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  1. #11
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    Re: What happens when two cars crash...

    Quote Originally posted by Gordo View Post
    Huh?
    You guys don't seem to get it. Modern vehicles are designed to be destroyed as that absorbs the energy as the vehicle deforms. This is good as it means the occupant is decellerated over a longer distance/time which means less stress on the persons body (lower "G").
    Easier way to visualise it is to consider the seat belt and the points where the seat belt is anchored. If in an impact the seat belt and the vehicle structure were very rigid, the anchorages would stop immediately, the seat belt would immediately stop the body in a manner similar to striking a wall at the speed one had been travelling.
    Now consider the vehicle still being rigid but the seat belt being of a material to stretch and absorb energy. The anchorage points would still stop immediately but the seat belt would stretch, giving time and distance for the body to slow down.
    When the vehicle deforms due to crumple zones, the anchorage points also move through a distance as the vehicle slows, effectively extending the distance/time for the body to slow down.

    Two vehicles crashing head long at 40 mph IS NOT like one crashing at 80mph - it is the same as one crashing at 40mph. It is very unlikely one would survive a direct crash into a barrier at 80 mph - unless one had a specially prepared car and restraint system - it's why injuries are so rare in motorsport.

    Go ahead and read my post again, I get the whole thing on how cars are being designed to protect a safety cage from deforming and thus keeping the people from sustaining damage and what not and I understand how they have some areas that do crumple to absorb energy, I didn't need any of that explained. What I was asking was if two of the same five star cars crashing into each other would be as safe as one five star car crashing into a much larger car but an older one that crumples more. Again I'm not up with my physics but a combined speed of 80 miles per hour is the same as a 40 mile per hour crash into a stationary object? Are you sure? I would still think the forces would be greater when you're hitting another object with a large mass and large amount of energy moving towards you and designed to be very strong and not deform much.

  2. #12
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    Re: What happens when two cars crash...

    Guys, theres only one car I'd want to be in, in a crash. And its this one:
    http://www.youtube.com/watch?v=amFJw...elated&search=



    *edit* Scratch that, i found another chinese car thats even safer!
    http://www.youtube.com/watch?v=f7rrk3ZjN-I
    Last edited by Cyph3r; September 12, 2007 at 08:04.

  3. #13
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    Re: What happens when two cars crash...

    Quote Originally posted by acarney View Post
    Go ahead and read my post again, I get the whole thing on how cars are being designed to protect a safety cage from deforming and thus keeping the people from sustaining damage and what not and I understand how they have some areas that do crumple to absorb energy, I didn't need any of that explained. What I was asking was if two of the same five star cars crashing into each other would be as safe as one five star car crashing into a much larger car but an older one that crumples more. Again I'm not up with my physics but a combined speed of 80 miles per hour is the same as a 40 mile per hour crash into a stationary object? Are you sure? I would still think the forces would be greater when you're hitting another object with a large mass and large amount of energy moving towards you and designed to be very strong and not deform much.
    Wasn't really aimed at you, Acarney, just in general*.
    Combined speed would only apply for identical cars both at same speed. As the energy is to the square of the speed, any difference between them would increase the sum energy to be dissipated and conservation of mass would result in a combined velocity one direction or the other.
    If you have two different vehicle masses and/or structures and/or speeds it would, IIRC my basic physics, it would be a case of calculating the initial and final velocities of the vehicles, the energy dissipated in the collision and an analysis of the deformation of the vehicle and elongation of the belts to work out the cange of velocity of the occupant over time. It could be done but you'd need additional information.
    I suspect it would be primarily down to the relative masses of the vehicles. For example, if I was driving a truck and trailer of a mass of 40 tonnes and I collided with a car of mass 1 tonne, travelling at the same speed, head on. I would lose a little over 2.5% of my speed but the car would have a change of nearly 197.5%. Regardless of how effective the restraint systems, the driver of the car would have to endure around 3000 times as much energy transfer. An extreme example, and one which doesn't take into account the likelyhood of the car being crushed, anyway.
    If the vehicles were of equal mass, one solid and one with a crumple zone, with identical seatbelts, both drivers would have almost identical stress levels but, as the centre of mass would be slightly more towards the crumpled car, it's driver would have slightly greater stress levels. Both drivers would have greater stress than if it was two crumple zone cars as there would be half the vehicles' energy dissipating distance.

    * I actually got some Government safety adverts pulled down here because they couldn't understand the basic physics.
    Last edited by Gordo; September 12, 2007 at 09:29.

  4. #14
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    Re: What happens when two cars crash...

    Quote Originally posted by acarney View Post
    Go ahead and read my post again, I get the whole thing on how cars are being designed to protect a safety cage from deforming and thus keeping the people from sustaining damage and what not and I understand how they have some areas that do crumple to absorb energy, I didn't need any of that explained. What I was asking was if two of the same five star cars crashing into each other would be as safe as one five star car crashing into a much larger car but an older one that crumples more..
    I see what you are asking... Basically the occupants in the two modern smaller cars will have a good chance of surviving.

    Obviously there is less room if the smaller cars collide, with less crumple zone, however there is also less weight being pushed through the cars, so the forces involved are lessened somewhat.

    I would say that ALL occupants involved in a "small car to small car" crash are safer than the ALL occupants in an "old large car/small car" crash.

    The more modern the car though, generally the better your chances.

  5. #15
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    Re: What happens when two cars crash...

    just 2 visions I have in my head -

    Granted the fact that 2 cars traveling @ 60 mph in a head-on collision - IS the same as 1 car hitting a concrete barrier @ 120 mph - physics.

    Modern car (golf, peugeot) = occupants inside of a golfball - packed inside of a snowball.
    ie - cars built on frames vs. chasies
    Chasis-built cars - a baseball of the same size, if the vehicles have the same mass - where the occpuants are inside of the rubber bands -

    now what would you expect when these 2 objects hit each other @ the same speed.

    - I was rear-ended in toyota 4X4 by a 206 peugeot when I was taking a left-had turn @ a standstill. the peugeot hit me hard enough that my seat broke backwards - however she had no front end left, no engine @ all - not to a surprise - her windshield didn't shatter - and met the top of the firewall down to the ground - nothing left in front. I can's say for certain that she didn't feel more of an impact than I - I can tell you that to this day - for about a week a month - I get excruating back pain from when my seat broke backwards. - MANY variables invloved in this - height of cars - bumpers/fenders - weight. There isn't any solid rule. But many modern cars are also built in production costs in mind. frames and plastic fenders are cheaper than a chasis and the extra steel. But a lot of the technology is auctually brought over from racing - if it saves lives - Im all for it.

  6. #16
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    Re: What happens when two cars crash...

    Quote Originally posted by Gordo View Post
    Two vehicles crashing head long at 40 mph IS NOT like one crashing at 80mph - it is the same as one crashing at 40mph. It is very unlikely one would survive a direct crash into a barrier at 80 mph - unless one had a specially prepared car and restraint system - it's why injuries are so rare in motorsport.
    1/2*m*V^2

    ex:
    1/2*m*(17.8816m/s)^2 = 159.87581J / kg (at 40mph, converted to m/s)
    1/2*m*(35.7632m/s)^2 = 639.50325J / kg (at 80mph, converted to m/s)

    which comes out to a 4x increase in energy for going twice as fast, assuming same mass in both sides.

    I suppose there could be a difference in crashing into an immovable object as opposed to another car which will likely have a different weight and possibly not be directed exactly down the direction of motion, translating to some sideways movement as well. This same concession would have to be made for the immovable objects too, though, as you may not collide with it directly head on.

    But, yes, a heavier car will retain more of its velocity in a collision with a lighter car, thus sustaining fewer G forces of deceleration. I suppose that could make up the difference or more than a more sophisticated crumple zone on a much smaller car, but the difference in weight might have to be pretty substantial depending on how much energy the crumple zone on the newer car can absorb.




    I think I was getting myself too confused on further investigation, so I'll leave what I already typed above and post this link to a decent explanation:

    http://www.newton.dep.anl.gov/askasc...0/phy00719.htm

  7. #17
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    Re: What happens when two cars crash...

    I'm really not sure if you guys are really getting what I'm talking about still. This is a total theoretical, identical cars of identical weight traveling at identical speeds hitting head on. Both these cars are the same, so both have been built to deflect as much energy as possible away from the passenger. Both do have some crumple zones since they are the same car however, that crumple zone can't absorb much energy because of the small size of the car, thus the energy must go somewhere else, possibly throwing the car more violently in the air or off the road?

    What I'm saying is it's like taking two brand new bricks that each have a little styrofoam on the fronts of them and then slamming them against each other, yes there is a little padding to deform and absorb some energy but the rest of the brick is built so damn strong as to not use any of the energy up in deforming. This is compared to a brand new brick with a little styrofoam being slammed against a bigger old brick with no styrofoam but that's become weak and crumbly. The old brick breaks and crumbles away, the new one has it's styrofoam deformed, both action absorb energy, I suspect the old brick actually absorbs some energy that WOULD have been transfered or originated from the new one while it crumbles away.

    See what I'm saying? If you have a force of 100 units and the old car ends up absorbing 70 units since two feet of it crumple away then the new car only has to take 30 units in it's tiny crumple zone and maybe only passes 20 units onto it's strong safety frame.

    However, if neither car gives way A LOT (like the old car will) then we can assume that each car must take 50 units. Again that crumple zone takes 10 of the units but that leaves 40 units being passed onto that safety zone OR being passed on in the form of movement (I.E. throwing the car off the road or upside down or something)

    I know that's all based on just what "seems to make sense" and really has no real numbers following any laws of physics, but just watching the crashes it looks like one car always take an F load more energy then the other, the old one that just smashes all to hell, leaving me to think that if that car wasn't the only taking the worst of the force, where would that force go and what result might it have?

  8. #18
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    Re: What happens when two cars crash...

    Quote Originally posted by IrishGI View Post
    just 2 visions I have in my head -

    Granted the fact that 2 cars traveling @ 60 mph in a head-on collision - IS the same as 1 car hitting a concrete barrier @ 120 mph - physics.
    No, no, no, no!

    Concrete barrier, no absorbtion of energy.
    One car at 60 mph has to get rid of its energy within it's own crumple zone etc.
    Two cars collide head on, each travelling at 60mph, each has to get rid of its energy in its own crumple zone.
    Remember, just as there are two cars worth of energy, so there are two cars to get rid of that energy.
    As has been said above, one car at 120mph into a barrier has 4 times as much energy to get rid of, therefore collision is much more severe.

    It's not unusual for a modern car to have its front bodywork destroyed and the windscreen still intact as the safety cage is strong and the bonded in windscreens are a strengthening member of the structure. You were lucky her car did absorb so much energy or you may have been quite a bit more seriously injured.

    Anyway, bedtime down here.

  9. #19
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    Re: What happens when two cars crash...

    im an appraiser in a body shop i see smashed cars all day everyday, if you wanna be safe.

    screw gas mileage and get a truck or big SUV, they take hits like no car ever could and are even still drivable,

    now some smaller cars do take a good hit and its true you are not likly to get seriously injured in a sub 50mph collision,

    now you get in a head on with a truck your in bad shape, and forget getting in an accident with a tractor trailer,

    ive seen a tahoe side swiped by a tractor trailer at 55mph + and the B pillar barly hit the seat and the people walked away, now if that was a car =( im sorry but your not walking out

  10. #20
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    Re: What happens when two cars crash...

    Quote Originally posted by Gordo View Post
    No, no, no, no!

    .
    So, according to newton's third law, it must be that 1 2006 1.9L Volkswagne golf hits head on a similar stationary car (ok not concrete, which would make the collision equally elastic) and it produces X ammount of kenetic energy. then - say 2 1.9L 2006 Volkswagen golfs at X velocity hit each other produces 2X of kenetic energy. - It's "physics" as the ammount of kenetic energy that needs be absorbed - not the impact each mass experiences.

    How the kenetic energy is absorbed to protect the inside passengers doesn't have an effect on newtons third law. It just what parts of the cars absorbe the impact - it doesn't according to newtons basic principles lessen the impact - just displaces it to different parts of the car. The total momentium during a collision remains constant throughout the collision - and is exactly doubled when another mass of identical charachteristics at the same velocity in exactly the opposite direction collides with it - this is the total energy that needs be absorbed - each car still experiences X ammount of force - but it becomes 2X as there are 2 masses. how the energy disipates within a system does not effect the momentium an isolated system carried before the collision began, or the ammount of kenetic energy the car needs absorbe on impact - just the way its absorbed. That means when you add all the momentum from each object before and after the collision, they will equal each other as long as another force does not affect the system - and in the case of a collision - that would require a thrid car. And if car A had a mass of 2x - car b would experience a kenetic force of 2x.

    Here is a thread basically identical to this one to explain what I mean (you'd think mechanics would know better ...)

    Newton's third law of motion is naturally applied to collisions between two objects. In a collision between two objects, both objects experience forces which are equal in magnitude and opposite in direction.
    (but as there are 2 masses - the total kenitic energy is doubled - but equal on the 2 masses)

    I assumed you knew what I meant by the snowball hitting a baseball anaology - a snowball is not identical to a baseball - such as a peugeot 206 is not equal to a shelby mustang gt500. - Otherwise - take it up with Newton.
    Last edited by IrishGI; September 12, 2007 at 14:44.

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