# Cause of Hajj stampede casualties

1. Oct 4, 2015

Would a crowd be able to push so hard to kill an individual?

Apart from all political disputes and different reports concerning the recent Hajj pilgrimage incident, I’m a bit curious as to some physical aspects of the incident.

Suppose some one million pilgrims are walking towards a particular point located farther along a road, which is all surrounded by lateral walls.

There is a thick wall made of glass on the way (so that it can’t be visible to the crowd), intersecting the road.

As the crowd, at the back, are not aware of what is going in the front, and therefore keeps moving, what would the physical consequences of a surmounting pressure be, regarding the safety of the pilgrims, in the front line, after the first line gets blocked at the glass wall?

Bearing in mind that this crowd is not a bunch of plastic bags filled with some shopping, one might question if the guys at the back would be able to tell the minute increase in the pressure just before it’s too late, while there is increase in density.
Still, if you push someone, you are pushed yourself and if that push is gonna kill someone, you are killed yourself!

In the real incident, there might have been all sorts of reasons for the number of casualties, but here, we’re only concentrating on the effects of a particular pressure.

I admit, put that question into numericals involves a lot of endeavours, it would be fine for me to just hear your guesses.

Thank you

2. Oct 4, 2015

### Bystander

Consider behavior of wildebeest at the Mara River crossing, https://www.google.com/?gws_rd=ssl#q="wildebeest"+"mara+river" . Consider tailgaters. Consider the "old bat" behind you in the checkout line at a store crowding you with a shopping cart to make the line move faster. This is more a question of animal behavior than of physics.

3. Oct 4, 2015

### Staff: Mentor

This happens disturbingly often. Consider if someone is pushing on you with a force of 100N. What is the force on you if someone pushes on him with a force of 100N? How about a hundred people?

4. Oct 4, 2015

### Staff: Mentor

From most reports I've read of previous deaths, many times the person is knocked down and trampled to death. Also, it would be feasible that if packed tightly enough, one could suffocate.
And in 2010, a holiday celebration in Phnom Penh, Cambodia left at least 353 trampled to death after a suspension bridge began swaying and thousands of revelers tried to flee.[/quote]

5. Oct 4, 2015

As has been mentioned in the OP, what is being asked here is not the reason of death by being trampled or knocked down; rather the question is if someone dies of pressure, that pressure which is equally distributed among the pressing bodies, i.e. other human beings, should theoretically kill every one in the crowd; assuming other variables being equal.

6. Oct 4, 2015

### Bystander

The pressure is not equal/uniform throughout the crowd.

7. Oct 4, 2015

### Staff: Mentor

No, that is not correct and if you read the articles I posted you would get some idea of what happens. The first link has a link inside it.

http://www.newyorker.com/magazine/2011/02/07/crush-point

8. Oct 4, 2015

No matter how many people there are in the crowd, if the next one to you is pushing on you with a force of say 1000N, then you are pushing on him with exactly the the same amount of force, i.e. 1000N. As such, both should suffer the same amount of injury.

9. Oct 4, 2015

Could you please elaborate. As we are only considering the effects of the 'pressure', then any other factor like a 'strike' is dismissed. A sudden increase in pressure made by a rush is considered a "strike".

10. Oct 4, 2015

### willem2

What happens is that the person at your back pushes on you with 1000N (and you push back with 1000N), but you push with let's say 1100N on the person in front of you, the difference comes from the friction with the floor. The person in front of you pushes with an even greater force on the person in front, etc..
If you have 50 rows of people doing that, people will get crushed.

11. Oct 4, 2015

In physics, sometimes, we try to set aside the non-relevant factors, and only consider the one we are after. So we construct our unique situation.
regards.

12. Oct 4, 2015

### Staff: Mentor

In addition, people in a crowd are rarely aligned in rows. Pressure distributes unequally depending on the relative positions and orientations of persons.

13. Oct 4, 2015

### Bystander

Throw in the mechanical advantage (wedge) of crowds pushing through bottlenecks, and you've crushed rib cages with very little force from the crowd behind.

14. Oct 4, 2015

### sophiecentaur

It is worth pointing out that it only takes a small amount of pressure on the chest to stop us breathing, I did a (supervised) experiment in a swimming pool which involved trying to breathe through a straight vertical tube as I descended the poolside ladder. I couldn't breathe in - my diaphragm wouldn't operate - even with much less than 1m above my head. That was less than 1/10 of an atmosphere (=10kPa) acting against the diaphragm. If your thorax is about 0.1m2 this would only require 1000N of force, which is only the weight of one large person, concentrated on your chest. Of course, this is only a ballpark figure but it shows that the forces involved don't need to be that great and, as has been pointed out above, the forces add up as you go further towards the front of the crowd. It is not surprising that people couldn't breathe in there.

15. Oct 4, 2015

Staff Emeritus
16. Oct 4, 2015

I quite agree with you on increasing forces as you move forward to the front, but the reason being, as I see it, is that every row contributes to sum total of the force that the next row is receiving and not the friction. In fact, I would have thought that the friction only hampers the transfer of force.
I wonder what you think.
Another interesting point to me, is that how would people at the back become aware of a halt in the front row; either by increasing the resistance of the body in front of them, or by diminishing paces. Obviously, there is more probability of such thing happen only when there is not lots of jerks and shakes, so that the minimal change of force, and/or the change of pace is discernible.

17. Oct 4, 2015

### Staff: Mentor

Please post the sources that you are getting this information from, I believe that you are reading nothing and just pushing your own mistaken ideas. Please post your sources now before you make any more statements, I want to see where you are getting your ideas from. We do not allow personal theories here.

18. Oct 4, 2015

### Staff: Mentor

That isn't correct. People are also pushing against the ground. Think about a game of tug-of-war for the same phenomena but in tension instead of compression. Each person adds their force to the rope and though the tension between to people provides the same force to/from each of them, it is not the same tension in front of a person as in the back.

For the tug-of war, the progressively increasing tension happens because people are leaning back. For progressively increasing compression, the people just have to lean forward.

19. Oct 5, 2015

The main part of what you have quoted me is a restatement of the third law of Newton:
" When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body." Wiki.
Are saying that it is not correct?
What I was saying was about two people pushing on together to start with. But when there are more than two people, and at the same time, there exist a different speed in movement, then that makes a lot of difference. In that case, as it's been said in previous posts, the pressure on the back is different from the pressure on the front of a single individual. That said, the third law of Newton, nevertheless, still holds between every two persons. That leads us to the conclusion that the compression in the back part of the body of a person is more than the compression in the front part.

20. Oct 5, 2015

Your manner of speaking is not only unscientific, but also quite unfriendly. This forum as suggests in the opening pages, is a place to communicate ideas and not a class room in which EVO is a professor.
You haven't got the gist of what I am talking about in the OP, and still, giving me all sorts of irrelevant links. There are gentleman and ladies here who have adressed the question quite relevantly, excluding you.
You should remember that in any scientific discussion what ultimately counts is the one's 'understanding of principles and laws' and not the principles and laws on their own.
In this case, you're never sure your deductions are based on Newton's views or just on your own interpretations of Newton's views. That's not a bad thing, anyway; through all these discussions you realize how well constructed your ideas are; keep the good ones, and get rid of the unwanted.

Last edited: Oct 5, 2015
21. Oct 5, 2015

### davenn

now, now, now, play nice
yes it is, an open place, but there are rules, and this it isn't a place for spouting personal theories ( reread the rules that you should have read when you signed up)
we like to keep PF as accurate as possible and within the bounds of currently understood physics

For this reason we like to see references to scientific papers to back up statements that seem offbeat

Regards
Dave

22. Oct 5, 2015

### sophiecentaur

I don't see any particular "personal theory" in penomade's post - just the (correct) application of the same theoretical arguments that are used to explain hydrostatic pressure and the tension in a heavy chain. Newton 3 is only relevant between pairs of crowd members because the force on the guy in front will be the force from the guy behind plus your own force. This makes things cumulative. If he has used words that do not connect directly with you then possibly he could rephrase his argument better. I certainly think I got his message right.
The above is based on a 'quasi static' situation. For a dynamic situation where people start off running, you would have to consider the speed of propagation of a reflected wave in a lossy medium (gruesome) and the forces involved with that. Beyond my pay grade, I think.

23. Oct 5, 2015

Hi Dave
It's easy to declare any piece of writing as ungrounded; but as much as a post is needed to be written in a proper and scientific manner, it is expected that such declarations, on their own, be based on facts, logic, and more importantly, be specific as much as possible. It's just like someone comes up to you, and in response to you explaining your idea with much fervour, just tells you to go and have more reading. In order to be helpful, that one person could at least, specifically point to where you've gone astray, and that's fair enough. Otherwise one wouldn't realize which part of his post(in this case), is afflicted with ungroundedness and things like that.
Best regards

24. Oct 5, 2015

### Staff: Mentor

For example, why would you think this?

Your thread title is "Cause of Hajj stampede casualties". I think if you read about the stampedes that have occurred, you would have a better understanding of what happens in these situations.

And no, I am not attempting to answer the physics part, I am trying to help you understand your other questions of what happens with the crowd.

Last edited: Oct 5, 2015
25. Oct 5, 2015

### Staff: Mentor

No, I'm saying you haven't applied it properly. There are THREE people in the analysis, not two: the force between a person and the person in front of him does not equal the force between the person and the one behind him.

You may want to draw a diagram of this.