Crash Physics: Deceleration & Material Properties

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    Crash Physics
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Deceleration is always present during a collision, as energy must be conserved, even in high-speed impacts like a plane hitting a building. The materials and speeds of the colliding objects significantly influence the crash physics, with no collisions being perfectly elastic. In the case of a plane crashing into a building, the initial impact may not visibly slow the plane due to its high momentum, but deceleration occurs as it penetrates the structure. The construction of the building and the geometry of the collision also play crucial roles in determining the extent of damage and deceleration. Overall, the physics of crashes is complex and involves multiple factors, including material properties and energy transfer.
  • #31
Its very difficult to show you where it deforms in a crash like this, becuase I don't keep a slew of plane crash physics videos to hand.

All I can do is talk you through the impact events, and the simulation vid seems to be vlid enough to talk through this.:

Nose hits and deforms inwards,
The fuselage is the stiffest is can possibly be being rammed through a wall at 90 degrees. So it shouldn't compress that much due to the impact.
The wings tear throug the outer wall of the building (releasing fuel) but are ripped off by the increased stress by going through a flooe and then finished off by the main supporting columns in the centre.
As this is happening the nose pitches down and the fueslage splits due to bending.
The tail remains intact throug the outer shell, but deflects to the side and comes off very soon after entering the building.

As the smaller sparts are moving through towards the central colums, the whole plane is torn to bits (there is no more accurate way of putting it than this).
About the same time as this the fuel hits a spark and the fuel ignites.

You won't see any of this in a video because a) it happens too quickly to see b) you arent superman and can't see through buildings.
 
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  • #32
alibongo said:
A wrecking ball is dense and heavy, it's designed to smash through walls.

Yes, this is obviously true, but your missing the point of why I made that remark. My point for making that comparison was to help you get an intuitive sense of why there is not a large deceleration of the plane upon impact. I suppose that there are better comparisons, but that's what came to my mind first.
 
  • #33
buffordboy23 said:
Yes, this is obviously true, but your missing the point of why I made that remark. My point for making that comparison was to help you get an intuitive sense of why there is not a large deceleration of the plane upon impact. I suppose that there are better comparisons, but that's what came to my mind first.


I am trying to understand, but your comparison makes me think if I drove my car into a wall at high speed, it might go through the wall without any deceleration.
Here's another clip.

 
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  • #34
xxChrisxx said:
Its very difficult to show you where it deforms in a crash like this, becuase I don't keep a slew of plane crash physics videos to hand.

Fortunately I have a slew of plane crash videos. The one I just posted has much better resolution than the others but I still can't see any deforming.
 
  • #35
You wont, not from any of these videos taken on the day. They are all too far away and can't be slowed down enough to show anything. All it'll show in a plane dissapearing into the side of the building. No matter how many you come up with you'll not see what you expect to.

To see the deformation you're talking about, you'd need a high speed camera positioned above the plane and to the side of the plane as it enters the building. This would also show that the plane decelerates. And by high speed i mean its shooting at least 1000FPS.
 
  • #36
Alibongo,
Do you have a video of the impact that is in slo-mo, and perhaps stabilised?
 
  • #37
hookes law said:
Alibongo,
Do you have a video of the impact that is in slo-mo, and perhaps stabilised?

I'll look for one, that one I posted today is the best quality I've found so far.
 
  • #38
you still won't see anything though, 25 fps is simply not a fast enough sample rate to capture what you want to see.

however we'll work with what we have and go from there, let us know if you find one.
 
  • #39
I've found it.
In the simulation, the tail drops down to fit into the hole, but I've tried slowing down one of the videos (the close-up one) and this simply doesn't happen in the video. There is enough frames of the tail going into show that (on the video at least) the tail does not drop down in such a way.
 
  • #40
The tail drops? No it doesn't, the entire back end drops as the floor below gives way.
Look at the very first video he posted, the top vid is from the actual crash. You can see the back end drop in the same way.

The other two vids are of the second crash, and the simulation is of the first.

The physics of the second crash are slightly different as the flight is more level and the contact isn't perpendicular to the wall.
 
  • #41
xxChrisxx said:
The other two vids are of the second crash, and the simulation is of the first.

Ah, I see.
Maybe we should stop trying to compare details of the videos of the second crash to a simulation of the first.
 
  • #42
xxChrisxx said:
The physics of the second crash are slightly different as the flight is more level and the contact isn't perpendicular to the wall.

It doesn't look perpendicular to me either. Not exactly anyway. Should we see some rotation in that case, or is the wall, in effect, simply giving way with no measurable resistance?
 
  • #43
i'm really not sure to be honest, i'd assume some rotational motion but not enough to see on the videos. i could be wrong though.
 
  • #44
xxChrisxx said:
i'm really not sure to be honest, i'd assume some rotational motion but not enough to see on the videos. i could be wrong though.

Yeah, I would assume some rotational movement as well. I'll see if I can find a good copy of the video and put my old computer skills to work :)
 
  • #45
I'm tired of waiting for Hookes Law to make a clip !
Here's a clip with the argument about the frame rate.



Although the 911 plane hit footage only has 25-30 frames per second, you can still see there's no deceleration, and no breakage of the plane.
Most film only has 25-30 frames per second, yet other crashes, such as in F1 racing, you can still see deceleration present.
I understand why the frame rate is such an issue. The plane doesn't slow down as it meets with the tower wall.
When should the deceleration present itself?
 
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  • #46
alibongo said:
Although the 911 plane hit footage only has 25-30 frames per second, you can still see there's no deceleration, and no breakage of the plane.
Most film only has 25-30 frames per second, yet other crashes, such as in F1 racing, you can still see deceleration present.
I understand why the frame rate is such an issue. The plane doesn't slow down as it meets with the tower wall.
When should the deceleration present itself?

The frame rate is not fast enough then for you to see any deceleration. You need a faster frame rate.

Here's another scenario. Wet a paper towel and have your friend hold it perpendicular to the floor. You now punch your fist through it as fast as you can. While there is a deceleration upon your fist during the impact, you will never see it.
 
  • #47
buffordboy23 said:
The frame rate is not fast enough then for you to see any deceleration. You need a faster frame rate.

Here's another scenario. Wet a paper towel and have your friend hold it perpendicular to the floor. You now punch your fist through it as fast as you can. While there is a deceleration upon your fist during the impact, you will never see it.

I punched the paper towel, I didn't notice any deceleration, but I did notice the towel break at the moment I punched it. The plane doesn't break at all as it enters the tower wall, it just glides in, shouldn't parts of the plane have broken off, especially the tail or wings? Also there's a delay of ignition, the fuel in the wings should have ignited the moment it hit, not a second or so after.
 
  • #48
Jesus christ...

The deceleration is VERY SMALL. YOU CANNOT SEE IT FROM A POXY VID LIKE THAT. END OF STORY. Yet the laws of motion are not avoided, it hits the building therefore it must slow down by some amount.

Your F1 crash analogy is fail, you see deceleration owing to the fact that they have brakes on and are ususally traveling though a gravel trap. A typical big F1 crash takes place at about 150mph at point of impact. A typical crash may be upwards of 80g deceleration (this is hitting solid armco) you get an event time of about 2 frames. or about 0.08 seconds. for the main impact event. The cars wegith a shade over 600kg.

A thin wall of the WTC will in no way provide that amount of deceleration when something traveling around half the speed of sound and weighing probably 80 - 100 tons. The momentums and kinetic energies involved are not even in the same league.

I really with people would stop arguing there useles case without providing any scientific back up for thie rather stupid statements.

Your stupid points were as follows:

The fuel in the wings should have ignited the moment it hit?
Explain why you think for some strange reasion this should happen? (real crashes are nothing like the movies)Why should bits have boken off the plane as it hit?

The plane's wings is a hell of a lot stiffer than the outer skin of the WTC. So you would expect them to snap off when it met the structural beams inside the building. which is precisely what we see.

you arguements are utter utter crap. we've given you all scienficic reasons for why they are. stop persisting with them. or try to back them up with some evidence, or at least some measure of reasoning.

'because i think so' doesn't count.
 
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  • #49
xxChrisxx said:
A thin wall of the WTC will in no way provide that amount of deceleration when something traveling around half the speed of sound and weighing probably 80 - 100 tons. The momentums and kinetic energies involved are not even in the same league.

Thin wall of the WTC?

Hmm quarter inch thick steel and 14 inchs in depth. Also 4 inch thick concrete poured over the steel truses on every floor, not to mention the steel core that the plane would come in contact with if it was possible to make it half way through.

Hello Chris, are you saying that if I was to hit an engine block with my fist fast enough, my fist could penetrate that engine block?

This thread was entertaining to read!
 
  • #50
Although those figures about the wall being thick sound quite impressive, you are failing to appreciate the energies and forces involved. The fist example shows this to be ture.

Lets look at a comparison of energys for a moment.

A large .50 cal rifle round has on the order of tens of kilojoules. (approx 20kJ energy)

A 12lb cannonball fired from a napoleonic era cannon has hundereds of kJ (0.25MJ)

A plane traveling at that speed has approximately 1.2 Gigajoules of energy. This is the same energy as 0.4 tons of TNT. (although the method of destruction between high explosives and kinetic energy weapons is very different)

The reason for pointing out the above is to show that the idea of a plane acting like a slow moving object such as a fist striking the wall is incorrect. The plane should be viewed an an enormous bullet interms of its destuctive capabilites.

This thread is conerning the 1st imapct, which takes place roughly perpendicular to the outer wall of the building.

The main strength of the wall comes from the concrete, which as a material stands up to static compressive loading very well indeed. However concrete is hard and brittle, and perfoms poorly under dynamic loading and bending. Which is the loading that a plane impact would put on the wall.

The 'thin wall' description was used in context of the problem, when firing a bullet at a sheet of metal, when is that metal considered thick? Is a 1mm sheet thick? 3mm? half an inch...? As a general idea, the wall is considered thin when its critical length (in this case thickness) is << than the critial length of the striking object (in this case the length of the plane).

The plane itsself is basically an aluminium tube. Tubes are very stiff when loaded axially which is how the plane would be loaded during the impact.

During the impact event there is very little way to gauge the forces involved on the plane, we know that the impact event of the outer wall destroyed the wall but not the plane. This however does not mean the plane held up unsctathed. If you look at the simulations run, its shows that although the plane survives the inital impact well, the result of that loading deformesthat plane, and eventually lead to it being turned into shrapnel as it moves through the building.

It also indicates that the plane did indeed reach the central column, but by this time its was basically scrap metal. The main thing that confuses me about the impact is why the wings weren't ripped off, I would have expected this to happen due to the distributed load along the wing gausing a large moment at the wing/fuselage joint.Now let's take a look at your fist analogy. Relative damage to impact bodies depends on energy of impact, deceleration (which gives the forces) and the relative strength of the bodies in contact.

The simple answer is that if you could get your fist moving fast enough you could indeed damage an engine block. Probably not penetrate it due to your fist being made of flesh and bone, and the block being cast iron the difference in strength is very high. With a material fist sized but a strength closer to cast iron would likely penetrate the block

Lets look at the kind of speeds your 'fist o' steel' would have to be traveling at to theoterically penetrate the block. We need to take something of a similar size, so the 12lb cannon ball with do nicely. Assuming your hand is infinitely stiff and won't get turned to a bloody mess on imapct and that your fist weighs approx 1Kg.

To get a KE similar to that of the cannonball (which we know penetrates an engine block) you would need to punch the engine at 1.8 times the speed of sound.
 
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  • #51
The force is equal and opposite so it would be the same as if the building was moving and the plane was stationary. The speed doesn't matter, the stronger denser object will always prevail. Except, as we see, in the case of 9/11 where basic laws were defied!
 
  • #52
Oh dear, you are talking total rubbish and don't seem to be realising it. The statements you make are correct, but you clearly don't understand the physics of the situation.The same object can have different 'effective strengths' depending on how its loaded. You cannot assert that object a is more dense than than object b so object b will survive better, as that does not hold true in all situations. Concrete is a perfect example of this, its compressive strength is very impressive, but it lacks tensile strength. It is also not a tough material and tends to fracture easily. This is the reason why concrete walls don't stand up well to bending and impact loads, and is the reason why steel reenforcement is used.

The walls of thw WTC were designed to withstand hurricane force winds up to , which impart much more force on the building that a plane does, so by your logic the building should have survived no problem. The difference is this load is distributed and not localised.

You also state the speed does not matter, this is also untrue. Dynamic loading is tricky business, but a simplified way of looking at it is the faster the impact (more correctly the greater the acceleration/deceleration of the object) the more localised the damage will be as the material doesn't have time to react and distribute the load.

The statement that it doesn't matter if its the building or the plane that's considered to be moving is true. The thing that matters is HOW the plane hits the building and the deceleration of the plane determines the forces involved. The total damage will depends on the KE at impact.The basic laws of physics were not broken in the case of the plane impacts.
 
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  • #53
Both the mass and the tensile strength of the tower are greater than that of the plane. There is to argue with someone who argues that airplanes can meld into steel and concrete building.

Your not fooling me so you must be trying to fool yourself, clearly you can't know what your talking about with responses like you have made!
 
  • #54
shure, might it have something to do with how the shape of the plane is very long, so there's lot of metal behind each 'last bit' to keep on 'puncturing' the building's outside as it comes up against it? It would be like a needle against cheese.

So instead of 0.25" of plane against say 1" of building, it's the entire length of plane (1000s" against 1" of building).
 
  • #55
Are you comparing the plane and the WTC to a neelde and cheese? Thats funny!

You must be familiar with the bug hitting a windshield question?

The bug would be the plane and the windshield would be the WTC. There is no way the bug would penetrate the windshield!
 
  • #56
Shure

Buy books,
Read books,
Learn.

That is all.Until you rise above your crass lack of knowledge on this subject, there is little 'arguement' to be had regarding this.
 
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  • #57
Ha, ha, you need to follow your own advice "xxChrisxx" since you seem to think a Boeing 767 can juts meld into a steel and concrete tower!
 
  • #58
The evidence shows that quite clearly a plane can penetrate the steel and concrete wall of a building.

'I don't think so' and 'I don't believe that can happen' is not a valid argument to the contrary, no matter how often you say it.As opposed to this pointless discourse, let me ask you a question for my own amusement.

What do you think happened to the planes and building at impact, and why?
 
  • #59
Evidence! You mean the video shows that a plane can penetrate a STEEL and CONCRETE building.

FACTS dictate that water freezes at 32°F. Facts also dictate that what we are seeing in the video is unrealistic.

With your logic I can say I can fly because I saw a video of Superman!
 
  • #60
Its dissapointing that the thread has gone in such a poor direction, with very little reasoned response from you. The crash was described using real physical principles, based on dynamic loading, crash geometry and the way materials behave. You have decided that 'no it doesn't is a better arguemnt'

What a shame.
 
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