The Magnus Effect Explained | The Coolest Physics You'll Ever See

Air is real thing. We usually ignore the air while solving numericals based on physics and gravity. But as we know, that's not the case in real life. Air has a lot of effects on stuffs around us that are flying here and there.

If you take a cylindrical paper, and then make it roll down an inclined plane, then notice what happens. You will notice a very unusual thing. Okay, first take a tennis ball and make it roll down an inclined plane. As expected, it will roll down and fall off normally. But if you take that cylindrical paper thingy and make it roll down, you will see that actually, that paper, once crosses the inclined plane, flies backwards! The ball went frontwards, but our special thing went backwards! How is this possible?

This is possible due to a very special phenomena in physics known as the Magnus effect. Whenever an object spins through the air, it experiences a 'Magnus Force' due to friction between the air and the object's surface.

Magnus effect is mostly seen in sports. Many sportspersons also know what this is but they don't call it Magnus force. That's the thing. This phenomena or rather, this force is responsible for the curving, swinging of balls when hit or thrown. This force or effect is known as the Magnus effect because Magnus was the first person to discover this phenomena when he was investigating why cannon balls curved as they propagated through the air.

What Magnus found out was outstanding. He saw that when a ball or a cylindrical object spins, there is a force perpendicular to the spin axis. If its spinning clockwise or topspin, then the force acts down (below the object) and if the object is spinning anti clockwise, then the force acts up.

Now, how does a magnus force effects a ball spinning in the air? You must've seen free kicks in football, right? When the player hits the ball with a certain angle in his foot, the ball flies with a small but beautiful curve and there goes into the goal. So, now, what is the science behind the Magnus effect?

As the ball is moving forward through the air and if it's spinning, then the air is flowing around the ball from the front to the back. The ball is spinning in the same direction as the air flow at the top of the ball but in the opposite direction at the bottom. Now, because of the friction between the air and the ball's surface, air is dragged around the top of the ball downwards to the back. But at the bottom of the ball, the air flows in the opposite directions.

Now, the air comes to a halt soon instead of deflecting upwards. The net result is that the air is deflected down due to Newton's third law where the air exerts an equal amount of force on the ball which is upwards.

Magnus Effect: A Deeper Look

The Magnus Effect is used in a lot of sporting techniques. It is used in tennis, table tennis, golf, football etc.

Let us again consider a tennis ball that is spinning in air. The air that comes and hits the ball does an interesting thing. The air below the ball hits and the air increases its speed.

(Important side note: In this case, we are considering the ball to be moving clockwise towards the air which makes the bottom part of the ball moving along the direction in the air. Also, the air is moving opposite to the direction of the ball)

As the ball spins, the bottom of the ball will drag the air in its direction and thus will eventually increase the speed of the air in that direction. But the case is contrary on the top side of the ball. On the top side of the ball, the top surface is dragging air in the opposite direction therefore the speed of the air on top of the ball will reduce and there will be what's called an "Air drag".

Now, pressure comes into the play. Basically in our case of the tennis ball, at the bottom, the pressure becomes low and at the top, the pressure is high. And now we clearly know that difference in pressure creates movement of air to balance the pressure. So this will create a net downward force and the ball will finally end up curving downward. This is what's called the Magnus effect named after Heinrich Gustav Magnus in 1852.

The "Magnus Effect" affects all the rotating balls and cylinders as they fly through air. Now as you might be thinking, does the Magnus effect has any non sport applications? Perhaps, it does.

In the past, sailors used the Magnus effect to sail boats across seas and oceans. At those times, the sail boats were kind off a lot different than todays ones. They had Flettner Rotors which took the place of the sails. What it used to do was that it deflected crosswinds, using the Magnus effect, to propel the ship forwards.

Another real life non sport application of the Magnus effect in the past was a plane with rotating cylinders instead of wings! Yes! Using the Magnus Effect, the cylinders actually generate more lift than traditional wings. But the problem was that it was very impractical to do so because it created way more drag. And therefore the consequence, the plane flew once and crashed.

But the thing is. The Magnus effect is kind off making a comeback! The E-ship 1 uses four rotating cylinders (Flettner Rotors) to increase its efficiency and reduce the fuel used. So, yeah, in the future, the Magnus Effect must have a lot of other cool applications rather than curving things in mid air. But that too is so cool.

I hope you took something new from here.

Thank you for reading!