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Crash physics question

  1. Mar 23, 2009 #1
    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?
  2. jcsd
  3. Mar 23, 2009 #2


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    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.
  4. Mar 23, 2009 #3
    Hi alibongo, welcome to PF!

    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.

    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.
    Last edited: Mar 24, 2009
  5. Mar 24, 2009 #4
    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.

    Last edited by a moderator: Sep 25, 2014
  6. Mar 24, 2009 #5
    The first one seems slightly more likely, the fuel wouldnt explode as soon as the plane touched the building. What you dont 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. Infact 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.
  7. Mar 24, 2009 #6


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    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.
  8. Mar 24, 2009 #7
    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,
    Last edited by a moderator: Sep 25, 2014
  9. Mar 24, 2009 #8
    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 thats what caused the fire.

    Imagine a razor hole (edit: slit) in a peice 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.
    Last edited by a moderator: Sep 25, 2014
  10. Mar 24, 2009 #9
    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...
  11. Mar 24, 2009 #10
    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 dont 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 cant see in it the vids is because they arent shooting fast enough and are too far away to see whats going on.
  12. Mar 24, 2009 #11
    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.
  13. Mar 24, 2009 #12


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    To take it to extremes, think of the 'straw in a hurricane' scenario. A regular stalk of hay or grass, travelling at a couple of hundred mph, can penetrate the bark of a tree. The aforementioned bullet is an unfortunate example as well.
  14. Mar 24, 2009 #13
    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!
  15. Mar 24, 2009 #14


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    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. :surprised
  16. Mar 24, 2009 #15
    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
  17. Mar 24, 2009 #16
    Why wouldnt 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.
  18. Mar 24, 2009 #17
    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...
  19. Mar 24, 2009 #18

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

    In the above explanation I did not assume this :rofl:

    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 :rofl:

    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. :tongue2:

    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.
  20. Mar 24, 2009 #19


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    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.
  21. Mar 24, 2009 #20
    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.
    Last edited: Mar 24, 2009
  22. Mar 24, 2009 #21
    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.
  23. Mar 24, 2009 #22
    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...
  24. Mar 24, 2009 #23


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    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.
  25. Mar 24, 2009 #24
    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 dont understand, whats all this talk of CGI?
    Last edited: Mar 24, 2009
  26. Mar 25, 2009 #25


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