What is the best shape for a soccer goal post?

[kshitij]

what weighting you place on each trajectory.

What do you mean by this? Does it mean something like how probable the ball is to come from any given angle? If so then I had assumed that it is equally likely to come from any angle

 

[pbuk]

What do you mean by this? Does it mean something like how probable the ball is to come from any given angle? If so then I had assumed that it is equally likely to come from any angle

Not just angle, it makes a difference whether the ball is heading for the front corner of the post or the middle, or somewhere in between.

So now you have two assumptions on which you are basing your calculations:

• The ball is of negligible size compared to the post (whereas in fact the ball is larger than the post)
• Shots from e.g. 10cm from the corner flag are equally as likely as shots from in front of the goal (which does not match my (very limited) observation of real play)

Are you still going to be happy with your conclusions?

 

[kshitij]

• Shots from e.g. 10cm from the corner flag are equally as likely as shots from in front of the goal (which does not match my (very limited) observation of real play)

I'm happy with this particular assumption because the play is randomly distributed around each post, that is, its allowed for any player to take a shot from anywhere on the field, so given enough time, yes I think that shots from the corner flag are equally as likely as shots from the front of the goal.

However there isn't anything I could do about the other assumption, I'll have to assume the ball to be a point mass, otherwise its impossible to calculate anything. But as mentioned earlier, I would still be happy to find out results about the point mass.

 

[jbriggs444]

Thus, I believe that this is the correct limiting case.

No. This is not the correct limiting case.

Here you have the ball striking the left hand goal post somewhere near the 45 degree line and rebounding into the right hand goal post exactly at it's 9 o'clock position. The ball will then continue, rebounding leftward at the same angle ($\Theta - \Phi$) above the horizontal as it arrived from below. That rebound angle is clearly more than good enough to score. You need a rebound angle that is only exactly good enough to score.

 

[kshitij]

No. This is not the correct limiting case.
View attachment 282714
Here you have the ball striking the left hand goal post somewhere near the 45 degree line and rebounding into the right hand goal post exactly at it's 9 o'clock position. The ball will then continue, rebounding leftward at the same angle ($\Theta - \Phi$) above the horizontal as it arrived from below. That rebound angle is clearly more than good enough to score. You need a rebound angle that is only exactly good enough to score.

That is the limiting case of $\theta$ for any fixed value of $\phi$

 

[jbriggs444]

That is the limiting case of $\theta$ for any fixed value of $\phi$

How so? Smaller angles of theta can result in strikes lower on the right hand goal post that still go in.

 

[kshitij]

How so? Smaller angles of theta can result in strikes lower on the right hand goal post that still go in.

For smaller angles of $\theta$ the ball would strike below the point D so how can the ball still go in?

 

[haruspex]

For smaller angles of $\theta$ the ball would strike below the point D so how can the ball still go in?

It is easiest to see it by running it in reverse.
Suppose the ball eventually hits the half way around point of one of the posts, but does so approaching from almost parallel to the goal line, making some tiny angle $\alpha$ to it.
Trace that back to its last bounce off the other post, and back again towards the first, and so on until it misses a post. Clearly, by making $\alpha$ small enough there is no limit to the number of bounces.
The algebra will be messy...
Edit: it goes like this. I'll describe it as though the ball starts at the halfway around point of one post and progressively bounces out of the goal.
Let the posts be 1, 2 and the distance between them along their line of centres, L, be D>>R.
From 1, it hits 2 roughly $D\alpha$ from L, at a point subtending angle $\frac DR\alpha$ to that line at the centre of 2.
It bounces from there at an angle $(1+2\frac DR)\alpha$ to L, do you see that?
Can you generalise it to n bounces?

 

[haruspex]

What do you mean by this? Does it mean something like how probable the ball is to come from any given angle? If so then I had assumed that it is equally likely to come from any angle

Yes, you can choose it to be equally likely from any angle within the pitch, and for a given angle equally likely at any lateral displacement from the line that would have it hitting the post along the normal to it (of those displacements for which it would make contact). That is what I did for my result in post #28.

But you could instead say, e.g., it is equally likely from anywhere within the pitch, and at any angle from that point that touches the post. This gives a different distribution since a shot from a wide angle is relatively unlikely.

In reality, shots that touch the inner part of the post are more likely than shots that touch the outer part because the player is attempting to get it in the goal.

 

[kshitij]

It is easiest to see it by running it in reverse.
Suppose the ball eventually hits the half way around point of one of the posts, but does so approaching from almost parallel to the goal line, making some tiny angle $\alpha$ to it.
Trace that back to its last bounce off the other post, and back again towards the first, and so on until it misses a post. Clearly, by making $\alpha$ small enough there is no limit to the number of bounces.
The algebra will be messy.

I'll need a figure to understand that but right now I have to go, maybe I'll come back to that later.

 

[jbriggs444]

For smaller angles of $\theta$ the ball would strike below the point D so how can the ball still go in?

The initial strike on the left hand pole was below the due-east point on that pole. Yet you busy are calculating how that strike can go in.

Now you have a rebound that strikes the right hand pole below its due-west point. There is still calculation left to do.

 

[Office_Shredder]

In reality, shots that touch the inner part of the post are more likely than shots that touch the outer part because the player is attempting to get it in the goal.

This is making an assumption about the accuracy of shots. My understanding is a significant fraction of shots don't even hit the net, so I would guess the 1" difference between the inner and outer part of the post is pretty insignificant for the probability distribution.

 

[haruspex]

This is making an assumption about the accuracy of shots.

No, only that the distribution is unimodal, but I did not state it correctly. I meant that of the parts of the post the player can see, there will be a bias towards the side nearer the goal mouth. Anyway, I agree it can probably be ignored.
Far subtler is the distribution of where shots are made from. Obviously the attacker would like to make the shot from in front of the goal, and the closer to the centreline the better; but the throng of defenders would lead to less ideal choices.

 

[haruspex]

I'll need a figure to understand that but right now I have to go, maybe I'll come back to that later.

Please see my edit to post #68.

 

[kshitij]

It is easiest to see it by running it in reverse.
Suppose the ball eventually hits the half way around point of one of the posts, but does so approaching from almost parallel to the goal line, making some tiny angle $\alpha$ to it.
Trace that back to its last bounce off the other post, and back again towards the first, and so on until it misses a post. Clearly, by making $\alpha$ small enough there is no limit to the number of bounces.
The algebra will be messy...
Edit: it goes like this. I'll describe it as though the ball starts at the halfway around point of one post and progressively bounces out of the goal.
Let the posts be 1, 2 and the distance between them along their line of centres, L, be D>>R.
From 1, it hits 2 roughly $D\alpha$ from L, at a point subtending angle $\frac DR\alpha$ to that line at the centre of 2.
It bounces from there at an angle $(1+2\frac DR)\alpha$ to L, do you see that?
Can you generalise it to n bounces?

So, I think this is the situation you are talking about (on decreasing the angle between the line joining the centres and path of the ball (red), number of bounces increases),

This infact, does contradict what I thought earlier the situation would be for a ball striking the far post below the midpoint,

But on closer inspection, I spotted a difference between the two,

We can quite clearly see that on extending the path of the ball, the red one passes above the centre, while the blue one passes below the centre.

So, I speculate this (below) might be the correct limiting case (feel free to correct me),

Here the path passes through the centre of the far post and on rebounding off the far post, the path of the ball retraces itself.

Am I getting it right this time?

 

[kshitij]

So, I speculate this (below) might be the correct limiting case (feel free to correct me),

Now I don't think that there is any smaller value of $\theta$ possible for this given $\phi$ because anything smaller than this will bounce away from the goal side after hitting the far post. So, I think that if we do what @haruspex said,

Edit: it goes like this. I'll describe it as though the ball starts at the halfway around point of one post and progressively bounces out of the goal.
Let the posts be 1, 2 and the distance between them along their line of centres, L, be D>>R.
From 1, it hits 2 roughly Dα from L, at a point subtending angle DRα to that line at the centre of 2.
It bounces from there at an angle (1+2DR)α to L, do you see that?
Can you generalise it to n bounces?

And generalise that to n bounces, (according to what I think) the (n-1)^th (*penultimate) bounce will be when the extended path of the ball just passes through the centre of either of the posts. So, we both should get the same result through both the methods. Doing the reverse process though will be much more complicated, so I think that solving the above critical case is the way to do it.

(*by penultimate bounce what I mean is that, we cannot keep on bouncing back and forth between the posts forever, there must be a time when the ball rebounding off one post will not bounce back onto the other post, thus the second last bounce is the penultimate bounce which you can picture by simply reversing the green path in the above image)

Edit: I do realize that if D>>R then there will be no limit on the number of bounces, but D=7.32m & R=0.06m, so I think we can't say that D>>R

 

[haruspex]

So, I think this is the situation you are talking about (on decreasing the angle between the line joining the centres and path of the ball (red), number of bounces increases),
View attachment 282741
This infact, does contradict what I thought earlier the situation would be for a ball striking the far post below the midpoint,
View attachment 282747
But on closer inspection, I spotted a difference between the two,
View attachment 282743View attachment 282744
We can quite clearly see that on extending the path of the ball, the red one passes above the centre, while the blue one passes below the centre.

So, I speculate this (below) might be the correct limiting case (feel free to correct me),
View attachment 282745
Here the path passes through the centre of the far post and on rebounding off the far post, the path of the ball retraces itself.

Am I getting it right this time?

You've gone a bit too far the other way. You could increase theta a little and still not score a goal.
E.g. if after the second bounce it travels parallel to the line of centres then after the third it will be moving away from the goal line.
It needs to keep getting a bit closer to the goal line each bounce. In fact, the angle to the goal line must reduce by a factor a bit greater than 2 (about 2+R/D.)

 

[haruspex]

I do realize that if D>>R then there will be no limit on the number of bounces, but D=7.32m & R=0.06m, so I think we can't say that D>>R

It is always true that there is no limit on the number of bounces. Running it backwards, the smaller you make the initial angle to the goal line the more bounces it will take to escape.

You don't think D>100R constitutes D>>R?

 

[kshitij]

You've gone a bit too far the other way. You could increase theta a little and still not score a goal.
E.g. if after the second bounce it travels parallel to the line of centres then after the third it will be moving away from the goal line.

I did try to visualise that and it seems correct.

It is always true that there is no limit on the number of bounces. Running it backwards, the smaller you make the initial angle to the goal line the more bounces it will take to escape.

You don't think D>100R constitutes D>>R?

Yes, I think you are right here.

 

[Jan Jaroscak]

@kshitij Did you manage to find any solution to the problem? I am trying to explore a similar question and I would probably make the same assumptions as you (even if that didn't lead to an answer that is representative of reality). I agree that the probability of the ball going in off a square post is 33% and 25%+ for the circular post, but I do not know how to figure out how much more than 25% it is.

 

[Orodruin]

@kshitij Did you manage to find any solution to the problem? I am trying to explore a similar question and I would probably make the same assumptions as you (even if that didn't lead to an answer that is representative of reality). I agree that the probability of the ball going in off a square post is 33% and 25%+ for the circular post, but I do not know how to figure out how much more than 25% it is.

Again, these assumptions are faulty even if the ball is infinitesimal in size. The answer must depend on the distribution of incoming balls.

 

[Jan Jaroscak]

Again, these assumptions are faulty even if the ball is infinitesimal in size. The answer must depend on the distribution of incoming balls.

The distribution of incoming balls is not relevant for the question that I (and from what I understand @kshitij as well) am asking, although it may be more representative of reality. My question is simply: Which goal post allows for a bigger angle for a goal to be scored after reflection – square or circular? And of course, find the exact critical angle. The image below explains why I believe the square post allows for a 90-degree angle resulting in a goal while the circular post allows for π/2 + θ. θ is what I am trying to find.

 

[haruspex]

The distribution of incoming balls is not relevant for the question that I (and from what I understand @kshitij as well) am asking,

In order to get an answer at all you have to assume a distribution. You must be doing that even if you don't realize.
There are two aspects: the direction of the ball and the lateral displacement. You could take a uniform distribution across the legal range for each independently. For a square post, that gives a 1/4 chance of a goal, not 1/3.
For round posts, in your diagram, the chance of hitting arc AB wth that distribution is (I think) $\frac 14-\frac 1{2\pi}$.
As noted in post #50 onwards, finding the critical angle/displacement in BC for round posts is quite tricky because there can be an unlimited number of rebounds between the posts before the outcome is clear. The distance between the posts is relevant.

 

[PeroK]

The distribution of incoming balls is not relevant for the question ...

You could consider only penalty kicks, for example. Otherwise, where is the ball being projected from?

 

[Jan Jaroscak]

You could consider only penalty kicks, for example. Otherwise, where is the ball being projected from?

That is actually a great idea! Instead of having to consider both the direction of the ball and the point of impact as varying factors like before, it is now possible to combine the two since the starting position will always be the same. In that case, am I correct in saying that the ball will end up in the goal when it reflects off the circular post anywhere to the right of the red ray in the diagram below? An example of this is the green ray. A ball (still assuming it is a point mass) traveling along the red ray would simply reflect in the same direction it came from as the ray is parallel to the normal. Any collision left of the red ray would cause the ball to reflect away from the goal line.

 

[Orodruin]

You could consider only penalty kicks, for example. Otherwise, where is the ball being projected from?

Even that is not exactly representative of reality because of curve balls.

 

[PeroK]

Even that is not exactly representative of reality because of curve balls.

Curve balls are in baseball, surely? We used to call them banana shots!

 

[Jan Jaroscak]

Right, but assuming the ball is a point mass, the interaction between the post and the ball would be a definite point, rather than an area, resulting in the reflected ray being straight. Am I missing something here?

Even that is not exactly representative of reality because of curve balls.

 

[jbriggs444]

Right, but assuming the ball is a point mass, the interaction between the post and the ball would be a definite point, rather than an area, resulting in the reflected ray being straight. Am I missing something here?

The concern was estimating the distribution of shot angles and/or displacements. One possible way to arrive at such a distribution would be to start with a distribution of points from which a shot was attempted.

But if you fail to account for "bananas"/"curve balls" then this will not lead to a correct distribution of arrival angles.

In addition, the reflected rays need not be straight if the balls rebound from a post with non-negliglble spin.

[It is not clear whether the idealization of the ball as a point mass is intended to do away with Magnus lift or to retain and idealize it as well]

 

[haruspex]

It seems to me now that the wrong question is being asked. The probability of a goal depends also on the thickness of the posts.
If we say circular posts are radius R and square posts are of side 2S then the question becomes what the ratio of R to S should be to make them equal.
To evaluate that, we need to consider the range of displacement of the trajectory from the centre line (the to the centre of the post) to be $\pm D$ where $D>\max\{R,S\sqrt 2\}$.