The Magnus Effect of a Ball in Sports
Spinning Tennis Ball
Sportsmen and sportswomen who play ball games such as cricket or baseball know all about spinning the ball in the air. They can use the ball not only to make it change direction when it hits the ground, but also make it curve in the air. In baseball, golf, cricket (except a fast bowler in cricket), all this is done with the use of spin by a technique that requires a lot of practice.
So why do and what makes a ball spin and curve in the air? Well this was a question answered in the early 20th century by the British scientist Lord Rayleigh (John William Strutt), who called it the 'Magnus Effect'. This was named after a previous German scientist Heinrich Gustav Magnus.
Generally, as a spinning ball travels in the air, it drags air around itself in the direction of the spin. The air moving past the ball is also speeded up on one side and slowed down on the other. Since air moving faster also exerts less pressure, the air on the opposite direction of the ball is at a relatively greater pressure. It is this specific difference in pressure that creates the force which makes the ball to curve in flight.
The Magnus Effect is evident in many particular sports. A lot of top tennis players can create a top-spin lob and which seems to be going 'out' but drops sharply before the baseline. A golfer imparts a spin automatically when driving the ball, so that the ball hisses away from the tee at around 160 kilometres per hour (100 mph) with a back spin of around 50 RPS (revolutions per second). The spin initially counteracts the effect of gravity which enables the ball to fly in a straight line rather than in a curved trajectory. With the highest-number irons which create better and greater spin in golf, the ball can even be seen to climb. If the shot is miss-hit with the club's head at an
angle, which many golfers realize, the same force can take the ball away sideways into the rough.
In baseball, a pitcher uses different kinds of hand grips to impart different spins, and thus different curves to the ball. The 'curve-ball' is a curve that breaks downwards and away from the batter. Some pitchers throw a so-called 'fade-away' or 'screw-ball', curving the ball towards the batter. A 'slider' breaks sharply at the last moment. These sort of curves require spins of up to 38 rps, and which can induce a swerve of some 2 feet in the 60 feet between the pitcher's mound and home plate.
In American football and rugby, the similar effect can be used as well with great skill and technique. An elongated ball kicked to spin around its long axis will go straight while traveling along the line of that axis. Only when it starts to drop, and the spin axis is different to the line of travel, does it begin to swerve. A professional player can therefore kick a ball parallel to the touchline, fully aware that it will curve out of play once it drops back to the ground.
In cricket, a curve or swing can be produced without the need of a spin. This is usually done by a medium to fast bowler making wise judgment of the single raised seam. The seam induces a slight air turbulence and causing the air to flow more rapidly over one side of the ball. As before, a different speed of airflow on the two sides will produce a difference in pressure causing the force that swings the ball in flight. A lot of skilled bowlers increase the difference in roughness between the two sides of the ball by rubbing and polishing one side of it on their trousers.