight line appear to "turn"? How can we play various loop balls well? Pang Ke will take you to find out today~

First, let's analyze the force applied to the ball when the racket hits the ball: When the table tennis ball is subjected to the force of the racket, it can be divided into four forces: impact force, friction along the tangent direction, gravity, and the force of the surrounding air. Since the mass of the table tennis ball is very small, and the force of the surrounding air is much smaller than the impact force and friction, the force of gravity and the surrounding air can be ignored. Therefore, the direction of the combined force of the impact force and friction can be regarded as the initial flight direction of the table tennis ball. Among them, the existence of friction is the key to making the table tennis ball "turn" (this is the key point, you must pay attention!).

After a table tennis ball leaves the racket, it is mainly affected by gravity and air resistance. Among them, the force of air on the ball is mainly divided into three aspects: the first is buoyancy, which is vertically upward and can offset the falling effect caused by gravity, so it can be ignored; the second is wind resistance, which is opposite to the direction of the ball's movement. Because the size of a table tennis ball is not small and its speed is very fast, the effect of resistance is very obvious; the third is the lateral force related to the "turn" of the ball (that is, the friction force mentioned above). It is the emergence of this force that leads to the occurrence of arc balls, and it is also the most direct application and embodiment of fluid mechanics in this sport.

To analyze the principle of curve ball generation from the perspective of fluid mechanics, let us first look at what the "Bernoulli principle" in fluid mechanics is. Once we understand the principle, the subsequent problems will be easily solved.

Bernoulli's principle is the basic principle used in hydraulics before the establishment of the theoretical equation of continuous medium in fluid mechanics. Its essence is the conservation of mechanical energy of fluid. In layman's terms, the key to its principle is to clarify the relationship between fluid velocity and pressure. We can review the scientific principle through a small scene commonly seen in life: hold the upper ends of two vertical A4 papers in your hand, let them hang vertically in a parallel posture, and keep an appropriate distance between the two. At this time, if we blow air from above to the area between the two papers, we will find that the two papers are close to each other.

Are you confused? Shouldn't they move away from each other? This is actually a practical demonstration of Bernoulli's principle. When you blow air between two pieces of paper, the local air will flow, so the pressure between the two pieces of paper will decrease, but the pressure on the outside of the paper will not change, so the two pieces of paper will move closer to each other.

In fact, Bernoulli's principle is widely used in life. For example, there are yellow safety lines on common subway (high-speed rail, train, etc.) platforms. This is because when a train is coming at high speed, the air near the train compartment will be driven to move quickly, and the pressure will decrease. If the passengers on the platform are too close to the passing train, there will be a significant pressure difference between the front and back of the body. In 1905, a tragedy occurred in Russia for the above reasons. A group of people were preparing to welcome the arrival of the train near the railway, but when the train was speeding, they fell onto the rails as if they were hit hard from behind, resulting in the tragedy of many deaths.

Source: Zhengzhou Railway Nanyang Train Depot

There are also frequent "roof-blowing" incidents in summer, which are also manifestations of Bernoulli's principle in life. When there is a strong wind, the air flow speed on the roof will become faster and the pressure will become smaller, while the air inside the house is almost stagnant, so the pressure inside the house will be greater than the pressure outside the house. The stronger the wind blows, the greater the pressure difference will be. Once the wind speed exceeds the critical value, the pressure difference will "whoosh" and blow the roof off!

Source: Strait Metropolis Daily

After understanding the principle, it is easy to understand the mechanism of looping. In the process of table tennis, the table tennis ball rotates in the air. At this time, the air moves backward relative to the ball on the one hand, and on the other hand, the ball will drive the surrounding air to rotate together through friction while rotating. After the two movements are superimposed on each other, the air speed on one side of the table tennis ball will become faster, and the corresponding pressure will become smaller. The speed on the other side is relatively slow, and the pressure will increase. Therefore, the pressure difference on both sides will provide lateral force on the table tennis ball. In this case, the trajectory of the ball will deviate, forming a loop.

Seeing this, Pang Ke discovered a suspicious point. In order to produce a loop ball, the ball must first be rotated in the air. So how can the ball be made to rotate?

In order to make the ball spin, there must be a "force". So where does this "force" come from? It all starts with the collision between the racket and the ball.

When watching a game, you can always hear the coach say to his players during a break, "You have to cut it hard!" In fact, this is not a sentence with too much emotion. "Cut, slice, rub, pull" are all professional action terms in the game, which means making the racket contact the ball in different postures to generate the required friction.

From the Internet

In addition, the "ball-holding" stage is also particularly important. "Ball-holding" refers to the process when the table tennis ball hits the racket surface, causing the rubber and sponge to undergo a slight deformation and then return to their original state. This is also the stage when friction occurs between the racket and the ball. Generally speaking, the greater the friction and the longer the ball-holding time, the greater the impulse torque generated by the torque corresponding to the friction, so that the table tennis ball can also achieve a better rotation effect.

In actual practice, athletes will also use their own explosive power to "swing the ball", which is actually to swing the ball out through the force exerted by the racket. In the process of swinging the ball, athletes will use the speed difference between the racket and the ball and the control of the force to appropriately extend the time of holding the ball, so that the ball can rotate quickly.

Source: Huaxi News

After reading this, do you have a good understanding of the movement principle of loop ball? Every rotation and arc of table tennis embodies endless wisdom and sweat. It is the application of these scientific principles that makes this sport more and more exciting. I wish the Chinese table tennis team can also achieve good results in the upcoming WTT Incheon Championships and continue to show the style and strength of the national ball.