Magnus Effect

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. 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 by the conservation of momentum it causes the Magnus effect.

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, but it was Magnus who got honoured.

Does the Magnus Effect depend on Bernoulli’s Principle?

The Magnus effect is often considered as a particular manifestation of 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 Magnus effect, the spinning ball creates a whirlpool of fluid (air) around it and experiences a force perpendicular to the direction of motion. The viscosity of the fluid is considered in Magnus effect whereas Bernoulli’s principle is applicable for fluid without viscosity. Therefore, the Magnus effect does not depend on Bernoulli’s principle.

How does the Magnus Effect work?

Magnus effect 1

In order to get the ball to turn left while moving in the air, you need to get the ball 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.

Magnus effect 2

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.

Magnus effect 3

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.

Magnus effect 4

 

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.

Magnus effect 5

Application of Magnus Effect

  • Magnus effect is mainly applied in games like football, golf, cricket, tennis, baseball and many more. This concept is important in understanding the physics behind many ball sports.
  • Some aircraft have been built that use 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.