# Mother Nature's juggling act

05. okt 2006 00:00

History says nothing about how good Isaac Newton was at ball games. Yet already several hundred years ago, he described some of the laws of nature that determine how you can be best with a ball.

Once you have learned to catch and kick or toss a ball, it is time for you to move on to more difficult things. It might be hitting a team-mate on the run or feinting, putting a screw on a ball or rebounding, or different ways of bouncing a ball on the ground. No matter what, however, you have to practise repeatedly to figure out how to put the best slant on the ball when you throw it at the ground in an effort to make it reach a team-mate or go into a goal.

Laws of nature

These tricks can be explained using science. The fact is that all moving objects follow certain laws. Those who have put these laws into words, have studied nature and tried to explain its behaviour.

Newton's laws of motion

Sir Isaac Newton was a scientist who became famous for arriving at three very important laws of motion. We now call these Newton's laws. We can use Newton's laws to explain what takes place in ball games.

Newton?s first law: Objects at rest will remain at rest unless acted upon by an external force. Objects in motion will remain in motion unless acted upon by an external force.Newton's first law of motion

Before kick-off, a soccer ball is at rest in the centre of the pitch. If no one were to touch the ball after the umpire blows his whistle, there would be no game. Objects at rest remain at rest unless acted upon by an external force, according to Newton's first law of motion.

Force of friction

The law also states that an object in motion will remain in motion unless acted upon by an external force. A ball rolling across the ground will ultimately stop by itself. This is due to the force of friction. You can try this yourself by rubbing your hand against different objects.

You encounter considerable resistance when you run your hand across something made of rubber, while a sheet of metal or a desk can be quite smooth. When your hand moves easily, there is little friction. The greater the friction, the faster the ball stops. Gravel or grass will stop a ball shorter than a slippery hall floor.

Newton?s second law of motion: Force is required to change the speed or direction of an object.Newton's second law of motion

If you manage to get your foot on a shot as it moves towards the goal, you may possibly be lucky enough to stop your opponent from scoring. You change the direction of the ball so that it misses. Although Newton was hardly a hard-core football fan, he explained this phenomenon in his second law of motion. He said that you have to exert a force to alter the speed or the direction of an object.

The ball is thrown to the ground.Corner

However, that force need not be your own. Good football players make use of the forces of nature for cornering, for example. You have probably seen players screw a corner shot right into the goal. The ball actually changes direction while in the air, without anyone touching it. To do that, the ball must be kicked in a way that makes it rotate at the same time as it flies through the air.

When the ball rotates, there is slightly less air on the one side of the ball. This gives rise to a force that curves the ball off to the side. You can test this force in a very simple manner: Hold a sheet of paper horizontally up to your mouth and blow hard along the top of the sheet. The sheet of paper will lift up because there is less air on the top.

A ball that doesn't bounce.Rebound and screw

If you throw a ball straight down on the ground, it will bounce back up again. That will only happen, though, if the ball is elastic and bouncy, that is, if it does not get too compressed or deformed when it strikes the ground. A bouncy ball that is thrown at the ground at an angle will follow the same angle up from the ground back in the direction of your throw.

A ball that is not elastic will not bounce well because it will get more compressed when it hits the ground. The compression slows it down. Accordingly, a football player wants all balls to have roughly the same bounce or elasticity so that they will behave in the same way every time.

A ball that bounces.Confusing the goal keeper

By putting a screw on the ball and bouncing it on the ground, players can genuinely confuse the keeper and the other players. The way in which the ball rebounds actually depends on how it rotates. It is hard for the keeper to see how much a ball is rotating, and thus to know where it will bounce. The only consolation must be that the player no doubt has to practice an almost infinite number of times to know him- or herself!

Translation: Linda Sivesind

*Published in 'Nysgjerrigper' no. 2/06*

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