The Magnus Effect Explains The "Weird" Behaviour of a Spinning Ball and "Impossible" Goals in a Football Game

The Beautiful Game has a lot of "magic" concerning how goals hit the net to the cheers of the supporting fans or boos of the opposing team.

^{Wikipedia Public Domain Images Source}

The star player knows the correct angle to kick the ball to score the magical free kick that looks even more entertaining in the slow-motion replay.

In another scenario, suppose you and your friends are on a weekend hiking trip with a bit of mountain climbing on the side. Somewhere at the cliff one of your more adventurous friends decides the trip would be more fun if he throws down the ball off the cliff and watch it land in the valley thirty meters below. First, he picked the first ball and dropped it off the cliff. As expected, the ball fell straight down. He chose the second ball and got ready to drop it, but instead of releasing to drop as he did the first time, he gave the ball a little spin as he dropped it.

Everything about the ball drop path changed, instead of dropping down on a straight line as the first did, the balled took a curved path that sent it several meters off the base of the mountain.

This phenomenon that made the ball follow the curved path is known as the Magnus effect.

The effect was named after the German physicist/ discoverer of Magnus Effect, Heinrich Gustav Magnus.

Bend it Like Beckham

If you are a football lover or soccer depending on your Nationality, you could see Magnus effect at play (no pun intended).

The effect is not only synonymous with the game of football as the fascinating importance of the effect are employed in basketball, golf, baseball pitchers, both lawn and table tennis, etc.

In baseball, the ability of the pitcher to throw curveballs or knuckleballs that requires some spin is of utmost importance in the baseball game.

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How it works

The spinning ball moves through air disturbing the streamlines of air around it.

The ball spins at an angle perpendicular to the direction of the motion.

When the ball speeds through the air, the spinning surface pulls some surrounding boundary layer air with it which clings to its surface. On one side of the ball, the passing air collides with the thin boundary layer of air on the ball. A high-pressure point is created by the decelerating air as a result of this collision.

Meanwhile, the opposite side of the ball has the boundary layer air moving in the same direction as that of the air eliminating collision between air and the boundary layer air; that makes the air to move collectively faster. A low-pressure point area is then set up on that side. Recall that the Bernoulli's principle states that the pressure of the fluid (air) decreases with increase in speed.

This pressure differential; high on one side of the ball and low on the opposing side of the ball, will create a lift force known as the Magnus effect which is in the direction of the force differential- high to low as shown in the figure on the left.

where
ω: Angular velocity
V_∞: Far air stream velocity
v₁: Upper side velocity
v₂: Lower side velocity
p₁: Upper side pressure
p₂: Lower side pressure
and F is the resulting magnus force direction.

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Application of the Magnus Effect

The Sports Angle

It is well known that athletes apply a lot of it while in the field of play. In football, it can be used to create sidespin, backspin or topspin since the Magnus effect can be applied in various directions. Footballers who do the "banana kick" especially during corner kicks and free kicks which makes for formidable players all employ the Magnus effect.

In table tennis, Magnus effects are employed for instance, in topspin, the ball rotates in the same direction as the ball's travel direction. That makes the ball accelerate on hitting the table or court's surface.

The more the spin, the more we experience ball "hang" during play.


In Ballistics

Well, it is not only in sports do we require the aid of the Magnus effect to take home that medal. The snipers need to consider the Magnus effect while shooting at a long range target. This is because a spinning bullet will encounter a Magnus force which may throw the bullet off course through travel.


Solving the "Bouncing Bombs" Problems During World War 2

The fiercely protected three dams of the Germans located in the Ruhr valley looks like a target hard to get. There were torpedo nets in the water to prevent underwater attacks while the aircraft guns protected against the bombers.

The dams produce electricity, destroying it would harm the German's economy.

A British engineer, Barnes Wallis, devised a plan to use Magnus effect to give an aerodynamic lift to the bombs dropped on the dam, thereby increases the number of bounces the bomb will make before it hits the dam wall for more effective damage

He devised a cylindrical, 9500 pound weapon that could be dropped at low level while rotating backwards at 500 rpm. Released from a height of 60 feet, about 450 yards from the dam, and at a speed of 230 miles per hour, the weapon would then skip along the water (and over any torpedo nets) until it struck the dam wall, the spinning maintaining the weapon's stability and slowing it down.

The backward rotation would then cause the cylinder to roll down the dam wall where it would explode at a predetermined depth. The wall would be weakened and the great weight of water would cause the dam to collapse.^{Dam's Raid Story}

Transportation

The rotor ship uses the Magnus effects to make propulsion possible. Though there was much interest in this type of vessel at the beginning of the twentieth century, the interest later died down. But in 2008, a German company built E-ship 1

The gigantic four Flettner rotors (cylindrical shapes on the ship)utilises the Magnus effect to achieve propulsion. The spinning body produces a force that is perpendicular to the airstream's direction.

From the sports to the sniper or bouncing bombs and the ship, the Magnus effect is a fascinating physics and will always thrill people every day as we watch it in action.

Thank you for reading

^References

• ^{"Impossible" Goal Explained}
• ^{The Human Kinetics: The Magnus Effect}
• ^{The Physics of Bending a Soccer Shot}.
• ^{Application of Magnus Effect}

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