Crash Physics: Deceleration & Material Properties

[alibongo]

Hi everyone, I'm trying to understand crash physics.

When a moving object collides with a static object, is there always deceleration present at impact?
If the moving object is designed to pass through something else (a bullet or missile) I assume there is very little deceleration if any.

Do the crash physics displayed depend on the material or speeds of the objects involved?
Is there any way a car for example could pass through a brick wall without any deceleration, or breakage?
If the car was capable of traveling at say 300 mph, might it pass through a wall with no deceleration or breakage? Are the properties of the objects and the speeds of the objects at all relevant?

 

[Danger]

Welcome to PF, Alibongo.
There will always be some degree of deceleration, unless the moving object is a neutrino or maybe a high-frequency photon. A freight train hitting a mosquito slows down by an immeasurably tiny amount.
Both the materials and the speed are extremely relevant, as are the shapes of the objects. In fact, those are pretty much the only factors that do matter.
By the way, a properly designed bullet stops completely within the target. If it goes through, a lot of energy is wasted rather than causing damage. That's what hollow-points are all about.

 

[buffordboy23]

Hi alibongo, welcome to PF!

Hi everyone, I'm trying to understand crash physics.

When a moving object collides with a static object, is there always deceleration present at impact?

In the macroscopic world, the answer is definitely yes. Energy must be conserved. For example, if you hear sound from the impact, energy from the system of the two colliding objects loses energy to its environment. As a consequence, some deceleration must occur.

Do the crash physics displayed depend on the material or speeds of the objects involved?

To my knowledge, no collisions are perfectly elastic although some collisions are well approximated by being elastic. Therefore, there will be some energy loss of the system to the environment, which may or may not be practical to consider based on the type of collision.

 

[alibongo]

Thanks for the replies.
My physics is so limited I'll show you what I'm trying to understand, rather than attempting to explain it.
Here's a clip with two fake crashes, which one is the most realistic in terms of physics?
The bottom one seems more realistic to me, with the deceleration, immediate ignition of the fuel, and breakage. I've never studied physics though, hence posting the question on here.

 

[xxChrisxx]

The first one seems slightly more likely, the fuel wouldn't explode as soon as the plane touched the building. What you don't see in the first one is that once it peirces the outer wall the plane is ripped to shreds, the engines and fuel tanks are ripped open inside the building. If you noticed what happened on 911 you'll see that the 'entry wound' to the building is actually very small. In fact I think the top video is real as it matches the sim exactly.

There was a decent combined simulation (plane and building) on youtube showing exactly what happened to the WTC and why it collapsed, it combines FEA and fire simulation into a single animation. Search for 911 simulation, its the top one.

 

[Danger]

The first one seems more realistic, except that I didn't see any damage to the building upon impact. It looks as if the plane is 'phasing' through the wall rather than punching a hole in it. (That might just be a problem with my eyesight, but I watched it 4 times.)
As Chris pointed out, there's no way in the world that poking a plane on the nose could rupture mid-ship fuel tanks, let alone spark them off.

 

[alibongo]

Danger, there's no problem with your eyesight, there is simply no damage to the wall.
I'm more interested in the deceleration though, shouldn't 500,000 tons of steel slow down the plane? At what point should the deceleration present itself?
(I'm not into conspiracy theories, I can't stand the 911 truth movement)
But this footage bothers me.
It's not just the Spell footage that shows no damage to the wall, no deceleration, and no breakage of the plane. Here are 3 more,

 

[xxChrisxx]

Thats the point its not 500,000 tomns of steel. The way the WTC is constructed its many many steel beams, that all together are incredibly strong but the plane only had to fly through a few of them. Take a look at that simulation I told you about, give me a sec and i'll link it.

The plane had the energy to smash through the first few, and planes are bloody stiff so its not surprising that it didnt break up instantly. As it flew through the structural beams inside it slowed down, that ripped the engines off and that's what caused the fire.

Imagine a razor hole (edit: slit) in a piece of paper, from far enough away you can't see it.

Edit and you can tell it had to fly into the building as the explosion came from the inside and blew out the side wall.

 

[alibongo]

Thanks Chris, but I'm curious about the transfer of energy. Shouldn't the plane slow down as it hits?
I didn't think planes were bloody stiff, I thought they were made with aluminium.
Shouldn't the plane have bucked immediately? Like Dangers comparison with the mosquito and the train.
Deceleration doesn't seem to be present in the vids I posted. If anyone can explain this please reply...

 

[xxChrisxx]

It does slow down as it hits, just not enough to notice on those videos. you can tell it slows down as it moves through the building as it takes longer for the debris to come out of the other side that it would take a plane to fly the distance.

Planes are made of aluminium, but its the construction (geomoetry) that makes them stiff. The plane deforms as it hits the building, but you've got the remember relaive to the width of the building the walls are paper thin and don't provide a large stopping force. Planes have to be incredibly stiff for saftey reasons, you can make planes much lighter than they are now but for crash saftey they are designed to stay together and the engine to snap off. (Which you will see in the simulation they did as it was into the biulding).

The reason why you can't see in it the vids is because they arent shooting fast enough and are too far away to see what's going on.

 

[buffordboy23]

I agree with xxChrisxx. When the initial impact occurs, you can think of the plane as 100,000-150,000 kilogram wrecking ball moving at a pretty decent speed; that's an incredible amount of momentum. So, any wall, even one with numerous reinforced steel beams, is not going to initially decelerate the plane that much to notice.

 

[Danger]

To take it to extremes, think of the 'straw in a hurricane' scenario. A regular stalk of hay or grass, traveling at a couple of hundred mph, can penetrate the bark of a tree. The aforementioned bullet is an unfortunate example as well.

 

[hookes law]

To take it to extremes, think of the 'straw in a hurricane' scenario. A regular stalk of hay or grass, traveling at a couple of hundred mph, can penetrate the bark of a tree. The aforementioned bullet is an unfortunate example as well.

Ah, but that's an urban myth!
The high forces of a tornado bend and twist telegraph poles and trees alike.
Near to the ground, a tornado can contain millions of flying pieces of grass, straw, twigs and things - it is simply one of these getting lodged into a bent tree or pole, the tornado passes, the tree bends back, and voila! A magic straw!

 

[Danger]

Ah, but that's an urban myth!

Not so. I don't dispute the rest of your argument, but the straw thing is real. You can demonstrate it yourself on a small scale by stabbing a potato very hard with a paper drinking straw. A fellow Canuck, whose name I can't recall, runs a lab that tests all manner of such things for the purpose of storm analysis. You wouldn't believe the amount of damage a 2x4 can cause when it comes out of the cannon at 300 kph.

 

[hookes law]

A fellow Canuck, whose name I can't recall, runs a lab that tests all manner of such things for the purpose of storm analysis. You wouldn't believe the amount of damage a 2x4 can cause when it comes out of the cannon at 300 kph.

Haha! Excellent!

Although having seen all the recent plane crashes on the news recently, I doubt a plane should have acted like that in the twin towers. Surely the tail section wouldn't have had the remaining force to cut through and leave a tail-shaped hole above where the rest of the plane has gone in? Not sure about the wing-tips. My gut tells me the same thing about them as well.

I would have expected a less plane-looking hole, if you see what I mean lol

 

[xxChrisxx]

Why wouldn't it? At that speed the tail would be like a knife going through the side of the building. After the initial impact the tail is torn off, the building is also structurally weakened by the front of the plane going in.

 

[hookes law]

Why wouldn't it? At that speed the tail would be like a knife going through the side of the building. After the initial impact the tail is torn off, the building is also structurally weakened by the front of the plane going in.

You know, I had a look at some of the videos, and the tail isn't torn off at all.
And it would have to be like a thinner, hollow knife going through a bigger, denser knife - I would expect to see it rotate downwards, or disintegrate against the wall.
There's something awry with some of these 911 videos. Hope no-one's making money from these...

 

[dE_logics]

When a moving object collides with a static object, is there always deceleration present at impact?

Yes.

Even if it collides with another moving object in the same direction there's always a retardation.

If the moving object is designed to pass through something else (a bullet or missile) I assume there is very little deceleration if any.

In the above explanation I did not assume this

Retardation is cause to third law, in this case, the third law didn't hold but it did apply, that means loss in momentum.

You know if a 7.62 mm bullet passes though Kevlar, it WILL slow down

Is there any way a car for example could pass through a brick wall without any deceleration, or breakage?

Then wall needs to be thin and there's NO WAY you can do that unless you're on a like......6000NM torque engine (i.e the engine needs to be running).


That too will be very less I think.

The answer lies not in the speed, or even the BHP but in the torque.

Since at higher speeds (cause of the lower gear ratio) the torque reduces by a huge amount, it should be preformed at a very low pace.

 

[Danger]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

 

[xxChrisxx]

You know, I had a look at some of the videos, and the tail isn't torn off at all.
And it would have to be like a thinner, hollow knife going through a bigger, denser knife - I would expect to see it rotate downwards, or disintegrate against the wall.
There's something awry with some of these 911 videos. Hope no-one's making money from these...

You arent taking into account the geometry of the situation. The tail may be hollow, but it went in for all intents and purposed at a right angle to the wall. This means the tail would have been the most resistant to bending it could have been and the wall the least.

The entire plance was mashed in the middle of the building so it may or may not have come off, i'll have to re look at the videos.

EDIT: If you look at the simulation the tail is ripped off just after its entry to the building.

 

[hookes law]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

Actually that's not true - the actual impact shots started appearing around midnight and then over the next few days, and the witness' statements varied from small plane to large plane to missile and even nothing...
But its just the impact videos that are interesting me at the moment.

 

[hookes law]

You arent taking into account the geometry of the situation. The tail may be hollow, but it went in for all intents and purposed at a right angle to the wall. This means the tail would have been the most resistant to bending it could have been and the wall the least.

The entire plance was mashed in the middle of the building so it may or may not have come off, i'll have to re look at the videos.

EDIT: If you look at the simulation the tail is ripped off just after its entry to the building.

You're quite right about the geometry, but I don't trust the simulations. The tail section in the impact videos stays straight and true throughout. I can't see any deceleration, the plane passes through the building in the same amount of frames it takes to travel its own length when in the air...

 

[Danger]

Actually that's not true - the actual impact shots started appearing around midnight

I admit that I might be mistaken on that issue. When I woke up at 7:30 in the morning (MST), it was on the news. My mother talked about it, as she had seen it at least half an hour prior. I still dispute, however, that there was time for CGI faking.

 

[xxChrisxx]

You're quite right about the geometry, but I don't trust the simulations. The tail section in the impact videos stays straight and true throughout. I can't see any deceleration, the plane passes through the building in the same amount of frames it takes to travel its own length when in the air...

Of course simulations with this magnitude of unknowns is inherently inaccurate, the general deformation of the plane is more or less correct. The simulation seems to match (roughly) time wise with the length of the impact and explosion, but without knowing the assumptions and the input data for the FEA its impossible to say.

And I don't understand, what's all this talk of CGI?

 

[Danger]

whats all this talk of CGI?

Computer Generated Images. Think of movies like 'Toy Story' or 'Spider-Man'.

 

[alibongo]

It does slow down as it hits, just not enough to notice on those videos. you can tell it slows down as it moves through the building as it takes longer for the debris to come out of the other side that it would take a plane to fly the distance.

The plane deforms as it hits the building, but you've got the remember relaive to the width of the building the walls are paper thin and don't provide a large stopping force.

The reason why you can't see in it the vids is because they arent shooting fast enough and are too far away to see what's going on.

You can tell it slows down because the debris takes longer to travel out the other side of the buiding?! Shouldn't you be able to see it slowing down as it impacts?
I ask again, at what point should the deceleration present itself?
Where in the videos I posted does the plane deform? The plane doesn't deform, it remains intact as it moves through the steel.
Are you saying the camera's frame rate isn't fast enough to capture the deceleration? You can see the plane fly into the tower the same speed it takes to fly flies through thin air. I'm not sure the frame rate is relevant.

 

[alibongo]

I agree with xxChrisxx. When the initial impact occurs, you can think of the plane as 100,000-150,000 kilogram wrecking ball moving at a pretty decent speed; that's an incredible amount of momentum. So, any wall, even one with numerous reinforced steel beams, is not going to initially decelerate the plane that much to notice.

A wrecking ball is dense and heavy, it's designed to smash through walls.

 

[alibongo]

One thing that people seem to overlook when critiquing reconstructions is that we all saw the original footage just minutes after the event. There was no time for editing, let alone CGI enhancement. Not to mention how many hundreds or even thousands of eye-witnesses. It's pretty difficult to fake such a public event.

Please, no talk of CGI enhancement. Why are you mentioning hundreds or thousands of eye witnesses? Eyewitnesses to what? Something probably did hit the towers, but the videos I posted look very odd.
What about the fuel ignition, I thought some fuel is carried in the wings. Shouldn't the fuel have ignited as soon as the wings broke up? Why can't we see the wings breaking in the crash vids I posted?

 

[xxChrisxx]

Computer Generated Images. Think of movies like 'Toy Story' or 'Spider-Man'.

I know what it is... :P I just don't see how you can 'fake' a ****ing enormous plane flying overhead. Especially the second hit, in which you can clearly see a huge plane.

If it walks like a duck and quacks like a duck...

What about the fuel ignition, I thought some fuel is carried in the wings. Shouldn't the fuel have ignited as soon as the wings broke up? Why can't we see the wings breaking in the crash vids I posted?

The fuel is carried by the wing, but nothing happens instantly in real life. And you can't see the wings breaking as the plane is in the building by the time they have come off. You've got to remember you can't see anything becasue the entire event (impact to explsion) takes place in about half a second. In shaky real time video you can't see squat.

The wings would hve taken a finite amount of time to break up, the fuel is is saftey cells to prevent a fire during a crash event, there would have required time to spread the fuel to an ignition source. Then it takes a finite amount of time for the combustion to begin.

EDIT: Aww I missed 100 posts :(

 

[alibongo]

There seems to be some confusion about the question I asked.
This thread is not about eye-witnesses, or how the plane could have been faked.

Can you post about the crash physics and show me where the plane appears to deform Chris?

Or make a new thread for discussing CGI, eyewitnesses, and how it could have been faked. I am also interested in these issues, but this thread is about the crash shown on the alleged footage I posted.

 

[xxChrisxx]

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.

 

[buffordboy23]

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.

 

[alibongo]

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.

 

[alibongo]

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.

 

[xxChrisxx]

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.