The Magnus Effect : The Physics Behind Bending It Like Beckham

February 7, 2017

Have you ever wondered how football players are able to bend the ball like they do? Perhaps you’ve noticed how other spinning solid objects seem to magically move sideways as they fall? Why is this? It all comes down to the wonder of the Magnus Effect. In this article, we’ll have a quick look at what it is and how you can see it in action. We’ll also show you some cool applications of the effect in technology.

Here we go.

So what is it?

Contrary to popular belief the Magnus Effect is not named after Icelandic journalist and former Mastermind presenter Magnus Magnusson. Ok, I made that up, it is of course named after German Physicist and Chemist H.G. Magnus.

In 1853, Magnus decided to experimentally investigate the odd effect of projectile deflection from firearms such as smooth bore cannons. Typically in science, he wasn’t the first to describe it. Isaac Newton, in 1672, correctly inferred the effect after watching tennis players at Cambridge. Similarly, Benjamin Robins, a British Mathematician, ballistics researcher and military engineer, also managed to explain deviations in musket ball trajectories to this effect.

Controversy aside, all of these prominent scientists, not Magnus Magnusson, worked out what exactly was going on. The Magnus Effect is a generation of a sidewise or perpendicular force on a spinning cylindrical or spherical object immersed within a fluid (gas or liquid). This only applies when there is a relative motion between the spinning object and the fluid. You’ll see it in action whenever you watch football matches or watch tennis players serve.

As the spinning object moves through a fluid it departs or deviates from a straight path. Pressure and air flow differences develop as the object passes through the fluid because of the velocity changes that the spinning object induces. The Magnus Effect is, in fact, a special case of Bernoulli’s principle which states that “an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy”.

Let’s look at an example

Let’s take the example of a ball spinning through the air. The ball will “drag” some of the air around it as it spins. From the point of view of the ball, air is whooshing past on all sides. The spin of leading side of the ball turning into the air flow “pulls” or deflects air in the direction of spin. Air travelling against the spin direction is separated from the ball, as you’d expect. The net result means air is dragged into the direction of spin with the ball “pushed” in the opposite direction. This is usually perpendicular to the path of the spinning object.

This causes the object to deviate in a noticeable arch away from the expected path. The following video from Veritasium shows this effect in all its grandeur.

Cool real world examples and applications of the Magnus Effect

You can see the Magnus Effect all around us, it often excites or upsets us (well if you’re a sports fan). It has helped to claw victory from defeat in the dying minutes of ball games or perhaps even saved your ancestor’s life on the battlefield in the past. Or of course, vice versa. Its effects have also sparked some really ambitious projects that could provide fantastic improvements in fuel efficiency or new ways of transportation.

Let’s take a quick look at some of these.

Flettner Rotor Ships

Resembling something that a child might cobble together out of ship model kits and straws, these fantastic ships use large vertical rotating cylinders to provide a potential method of propulsion for ocean-going vessels. These ships, first built by German Engineer Anton Flettner, use rotor sails powered by motors to take advantage of the effect. Flettner applied his tech to build the first Magnus Effect propulsion ship, the Backau. The ship looked a bit odd but it was a fantastic “outside of the box” application of the theory. Enercon GmbH uses this today on their E1 ship.

The Buckau, the Flettner Rotor Ship, photographed in 1924 [Image Source: Wikimedia Creative Commons]

Planes – Rotor Wing

Applications aren’t exclusive to the high seas. Inventors have tried to take advantage of this in flying machines too.

Engineers have tried to see if lift can be generated from the rotating cylinders when placed on the leading edges of wings. In theory, this would allow flight at lower horizontal speeds. One of the earliest attempts to do this was in 1910 by Butler Ames. Ames was a US Congressman who built a heavier than air aircraft.

Today the iCar 101 Ultimate is a proposed project using Flettner rotors in a roadable aircraft design to combine compactness and increased lift potential, pretty cool.

The Plymouth A-A-2004, Flettner rotor aircraft [Image Source: Wikimedia Creative Commons]

Bend it like Beckham

The Magnus Effect helps explain the common observations seen in ball sports. This usually provides fantastic seeming tricks, shots or curve balls seen in sports ball trajectories. You’ll notice it most dramatically in football. Great examples would include goals or free kicks taken from the likes of Ronaldo or of course David Beckham.

Interestingly, there was a controversy in 2010 during the FIFA world cup. The Magnus Effect caused some criticism of the match ball during this tournament. The argument goes that the balls had less swerve control but flew further.

Pitchers in baseball often take advantage of this phenomenon as well. As they pitch they impart different spins on the ball resulting in it curving in the desired direction. Major League Baseball uses the PITCHf/x system to measure the change in these trajectories all the time.

Projectile Weapons

Any spinning bullet is also at the mercy of this effect during flight. Although less significant when compared to gravity, crosswinds or air resistance, the Magnus Effect nevertheless plays it part. Even on a completely calm day, the projectile will suffer from small sideways components of wind, tilting the bullet nose slightly out of the direction of travel. The bullet effectively “skids” through the air. This yawing creates Magnus Effect forces that affect the vertical trajectory of the bullet and alter its intended final landing/impact point.

Sources: Britannica

[Featured Image Source: Pixabay]

SEE ALSO: Strange Things Happen to Basketball due to Magnus Effect


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