欧美极品 — Table tennis players wield their paddles with an intuitive command of the forces of nature. With Newton at their sides, skilled athletes can force a ball to dip, spin, and hop in a constantly shifting strategic game of deception, power, and accuracy.
Curve balls may help a pitcher strike out batters in baseball; and some nasty spin can make an opponent sweat to return a tennis serve. But more so than in any other ball game, in table tennis 鈥� where the ball is so light and so small 鈥揹edicated players must master the physics of spin.
鈥淲hile [the spin] strategy is also employed in baseball, tennis, cricket, and sometimes soccer, it absolutely dominates the game of ping-pong,鈥� says David R. Dowling, professor of mechanical engineering at the University of Michigan in Ann Arbor. 鈥淎 straight-up fire-throwin鈥� fast ball pitcher may be successful in baseball. However, the equivalent table tennis player, who does not use spin to make the ball鈥檚 trajectory curve, may easily be defeated by a table tennis player who does use spin.鈥�
The reason spin is such a dominating force in ping pong can be seen when you consider that the ball is actually traveling through a fluid, in this case air. In physics terms, when a ball travels without spin, the air moves over the top, bottom, and sides at the same speed and the pressure forces are balanced. However, when a ping pong ball spins as it travels through the air, these pressure forces become imbalanced, causing the ball to veer off to the side or move higher or lower as it flies. Ping-pong players use these imbalanced forces to control a ball鈥檚 path.
For example, imagine a ping-pong ball somersaulting through the air with top spin. In this case, the top of the ball is moving 鈥渋nto鈥� the wind and the bottom is moving 鈥渨ith鈥� the wind, so the wind鈥檚 speed is relatively greater on top of the ball than under it. The faster and slower wind speeds translate to higher and lower fluid pressure on the ball. This pressure difference causes a spinning ball to curve toward the side with lower pressure. In the case of a ball with top spin, that means downward.
The phenomenon, which fluid dynamicists call the Magnus effect, can send a ping-pong ball curving up, down, left, or right 鈥� always at a right angle to the direction of motion 鈥� depending on which way the ball is spun.
Fluid dynamics鈥� importance for the sport becomes clearer at high altitudes, where there are fewer air particles for a ball to collide with. Here, the thinner atmosphere reduces an object鈥檚 air resistance and weakens the Magnus effect, causing a ball with top spin to travel farther than it would at sea level, says environmental fluid mechanics expert Jorge Escobar, an assistant professor at Javeriana University in Bogot谩, Colombia, and a competing table tennis player for 15 years. In his city, 9,000 feet (2,745 meters) above sea level, even experienced players might find their returns flying far beyond the edge of the table, Escobar says.
Play depends on more than the density of air; the shape and weight of the ball, as well as the distance it has to travel, also affect how much control a player can exert. A larger or heavier ball could travel the relatively short field of play of a ping-pong table without much spin-induced trickery.
But the smaller and lighter table tennis ball is at the mercy of the Magnus effect 鈥� and inexperienced players are at the mercy of opponents who can skillfully exploit it.
Of course, players don鈥檛 have to understand the physics behind these effects to take advantage of them in a game. But 鈥渋f you know a little bit [of] the science behind it, you have a better sense about what you鈥檙e doing,鈥� says Hassan Masoud, mechanical engineer at the Georgia Institute of Technology in Atlanta and a table tennis player himself.
To win at ping-pong, explains Dowling, players have to do something that their opponents are not expecting. Sometimes it鈥檚 a question of just keeping the ball on the table. Sometimes it鈥檚 an offensive or defensive move designed to confuse an opponent. Players can 鈥渃hop,鈥� for example, imparting spin to the ball by swinging their rackets so that the ball almost rolls along the surface of the rubber. Or they can 鈥渂lock,鈥� stopping a spinning ball and sending it back at the offensive hitter with whatever spin they used themselves. Aggressive players will impose 鈥渃razy, crazy spins鈥� on a ball, Escobar says; they might even try to fool their opponents by faking a hand or wrist movement to hide the type of spin they have imparted.
For Olympics spectators interested in watching the fluid dynamics of ping-pong in action, a birds-eye view can reveal the side-to-side curving of a well-spun serve. Dowling also recommends keeping your eyes on the receiver rather than the hitter. In a game that moves so quickly, players must pay attention to how their opponents are hitting the ball and what type of spin they use.
鈥淭he player who鈥檚 about to hit it has already decided what they鈥檙e going to do,鈥� Dowling says. 鈥淭he receiving player has to be in position and ready, or they鈥檙e going to be aced.鈥�
Dowling and Masoud are both members of the American Physical Society鈥檚 Division of Fluid Dynamics.
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