When I was a kid, one of my favourite programs on TV was called HOW! it was a kids science program and it was presented in such a way to make complex scientific principles understandable using everyday situations. Totally brilliant!

Yes, one of my childhood heroes, Jack Hargreaves really did smoke his pipe in the studio on Kids TV!

So for today...

HOW! Can I bend it like Beckham...

...and so to start, please watch the little video and see exactly what I'm talking about.

This is David Beckham, not normally known for his scientific abilities, using The Magnus Effect to score an absolutely fabulous goal against Greece and if you haven't worked out what it is yet, the Magnus Effect is the effect, documented by German Physicist, Heinrich Gustav Magnus, that causes a spinning spherical object to arc or curve through the air as it moves!

It was actually originally observed by Sir Isaac Newton when watching a game of tennis back in 1672 but let's be honest, he already has enough fancy science stuff named after him!

The Simple Science of how it works

Now obviously this doesn't just apply to football, it applies to any spinning object moving through the air (or a liquid) so there are many applications in sports when there is a ball involved!
The simple explanation is that when something spins and passes through air, the object pushes through the air, moving the air out of the way which of course has to travel around the object. After it has passed, the air, which was at first all nice and still and orderly is still moving around as it gets back into line again and we call this a 'wake'. Now because the object, in our case a ball, is spinning, the wake becomes uneven and we get the effect as seen in the diagram below;

source www.fizzics.org

Now look closely at the shape made by the ball and the space behind it. Look familiar? Of course, it's exactly the same shape as an aircraft wing which we normally refer to as an aerofoil.

source www.explorabox.org

Now in the first picture, imagine you are looking down on the ball, thus the ball travels on a horizontal plane and the effect causes the ball to move left or right depending on the spin direction. Anti-clockwise spin makes it move left and clockwise spin makes it move right! Our aircraft wing, of course, moves either up or down depending on whether it is angled up or down. The same effect basically, changes in air pressure caused by the 'wake'!

Hang on! Air pressure, what does that mean?

We all know that the earth is surrounded by 'air' which is made up of the elements Nitrogen, N (about 78%)and Oxygen, O (about 21%) plus 1% of loads of other bits and bats of different gases. As such, even though we can't see it, it has mass, it actually weighs something and so, to move through it, you need to push this air stuff out of the way. It's not heavy or difficult to do, that's why it spends most of the time just floating around us being held in place by gravity, but it still creates effects on everything that passes through it albeit a tiny effect.

I mentioned Sir Isaac Newton earlier and it's his third law of motion that says that 'every action (pushing through the air using our force) has an opposite and equal reaction' (the air 'pushes back' using a force that we need to overcome to move through it!). This law practically governs the whole of the universe and can be applied to almost anything!

It's these tiny differences in air pressure, higher and lower, above and below the aerofoil that creates the lift so an aircraft can go up or down and in the case of our ball we are looking for the movement from left to right and vice-versa!

Now, the eagle-eyed amongst you will have realised that if the ball doesn't perfectly spin along the horizontal plane, it might also travel upwards or downwards depending on the angle of spin!

If you did, kudos to you and that's exactly what sportsmen use to make the ball curve exactly to the location they want it to go!

Free kick! - HOW?

Now we started with Goldenballs himself, and he uses left handed-swing to score that goal and to that, he approaches from the left in an arc, hits the ball slightly with the inside of his instep and follows through to get the ball spinning which then basically travels as a continuation of his run-up.

Watch the slo-mo at the end of the video, which way is the ball spinning?

Much more technically difficult in my opinion is an out-swinger. Ricardo Quaresma is a well-known expert in the 'trivela' and this is new to me! I didn't know it had a special name but watch the video and see how effective it is.

Then finally, we have top-spin which allows a player to get the ball up and then down again more quickly than its natural trajectory (flight path) might suggest! This is useful for getting a ball over a defensive wall for example and it's done by coming onto the ball on a slight angle and planting your non-kicking foot at the side of the ball pointing towards the target. Your kicking foot needs to connect straight on at the top of the instep, aim for contact with your laces! The ball should then have topspin which will make it fly up then quickly down again, great for short ranges free-kicks!

As I said at the beginning, this effect is used in every sport that has a ball and its effects change with the size and smoothness of the ball which dictates in turn, how accurately you can control that ball. In cricket, the ball is spun with the hand (a spinner) to make the cricket ball swing to try and confuse the batsman. In tennis, spin is applied to the ball by the angle of attack of the tennis racquet. Even in rugby, with their odd-shaped balls, I was taught to put a spin on the ball as I passed it to keep it flying true and faster to make up for its funny shape!

So that's HOW! Anyone with a little practice can bend it like Beckham

Marching On Together

@nathen007

Additional sources other than the ones shown come from Wikipedia. All copyright respected and images and clips used are used for educational purposes.

Posted with STEMGeeks