physics question - do forces propogate through a crowd?

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K

Kev

Lifer
Dec 17, 2001
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Say a crowd of people is pushing on a large glass door (like a mall entrance), does the overall strength of the crowd increase as the crowd gets larger?

I was arguing with someone about this (an engineer, so he is probably right). He said that the only force exerted on the door is the force exerted by those at the front of the crowd. The force from the people behind them would have no effect on the door. This doesn't seem right to me.

I have decided to ask the collective brain of ATOT to settle this dispute.
 
RaistlinZ

RaistlinZ

Diamond Member
Oct 15, 2001
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I guess it makes sense that some energy is lost/absorbed by the person in front of you - so that the total force applied to the door isn't increasing linearly with every additional person added to the crowd. Some force is also being lost through the peoples' feet through the ground too I suppose, since that force is travelling downward.

The real question I guess would be how many bodies deep would it take for the force being applied by the person at the rear to be completely absorbed/lost by the bodies of the people in front of him?

But I'm no physicist. I'm sure I'm way off.
 
PieIsAwesome

PieIsAwesome

Diamond Member
Feb 11, 2007
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The only force exerted on the door by the crowd is by those at the front actually touching the door, yes.

But the force they exert on the door can depend on the people behind them pushing them. Pretend its not a crowd. Pretend its a backhoe, pushing a guy against the door. Certainly the force the guy exerts on the door with the painful assistance of the backhoe is greater than if it were just the guy by himself pushing against the door.

If a person exerts a force on the door, the door in turn exerts a force on him, and in order for him to remain stationary, an equal and opposite net force must be exerted on the person. If its just the guy, this other net force is made up of the reactive force from the ground, friction at the point of contact with the ground, and the guy's weight. Or actually, lets ignore weight, assume the forces involved are horizontal. But if there is a crowd pushing him, that lifts him off the floor, and assume his weight isn't doing anything, then this net force will be the force exerted on him by the crowd. So the force exerted on the door by the guy will be more or less the same as that exerted by the crowd on the guy.
 
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Jeff7

Jeff7

Lifer
Jan 4, 2001
41,596
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If you are leaning against a wall with a certain amount of force, and someone pushes against you, the force exerted against the wall will increase.

Yes, there will be a damping effect, so it won't be linear, as RaistlinZ said. If you have 10 people in a line and someone at one end falls over, the guy in the front might receive a nudge of some sort. If you have 1000 in a line though, the person at the front won't know anything about it, unless you're all jammed together. Pack people in tight enough, and you've pretty well got a bunch of bony sacks of saltwater, which will be able to transmit forces.

So yes, the # of people in a crowd, assuming they're all exerting a force in the same direction and pushing against the person ahead of them, will increase the force acting upon the wall. At that point, it's limited by the compressive and/or buckling strength of the loaded bones in the first person's skeleton. :)


Another way of imagining it: Toss a fully-thawed chicken in a box, under which is a force gauge. Then keep filling the box with more chickens. The force on the gauge will continue increasing as you add chickens.
 
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D

Deleted member 4644

People up against a poll/railing have ben cut in half by crowd force, so figure that.
 
her209

her209

No Lifer
Oct 11, 2000
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Using that logic, only the force being applied by the two opposing people closest to the center in a game of tug of war matters.

:confused:
 
TecHNooB

TecHNooB

Diamond Member
Sep 10, 2005
7,458
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Basically, this depends on how well the force transfers as you add additional people to the back of the line.
 
C

Cheesetogo

Diamond Member
Jan 26, 2005
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I can't find it now, but I recall an article on how fluid simulations were being used to model crowds in situations like this.
 
M

Moonbeam

Elite Member
Nov 24, 1999
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It all depends on the charge on the people. Positive people attract each other and negative ones repel each other so if a lot of negative people collect around one side of a door the people on the other side of the door will reinforce it to the hilt.

Lots of folk crowding a door is called a door jam.
 
T

torpid

Lifer
Sep 14, 2003
11,631
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Kev said:
I was arguing with someone about this (an engineer, so he is probably right). He said that the only force exerted on the door is the force exerted by those at the front of the crowd. The force from the people behind them would have no effect on the door. This doesn't seem right to me.
Click to expand...

I'm curious to hear this so-called engineer's take on tug-of-war based on this logic.
 
J

jchu14

Senior member
Jul 5, 2001
613
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her209 said:
Using that logic, only the force being applied by the two opposing people closest to the center in a game of tug of war matters.

:confused:
Click to expand...

True if only the people closest to the center is pulling the rope and the rest of the people are pulling the person in front of them (as opposed to everyone pulling on the rope).

The discrepancy between the OP and the engineer is due to the understanding of 'force exerted by'. Now imagine if one person is able to exert 10 pounds of force by pushing the ground with their shoes. When you have one person, the door is feeling 10 pounds of force from the person pushing on the ground. Now when you add an additional person, the 2nd person is exerting 10 pounds of force on the first person's shoulder. The first person's arm now has to exert 20 pounds of force to the door. Now add the third person, and the same thing happens. The first person's arm has to exert all that force.

Whether you consider that 'by' the first person is just wording. If you remove people behind the first person, he can't exert that much force on the door any more, but if you place a scale between the first person's hand and the door, you'll get force = N*10lb where N is number of people.
 
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K

Kev

Lifer
Dec 17, 2001
16,367
4
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torpid said:
I'm curious to hear this so-called engineer's take on tug-of-war based on this logic.
Click to expand...

actually he's an engineering student. I was arguing with him pretty vehemently but I conceded because I assumed he had discussed this in class with an actual physics professor.
 
DrPizza

DrPizza

Administrator Elite Member Goat Whisperer
Mar 5, 2001
49,601
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www.slatebrookfarm.com
Tell your engineering friend to draw a free body diagram. He's wrong.

edit: btw, the paper linked to above - I'm guessing that someone else here linked to it quite a while ago; I remember skimming through it before.
 
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O

OCGuy

Lifer
Jul 12, 2000
27,224
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What OP is trying to ask, is if the engine in a MMF 3-way train gets pounded twice as hard?
 
M

Moonbeam

Elite Member
Nov 24, 1999
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Kev said:
actually he's an engineering student. I was arguing with him pretty vehemently but I conceded because I assumed he had discussed this in class with an actual physics professor.
Click to expand...

Damn, I would have assumed the professor didn't know his ass from a hole in the ground. I like your modesty.
 
M

Moonbeam

Elite Member
Nov 24, 1999
74,736
6,759
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DrPizza said:
Tell your engineering friend to draw a free body diagram. He's wrong.
Click to expand...

You might also tell him to try another profession. If it isn't obvious that a hundred million ton crown colliding with a door at one mile an hour isn't going to flatten that door, nothing ever will be to him.

If you want to be safe in a tsunami, just zip up the mosquito net on your tent.
 
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TuxDave

TuxDave

Lifer
Oct 8, 2002
10,571
3
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What your engineering friend said reminds me of something I read in the math section of Childcraft. Now I need to go dig up that book to see what it said.

Edit: Ok, work with me here.

1) Static model so nothing is moving
2) No one except the last person in the line is pushing with their feet (so no force is getting pushed against the friction of the ground). Everyone except the guy in the back is standing upright or whatever.

So if the guy in the back pushing with his feet is asserting force A, the guy he's pushing on is pushing back with force A and thus had to push forward with force A, etc.... all the way to the front.

Wouldn't that cause the guy in the front to only feel force A?
 
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K

kami333

Diamond Member
Dec 12, 2001
5,110
2
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Are you trying to solve it from a real life perspective or a "spherical cow" perspective?

If everyone had their arms out and locked and were pushing then larger crowd = more force, but in real life there's going to be a lot of elastic collisions so as RaistlinZ said there's probably a non-linear relationship with a upper threshold.
 
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