Magnus Effect

The Magnus effect is described as a phenomenon that is mainly characterised by a spinning object that is moving through a fluid (gas or liquid) wherein there is relative motion between the spinning body and the fluid. When the Magnus effect takes place, the path of the spinning object is usually deflected in a way that is completely different from when the object is not spinning. The deflection that occurs can be described by the difference in pressure of the fluid that is present on the opposite sides of the spinning object.

In simple terms, there is a generation of a sidewise force on a spinning object. The Magnus effect is greatly affected by the speed of rotation or we can say that it is dependent on it. The Magnus effect is why a football player is able to bend a football into the goal around a 5-person wall, and the movement of the cricket ball as seen in conventional swing bowling.

The effect is named after the German physicist Heinrich Gustav Magnus, who described the effect in 1852. Other scientists like Sir Issac Newton have explained this effect before Magnus, however, it was Magnus who got honoured. We shall learn more about this topic below.

Observation

Have you ever wondered how a football player puts a goal bending the ball around a five-person wall? Well, the answer is Magnus Effect. Curving the ball in the air is not as easy as it looks. There are some physics concepts that make it possible. The Magnus effect occurs on spinning objects that are spherical or cylindrical. The effect that we can observe is that the moving spinning object bends away from the intended direction of travel. The spin of the object alters the airflow around the body, and due to the conservation of momentum it causes the Magnus effect.

Does the Magnus Effect depend on the Bernoulli’s Principle?

The Magnus effect is often considered a particular manifestation of the Bernoulli’s principle. According to Bernoulli’s principle in a non-viscous fluid, the pressure decreases when there is an increase in the speed of the fluid. However, considering the example of a spinning ball in the Magnus effect, the rotating ball creates a whirlpool of fluid (air) around it, and experiences a force perpendicular to the direction of motion. We consider the viscosity of the fluid in the Magnus effect whereas the Bernoulli’s principle is mainly applicable for fluid without viscosity. Therefore, the Magnus effect does not depend on the Bernoulli’s principle.

How does the Magnus Effect work?

In order to get the ball to turn left while moving in the air, you need to get the ball to spin in an anticlockwise direction while moving forward. If you are kicking with your right leg, you need to kick it hard from the inside of your foot so that the ball spins anti-clockwise while moving forward. Basically, you need to kick it off-centre. When the ball moves forward, the ball faces air coming from the opposite direction. Now, the air on the left side of the ball moves along the direction of the spinning ball. This column of air moving on the left side of the ball gets accelerated and turns towards the centre of the ball.

The air on the right side of the ball moves in the direction opposite to the spinning ball. This column of air moving on the right side of the ball gets slowed down and continues moving straight. The air on this side of the ball doesn’t move towards the centre.

So, now you see that the movement of air around the ball is not symmetrical to the original direction of the ball. There is a net force exerted towards the direction shown by the arrow in the figure.

Now, Newton’s Third Law of Motion comes into effect. Newton’s third law of motion states that every action has an equal and opposite reaction.

Just like how a rocket accelerates upwards when the gas is pushed downwards. In this case, the force depicted by the violet arrow in the figure causes a counter-force in the opposite direction depicted by the pink arrow. So, now there is a direction change caused on the ball. The process repeats itself to cause further turning of the ball as it moves ahead.

Application of the Magnus Effect

The Magnus effect is a very important phenomenon especially in the study of the physics of many sports that make use of a ball. Additionally, it is essential in the defence field where this effect helps in the study of the effects of spinning on guided missiles. Alternatively, it has some engineering uses, particularly in designing aeroplanes and rotor ships.

• The Magnus effect is mainly applied in games like football, golf, cricket, tennis, baseball and others. This concept is important in understanding the physics behind many ball sports.
• Some aircraft have been built that use the Magnus effect to lift using a rotating cylinder at the front of a wing, this allows the flight at lower horizontal speeds.
• It is used in external ballistics. The combined sideways wind component of the wind causes a Magnus force to act on the bullet.
• Rotor ships use Flettner rotors which are mast-like cylinders that are mounted vertically on the ship’s deck. These help in propulsion. As the wind starts blowing from the side, due to the Magnus effect, a forward thrust is created.

Read More: Fluid Dynamics

Magnus Effect in Fluids – Video Lesson