You might find this easier to understand if you carefully watch some detailed slow motion video footage of such kicks. You also could experiment with a toy super ball that is so coloured that it is easy to see its direction of spin.

Assume that the kick has not put much spin on the ball. For the ball to be deflected sharply downwards, it has to have hit the crossbar fairly solidly, though not squarely enough to rebound, of course. Hitting the crossbar like that would cause its top half to decelerate violently, far more strongly than the bottom half. In fact the point of contact with the crossbar would act as a fulcrum,  the resulting leverage accelerating the bottom half forward. If the ball were moving steeply down, its top surface actually would be moving backwards.

The effect of such an impact is to put a sharp backspin on the ball and bounce it towards the ground. Obviously, when it hits the ground, as long as it does not slip but has good traction, the ball will bounce neither forward nor straight up, but in the direction in which it is spinning.

And with backspin, that direction is the way out!

Go well,

Jon

I have two theories on this.

1. As the ball is approaching the bar from the goal kicker's side... it is more likely to rebound in that direction.

2. More interesting though is the possibility that the ball behaves like superball/highbounce balls do. When bounced between two surfaces they return to you! This has something to do with the nature of the rubber they are made from where it grips and effects the rotation of the ball. This is a bit like the backspin some golfers put on their balls as they drop on the green to make the ball stop or roll back.

There are three possibilities for a football when it hits the crossbar. 

1. It hits with the bar in the middle of the ball - the ball then rebounds straight away from the goal

2. It hits the bar with the lower half of the ball - the resulting trajectory has an upwards component - if the ball hits the bar at the right place then the resulting trajectory will be vertically upwards

3. It hits the bar with the upper half of the ball - the resulting trajectory has a downwards component not caused by gravity - if the ball hits the bar at the right place then the resulting trajectory will be vertically downwards as in Hurst's shot.

Now in the third case we need to consider what happens to the ball - the bar hitting the top half of the ball causes the top of the ball to decelerate faster than the bottom and the ball starts to spin top backwards - back spin.

When the ball hits the ground under the posts this backspin causes the ball to bounce away from the goal.  The harder the shot the greater the force differential causing the back spin and the more pronounced the effect.

However, this does not answer the question as to whether Hurst scored or not as a ball which hit just lower than the vertical "sweet spot" would still have a back spin even when it's trajectory took it down and slighty forwards - far enough to score but with enough spin to bounce back out of the goal - hence the controversy.

Good Grief!!!

How do you like that for putting a hex on a team? Here we were innocently discussing ball physics, and there a classic example turns up on cue in the World Cup, with which team playing which? Which team kicking into which goal???

Who said "Shades of '66?"

Honest officer, it wasn't us, and anyway Mr Horseman didn't mean it...

Well, at least this time there was no doubt of the fact that it was a robbery. TV showed this plainly. Maybe it is time they imported hawkeye into soccer.

Actually, I reckon it is over time for them to change the rules and turn it into an interesting game...

This is simply just statistics and the square root of two, honest. You might want to draw your own diagram.

Consider an incoming ball flying horizontally, no spin, no dip, and at a height that will hit the crossbar somewhere (doesn't go straight over, doesn't go straight in). Have the sum of the radii of ball and crossbar equal to R.

I would assume that for a large number of shots, the ball centres are evenly distributed vertically in the space from R above the centreline of the bar (the highest possible hit) to R below (the lowest possible hit).

If, at the moment of impact, the line from the centre of the ball to the centre of the crossbar is at 45 degrees below the horizontal (when viewed from the side) the ball will bounce vertically downwards (equal angle of reflection). If the ball was higher than this, it will bounce back into play, if lower it will bounce down behind the goal line. If it hits above the centre line of the bar, it will likewise bounce back or over.

But the relation between height of hit and angle is not linear - it is a cosine thing. For the ball to bounce back down into play, it can hit the bar anywhere between level and 0.7071 * R (that is, R / sqrt(2)) below the bar. So it is about 70% likely to bounce back into play either high or low, 15% chance of deflecting over the top, and only 15% chance of scoring.

You might bend it like Beckham, but I prefer to figure it like Pythagoras.