The More You Know #5 – The Physics behind the PERFECT free kick

In Physics, Sport by Mystifact

Imagine being able to pinpoint exactly where you want the football when taking a free-kick. Surprisingly, you can use Physics to give the ball a specific path to ensure it hits the back of the net. The Magnus effect can be used to its full potential in various ball games such as football. When taking a free kick, putting some bend or dip on the ball has proven beneficial by confusing the goalkeeper into thinking the shot is initially going wide, for example. Having said that, how can we use spin to affect the movement of the ball?

The Perfect Free Kick

Before we can practically apply what we are about to learn, we need to understand the Physics of the Magnus Effect. Don’t be intimidated by it – it’s very simple!

The Magnus Effect

The Magnus effect is simply how a spinning object behaves in the air [1]. Generally speaking, when something is spinning, it tends to drag air faster around one side of it which creates a pressure difference. This pressure difference changes the path of motion of the object, towards the low pressure side. This force that changes the path of an object is called the Magnus force. In conclusion, the Magnus force acts in the opposite direction to the direction of spin.

Referring back to taking a free kick; if you put top spin on the ball (where the top surface of the ball spins forward) the Magnus force acts downwards thus making the ball dip. If done properly, the ball will initially seem to travel much higher than the target, and eventually dip enough to hit the back of the net. This tricks the goal keeper thus raising your chances of scoring a goal!

Pretty neat, right?

Same applies with:

  • Back spin: the Magnus force acts up, thus making the ball travel upwards (before gravity takes over and brings it back to the ground)
  • Left spin: the Magnus force acts to the right, thus making the ball travel rightwards
  • Right spin: the Magnus force acts to the left, thus making the ball travel leftwards (as shown in the free kick taken in the first image)

Therefore practising this, can help you determine the path of the ball, thus, executing the perfect free kick.

The Magnus effect is seen over and over due to it having many applications. It is important in different ball sports which involve a spinning ball. It is also used in the design of rotor ships and Flettner aeroplanes, where the rotor is composed of a spinning cylinder, which spins backwards to allow the plane to be lifted up into the air [2].

On an additional note: the video below shows a great example of the Magnus effect in action. You can see the basketball pretty much fly against gravity just by adding a tiny bit of back spin as it is being thrown. At 28 seconds of this video you can see he drops the ball without spin, then he proceeds to do it again, with a tiny bit of spin, see for yourself [3]!

The reason the ball seems to ‘fly’ is because the Magnus force acts upwards as the spin the ball was given was backwards.

If you have any questions, leave them below and until next time, take care.

~ Mystifact


References:
[1]: http://www.humankinetics.com/excerpts/excerpts/magnus-effect-
[2]: https://en.wikipedia.org/wiki/Flettner_airplane
[3]: https://www.youtube.com/watch?v=QtP_bh2lMXc

Please note; no copyright infringement is intended. All images used have been labelled for re-use on Google Images. If any artist or designer has any issues with any of the content used in this article, please don’t hesitate to contact me to correct the issue.

Previous ‘The More You Know’ posts:
The More You Know #1 – How to lose weight if you are 100% desperate
The More You Know #2 – Bees have spidey senses!
The More You Know #3 – The Human Race in a Sugar Cube?
The More You Know #4 – Human vs. Hydrogen Bomb