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Magnus effect

 

The Magnus effect (often called the Magnus force and named after its 1852 discoverer Gustav Magnus) is a lift force of tremendous importance to all athletes who want to bend the flight of a ball. You see the Magnus effect at work in the curved flight path of balls that are thrown, hit, or kicked and at the same time are given a spin. Golfers, baseball pitchers, and soccer, tennis, and table tennis players all employ this effect to curve the flight path of the ball. The game of baseball in particular is made more fascinating by the Magnus effect. The ability of a pitcher to throw curveballs, sliders, screwballs, and knuckleballs that have very little spin—and then have a batter hit these pitches—is the essence of baseball.

The Magnus effect operates in the following manner. As a spinning ball moves through the air, it spins a boundary layer of air that clings to its surface as it travels along. On one side of the ball the boundary layer of air collides with air passing by. The collision causes the air to decelerate, creating a high-pressure area. On the opposing side, the boundary layer is moving in the same direction as the air passing by, so there is no collision and the air collectively moves faster. This sets up a low-pressure area. The pressure differential, high on one side and low on the other, creates a lift force (the Magnus force) that causes the ball to move in the direction of the pressure differential (i.e., from high to low) (see figure 6.24).

The Magnus effect can be applied in any direction, and in this way an athlete can create backspin, topspin, and sidespin. Soccer players are well known for the way they use “banana kicks” (i.e., the Magnus effect) to curve free kicks and corner kicks around defenders and into the goal mouth (see figure 6.25). Tennis players and volleyball players use the Magnus effect when they apply topspin to make the ball drop suddenly while in flight. Elite golfers apply Magnus forces to produce draws and fades, and the weekend hacker unwittingly applies spin and uses the Magnus effect to slice the ball off to the left and right.

The Magnus effect can combine with the force of gravity or fight against gravity. A topspin combines with gravity’s downward pull, and this is why topspin forehands in tennis (and table tennis) arc viciously over the net and down toward the court. A topspin rotates in the same direction that the ball is traveling, and the spin causes the ball to accelerate once it hits the court surface. A backspin, on the other hand, fights against gravity. The more spin, the more the ball will “hang” in the air. Because the backspin is rotating in the opposing direction that the ball is traveling, the spin causes the ball to slow down and even jump backward once it hits the court surface. Experienced players are able to “read” the spin on the ball from the motion of their opponent’s racket.

A golf club like an eight or nine iron is steeply angled to give the ball tremendous backspin. The spin helps the ball fight gravity and gives it terrific lift, plus the possibility of stopping dead or rolling backward after it lands. The raised stitches on a baseball produce the same effect as dimples on a golf ball. The seams grab a thick boundary layer and so a spinning baseball gets plenty of help from the Magnus effect. Pitchers throw curveballs with a powerful snapping action of the wrist, which gives the ball terrific spin. The more spin, the greater the Magnus effect and the greater the curve. Pitchers frequently combine topspin and sidespin so the ball not only drops but also moves laterally across the plate. Spin, gravity, and drag forces all work together to produce this effect (see figure 6.26).

What happens when hardly any spin is put on a baseball? A pitch with a slight spin is called a knuckleball, and in volleyball a serve with hardly any spin is a floater. The characteristics of a knuckleball and a floater can be summed up in one word: unpredictable. In baseball not even the pitcher knows for sure where the ball is going to end up. Both the knuckleball and floater shift and flutter around in flight. It is this erratic movement that is so confusing to the batter in baseball and to the receiver in volleyball. A pitcher gives a knuckleball a lobbing or pushing release, so it is traveling slowly and maybe spins one full revolution by the time it reaches the batter. During flight, air flowing past at one instant grabs at the seams and at another instant contacts the smooth surfaces of the ball. The ball may go straight for a while, then suddenly veer to the right or the left and possibly back again. Pitchers have learned that a ball released at a certain speed will first demonstrate a regular flight pattern. Halfway to the plate the ball slows down to a critical level, at which point drag forces build up dramatically and the ball suddenly drops. All this is meant to confuse the batter. But if the pitcher makes the error of throwing the ball with too much spin or too much speed, a knuckleball becomes an easy target for the batter.

It is well known that greasing, cutting, scuffing, or wetting the surface of the ball can produce the strange antics of the knuckleball. A ball that has been treated in this manner will act as a knuckleball but at a faster speed. In a sport already dominated by the pitcher, this strategy gave the pitcher further advantage and as a result has been outlawed.

All the pitches thrown in baseball are affected in one way or another by environmental conditions. As a generalization, dense air helps move the ball around, whereas thin air at high altitudes makes it easy for the ball to go faster. Knuckleballs become less deceptive at high altitudes and as temperature rises. In these conditions, the game belongs to the slugger and the fastball pitcher.


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