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Three car crash physics

  1. Jan 18, 2010 #1
    Please help me answer some of the physics questions related to a 3 car accident....

    I was the middle car in a three-car crash during heavy morning rush hours (stop n go) traffic. The truck in front of me and I came to an almost complete stop when all of a sudden I was hit from behind by a little Subaru and push me forward to hit the truck in front of me. Before the accident happened, I would say the distance between the truck and I was about half a car length.

    Normally, this would be an easy case – the last car (Subaru) would be liable for all the damages. However, during the police report, the truck in front of me said she although she is not 100% sure, she felt 2 almost simultaneous impacts. I know for certain that I didn’t hit the truck in front of me without being hit/pushed by the car from behind. I want to turn to all the smart people to help me solve this physics problem…. Is it possible in a 3-car accident that is caused by the 3rd (last) car, the front car would feel 2 almost simultaneous impacts?

    I tried to get the video camera tape on the highway to prove my innocence but was told those cameras don’t record (only live images). Now I can only rely on physics explanation to help me prove it in court. Any feedback will be highly appreciated. Thank you.
  2. jcsd
  3. Jan 18, 2010 #2
    This is conjecture but it is possible imo she might have thought she felt two impacts that were very close together because she probably experienced two forces very close together. The first force, your car impacting hers, would of course accelerate her forward. Then depending on the angle her vehicle got shoved, she might have felt a really quick halt from the force of friction in the opposite direction. She may have slid a bit and the gripped very tightly very suddenly. This would force would be backwards as would the acceleration.

    Sometimes your body is not this best sensory mechanism for telling you exactly what kind of physics is going on.

    Just a thought. I may be completely off. I am relying on what someone else told me in a very similar situation and how I explained it. This person believed my idea to be correct after it was explained.
  4. Jan 18, 2010 #3
    How on earth can you distinguish between two simultaneous impacts that both happen from the same direction and a single larger impact? There are always two 'jolts' in an impact, the moving and the stoping. There is also the possibility that he shunted you forward and then carried on, but those would be two seperate impact events.

    Nothing we say here is even remotely worth bringing up in court, as it is neither detailed enough nor likely to be remotely accurate for a real crash. The only person that can help you is a professional accident reconstructer.
  5. Jan 18, 2010 #4
    Goes without saying about court. I would not know what the heck a judge would accept. I am just trying to give the OP my take. I would use a lawyer in a court. The lawyer would get the appropriate "accident investigator" if they thought they needed it.

    My only intent was to explain to the OP what the truck driver might have thought she experienced and why.
  6. Jan 18, 2010 #5
    First....2 simultaneous impacts? What does that even mean? Even if we figure out what it means it doesn't make any sense. It's either 1 impact, or 2. Not "2 simultaneous." Why didn't truck driver just go all the way and claim she felt 50 "simultaneous" impacts and get everyone on the whole road involved...

    Second.... what are the chances of getting in an accident? Not very big, but obviously plausible. Now, truck driver is claiming that she got into 2 (independent) accidents! Simultaneously! I'm not even going to try to calculate the probability of this, but truck driver might as well have said "I'm not 100% sure, but I think I also simultaneously won the lottery", and the chances wouldn't be that much different.

    So obviously truck driver was delusional and/ or crazy. But even when you're delusional, most of the time you know when to keep your mouth shut, so that the people around you don't know. She didn't, and she just potentially unnecessarily overcomplicated the situation. For everyone. I just hope judge or whoever will be intelligent enough t to just pay no mind to anything she said.
  7. Jan 18, 2010 #6
    Lsos, it's more than likely that she just didnt really understand the meaning of simultanous.

    She obviously felt 2 jolts, and it must have felt like 2 impact events. However this is more than likely from 1 accident. What she said was probably exactly what she felt, which is all that can be expected. She never implied that there were two seperate accidents, just two seperate jolts that occured very close to each other.

    Clearly the most obvious and likely reason for this is the first (middle) car being shunted forward and impacting, and then the second smaller impact of the 3rd car. It is however not beyond imagination, that the 2nd car drove into the rear end, then then 3rd car did the same.
  8. Jan 18, 2010 #7
    Yeah I guess I got a little carried away :P
  9. Jan 18, 2010 #8
    Look at skid marks for the Subaru, and the debris (dirt, plastic pieces, etc.) on the roadway to determine whether there were two collision points, the first one being 1 1/2 car lengths behind the truck.
    Bob S
  10. Jan 18, 2010 #9
    Wow, I really appreciate all of your prompt feedbacks! I did call a professional accident reconstructor today... $3,500 upfront and min. $220/hour?! I just can’t justify that kind of expense for this relatively smaller scale accident.

    I wanted to thank you all again for helping me walk through different scenarios that could have possibly caused the front driver to “feel” 2 (simultaneous) impacts. Pgardn– I agree it is likely the front driver might have confused what she “felt” with what really happened. xxChrisxx – “Jolt” is a perfect word to describe this and thank you for validating the possibility of my car being shunted forward and impacting, and then the second smaller impact of the 3rd car. But I definitely agree with Lsos – the front driver’s statement unnecessarily complicated the situation and because of that confusing statement, I now have to defend myself to not only the insurance company but also to court! (Sigh… am I not the biggest victim in this accident with both front and back damages?)
  11. Jan 19, 2010 #10
    I don't get why you have to defend yourself in court, the insurance company should pretty much be doing all of this as it's in their best interests to not pay out.

    You just go and tell them what happened. Don't tell the court anything but what you felt from your perspective, for example don't say that the rear car may have been going too fast or anything like that. Just that you were stopped, and you got hit from behind and pushed into the back of the truck, you had the brakes on and there was nothing more you could possibly to do avoid an accident as you were (almost) stationary at the time and you left a reasonable gap of half a car length.

    Also as an interest this is what witnesses are for at the scene, it's also why you should carry at least a disposable camera in the car for when you get into a crash and take photos of everything.
    Last edited: Jan 19, 2010
  12. May 23, 2011 #11
    my educated guess is that the car behind you hit you and slowed, while you lunged forward and hit the car in front of you, thus stopping you, then the car that originally hit you crept up behind you hitting the two of you from what ever momentum was left over.
  13. Jul 19, 2011 #12
    We had an exactly same accident. The car behind us changing lane and rear ended our car pushing our car to hit the car in front of us. The car before us felt 2 impact. The insurance company for the car behind us insisted there are 2 seperate accidents so only agreed to pay 50% of the injury for the one before us. Absolutely there is no other accident except the car behind hit us. We felt same pain as "apcpudding" being a victim. How to prove the truth now become so difficult. Curious how the accident was finally settled.
  14. Jul 19, 2011 #13


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    Was there any debris (broken lights etc.) on the ground from the crash between subaru and you? Assuming that the big truck didn't move much, it could show how far away from the truck he hit you, and that he hit you before you reached the truck.
  15. Jul 19, 2011 #14


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    There are two things you can say about the driver in front.
    1. She has not had many tail end shunts so she is not an ''experienced witness.
    2. She may have Heard one bang and then Felt and heard another bang, as your car hit hers. That could easily have given the impression of two collisions.
    As a witness she could not provide as good evidence as a photograph of the scene immediately after the incident, including the road.

    Also, there is always the possibility that a crumple zone could have extended the time profile of the impact and made it feel like two.
  16. Jul 20, 2011 #15
    Thank you for the replies. The comment about any debris on the ground reminded us a picture we took right after the accidient. It clearly showed the angle our car was hit from the back.
    Again, from the science of physics, the car hitting us from the back was rearing ended us at about 45 degree an angle (she was changing lane from left cutting into our lane on her right and hit our car at right cornor, pushing our car turning forward left, keeping on going and finally stopped almost on the left side of the front car). Could there be 2 impact for the front car?

    We know for certain that we were hit/pushed by the car from behind and that is the only accident. Yes, the front driver’s statement complicated the situation. However, we kind of trust the person before us as he is also the victim in this accident and there is no reason for him to lie how he felt. So is 2 impact reasonable and possible because of 1 accident?

    Thank you!
    Last edited: Jul 20, 2011
  17. Jul 20, 2011 #16


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    As you have introduced the idea that there may have been some rotation, perhaps the double impact sensation was due to an initial blow as a corner came into contact then a second blow as the two flat faces came together.
  18. Sep 21, 2011 #17
    I am having the exact same problem right now. In fact, I wanted to ask similar questions, which is why I joined this forum. Here is what I have come up with on my own.

    Car One - the car in front of me
    Car Two - me, the middle car
    Car Three - the car behind me

    Cars One and Two were sitting at a red light waiting for it to change. Car Two was six to eight feet behind Car One, fully stopped. The light turned green, but Cars One and Two remained stopped as pedestrians were crossing the road and Car One was going to turn right. Car Three slammed into Car Two, causing Car Two to propel forward into the back of Car One.

    I have pictures of all the damage to all the cars and showing where each car stopped after colliding. The front of Car Two is smooshed up against the back of Car One, with extensive damage to the front of the vehicle. Car Three is six to eight feet BEHIND Car Two, with minimal damage to the front of the vehicle.

    Car Three's driver spoke to Car One's driver and they figured that, based on the damage to Car Two, Car Two "must have hit Car One first."

    Pause here so I can calm down from how absolutely LIVID this level of "logic" makes me.

    The problem I have with their version of the story is that the physics and the physical evidence do NOT support their claims.

    1. IF Car Two hit Car One and THEN Car Three hit Car Two, why is Car Three so far BEHIND Car Two? (No, Car Three did NOT back up.) Wouldn't Car Three's front bumper be smooshed up against Car Two's back bumper the same way Cars One and Two ended up?

    2. IF, as Car Three's insurance company claims, the only way all that damage could have happened to Car Two was if Car Two hit Car One first, then WHY is there NOT similar damage to Car Three's vehicle? Wouldn't Car Three also have extensive damage to its front due to colliding with Car Two (which supposedly had already collided with Car One) and suddenly stopping?

    Here is where I need help/someone to explain the math to me. I KNOW that Newton's Second Law of Motion somehow supports what I intuitively comprehend here. Had Car Three slammed into two stopped vehicles, the mass of those two vehicles would have caused a faster deceleration rate to Car Three, which would have resulted in far greater damages than Car Three sustained.

    This link: shows what I am saying. At 6:04 (approximately) he throws two eggs at differing stopping forces with differing stopping times. The solid wall breaks the egg because of the shorter stopping time. The sheet leaves the egg intact because of the longer stopping time. Same principle applies here. Car Three had less damage because Car Two did not stop Car Three immediately. Thus, there is no way Car Two was already crashed into Car One.

    So, how do I show this with Physics/Math?

    Thanks! :)
    Last edited by a moderator: Sep 25, 2014
  19. Sep 22, 2011 #18


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    What you are discussing is the quantity called Impulse (=force X time the force is applied). This tells you how much the Momentum of an object has been changed by a combination of force and time values - small force for long time or big force for short time. (It's really only an alternative statement of Newton 2).

    I see where you're coming from. For the Physics to be applied meaningfully, you need to assume that the three cars are of similar masses and that they have similar construction.
    All things being equal, you would expect that, after impact, the two rear cars would have half the velocity of the rear car (sharing the momentum). So the next collision would be at a lower velocity - probably implying that you'd expect less damage. This is the most common situation, I think.
    However, if your brake was not on hard and the impact was low speed, you may have been accelerated during rear impact, which would have spread the impact over time. The front car may have had his brake applied harder (possibly because he heard a crash behind?) so the front impact took less time to complete. I don't think the highway code says anything about how hard you need to be applying your brake when stationary - except it must keep you from rolling.

    Either way, the standard way of treating this sort of shunt is to load all the blame on the last car to arrive on the scene, isn't it? I think that the other drivers would need some harder evidence (film, for instance) in order to convince anyone of their scenario.

    The other alternative could be that the car that rear-shunted you was actually driving (accelerating) you both forward once it had hit you so that the front impact was at higher speed than the rear impact.

    In any case, my commiserations.
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