by Yuanzheng (Bruce) Mao
Figure 1 Toni Kroos was taking a strike and he scored
in the 95th minute. (Source: https://indianexpress.com/article/fifa/fifa-world-cup-2018-tracking-the-path-of-a-curveball-5240657/)
Since the trajectory of the ball, in horizontal direction, is a curve, there must be a force that pushes the ball this way. Otherwise, the trajectory will be a straight line, according to Newton’s First Law of motion. The answer is—the air. It is the air that pushes the ball. For a fluid (air), if the speed increases, the pressure (force per unit area) decreases. This is called “Bernoulli’s principle”, published by Swiss mathematician and physicist Daniel Bernoulli in 1738. [3][4] For the ball, the air move in the opposite direction relative to itself. Because, if a football could see the air, it saw air moving backwards as it is advancing, same as the scenario where you are sitting in a car that is moving forwards and what you will notice if you look out of the window—the trees are moving backwards! When the ball is also spinning, it drags some air around it. This bit of dragged air is slowed down if the total air flow is in the opposite direction and speeded up if the total air flow is in the same direction. Therefore, the pressure (force) on one side of the ball is bigger than the pressure (force) on the other side. So, the ball is pushed by the air and bends. This is called “Magnus effect” in physics. [5][6][7](Figure 2)
 |
Figure 2 Magnus effect on a football. There is a
pressure difference on the ball, so the ball deflects from the original path. I
drew the picture myself. (Source: I drew this picture myself)
How can we bend a ball?
There are many fantastic
and free tutorials on the Internet for you to find out. Here are some basic
tricks to bend a ball: [8]
· Make sure you have enough angle on your run up.
· Kick the ball using the area between the instep
and inside of your foot.
· Lock your ankle and point your toes upwards.
· Hit the lower half of the ball slightly on the
side.
· Get help from your hip when kicking.
· Your non-kicking foot point slightly
off-target.
· Continue the follow through after kicking.
· Lean over the ball a little bit.
Most importantly,
practice makes perfect!
𝛄: large
contribution
𝛄 𝛄: enormous
contribution
[1]: FA editor (2009),
David Beckham’s free kick against Greece [online] Last accessed 16
January 2022: https://www.youtube.com/watch?v=t0GESlaVNdE
[2]: Felix Keith
(2021), “The moment England forgave me” – Why David Beckham’s iconic Greece
free-kick meant so much [online] Last accessed 16 January 2022: https://www.mirror.co.uk/sport/football/news/david-beckham-greece-free-kick-25149826
𝛄 [3]: Wikipedia, Bernoulli’s
principle [online] Last accessed 16 January 2022: https://en.wikipedia.org/wiki/Bernoulli%27s_principle
- cite_note-3
𝛄 [4]: The Editors
of Encyclopaedia Britanniaca, Bernoulli’s theorem [online] Last accessed
16 January 2022: https://www.britannica.com/science/Bernoullis-theorem
𝛄 [5]: Wikipedia, Magnus
effect [online] Last accessed 16 January 2022: https://en.wikipedia.org/wiki/Magnus_effect -
Physics
𝛄 [6]: The Editors
of Encyclopaedia Britannica, Magnus effect [online] Last accessed 16
January 2022: https://www.britannica.com/science/Magnus-effect
𝛄 𝛄 [7]: 李永乐老师官方, 2018世界杯C罗绝技电梯球上演帽子戏法，精妙绝伦！李永乐老师讲解如何踢出电梯球 香蕉球和落叶球 [online] Last accessed 16 January 2022: https://www.bilibili.com/video/BV1ks411L7a7
[8] Unisport, How to curve the ball | Learn bending free kick [online] Last accessed 16 January 2022: https://www.youtube.com/watch?v=vR2Grj1f0VE
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