The Physics Behind Soccer and Goalkeeping

I have played in multiple district level, and interschool level soccer matches, where I was the lead goalkeeper- our team has won several competitions, and I personally was awarded the golden glove for outstanding goalkeeping performance in 2018-19; a goalkeeper always plays an important role in leading the team throughout the match and guiding the defense according to his convenience. Even more so than the defenders, midfielders, and attackers, the goalkeeper must be smart enough to know the exact physics behind his sport, to maximize efficiency, and success rate. Earlier, I was not totally sold on the idea of studying the concept behind a sport, but through the years of experience in playing and reading sports physics blogs at Physics Diaries, I have realized how drastically a small piece of knowledge can impact your performance on the field. I take this opportunity of being featured at Physics Diaries to share a blog on soccer physics, so you don’t have to learn the hard way, the things that I had to as a passionate and curious soccer player.

Ball Spin- The Magnus Effect

When the football is kicked at an off-center position, it spins in a particular direction, very much like the swinging of a cricket ball (click here to read a detailed post on swing bowling in cricket).

This deviation in the trajectory of the ball is observed due to the Magnus effect. Through experiments, it was found that Magnus effect is not observed in vacuum, and hence it could be concluded that the Magnus effect is observed due to the interaction of the ball with moving air or wind.

Studying only the theory does not really help understand the applications and implications of the concept. So, let me introduce a unique perspective with respect to the Magnus effect.

For a goalkeeper, it is extremely important to save penalties and the only sensible way to predict the direction of the ball before it is kicked is by observing the attacker’s foot.

Here’s a video of an amazing play by a penalty taker, but an even smarter save by the goalkeeper-

Let’s analyze this clip frame by frame. Thanks to the multiple view angles, we can completely decode what goes on in the penalty taker’s and the goalkeeper’s mind.

At this instant, the penalty taker decides to kick the ball slightly to his right. This means that he is applying force at the off-center position of the ball, and hence the magnus effect comes into play. If the penalty taker kicks the ball slightly towards the right, the ball interacts with the flowing air in such a manner that it slightly deviates towards the left, and this phenomenon is termed as the ‘Magnus spin’. This is quite intuitive as you would generally expect the ball to spin towards the opposite direction of the direction of the force, but this spin is only possible if the attacker keeps his foot in a straight direction in the follow through- the spin may be affected of the follow through is different.

In this case, the goalkeeper reacts brilliantly, which means he can predict the ball’s direction just by keeping an active eye on the penalty taker’s foot, and hence bends towards his right. The penalty taker acts smartly by observing the goalkeeper’s move and adjusting his shooting strategy. He knows that the goalkeeper has already started bending towards the right, and hence the penalty taker shoots the ball almost at the center. This means that the Magnus effect is minimized, and the ball approximately stays in the straight path after shooting.

At this instant, we can almost conclude that the penalty taker has outplayed the goalkeeper, and in most cases, this would have been a clear goal. But this is where athletics and presence of mind of the player comes into play.

Quite unexpectedly, the goalkeeper saves the penalty. This next frame answers how-

In this next frame, two very important observations can be made.

Firstly, note the penalty taker’s foot. We can clearly see that his follow through is exactly in the direction in which he kicked the ball i.e., slightly off-center towards the goalkeeper’s left.

Secondly, we can see that although the goalkeeper has been outsmarted by the penalty taker, he tries his level best to somehow keep the ball in his physical reach. His outstanding athletics helped him to slightly kick the ball by his foot. This makes the ball deflect, and one of the best saves in the history of football is recorded.

With this, we can conclude today’s write up. In conclusion, I just want to emphasize the importance of athletics, and presence of mind in a sport like soccer. And it goes without saying, that you should always try to understand the science behind YOUR sport.

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