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Car accident

  1. Jun 23, 2008 #1
    ok so i have a question i'm really hoping someone can help me with. i got in a car accident recently and i want to prove that the person who hit me was speeding but i'm not sure how to do it. i am actually an engineer so i really should be able to do this on my own but i'm a civil engineer and so i haven't taken a physics course for a long time or applied any physics for a long time.

    anyway, here's the deal. i was pretty much perpendicular to the impact so i got hit in the side of my car right above my back tire. so i was going to use the front tires as the pivot point. i did almost a complete 360 (let's say i did about a 300 degree rotation) and the speed limit was 35 mph on that road. judging from the impact, the damage to my car, the fact that i almost did a 360 and seeing how fast he was going, i am positive he was speeding. but can i prove it mathematically?

    also i gathered some data such as car weights and wheelbase lengths, etc. so i have that info. my car outweighed his by almost 1,000 lbs. again, further proof that he was speeding i think.
  2. jcsd
  3. Jun 23, 2008 #2
    Conservation of angular momentum/energy? Or, thats what you'd do if it was an ideal situation.
    Its pretty complicated, b/c there are a lot of areas where the calculations are hard to do "realistically" (opposed to ideally)
  4. Jun 23, 2008 #3
    ugh yeah that's what i was afraid of. i was hoping maybe there was something fairly simple that i could use but maybe there isn't...
  5. Jun 23, 2008 #4
    Well, its impossible to do any serious calculation with real crashes, mostly because parts of cars deformate to "use" as much kinetic energy as possible... if you somehow know how much friction there was during your rotation, you might be able to estimate his speed, but that would be very crude estimation, nothing better than just looking at the accident and guessing his speed. Also, I would guess your car was moving at the time of collision, thats another complication.
    Result - do not dream about getting any useful figures :) Just try to convince policemen/judge/... that rotation for 300 degrees required significant impact, more than 35mph would do.
  6. Jun 23, 2008 #5
    ahh that's a good point too. didn't really think of that. well damn, i guess that's probably that then. i have pretty much the worst luck ever so if it comes down to me trying to convince a judge of something it probably isn't going to happen. thanks for you responses anyway guys.
  7. Jun 23, 2008 #6


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    Welcome to PF!

    Hi packthe9! Welcome to PF! :smile:

    Even 35mph should do it.

    The only way you can really work out the speed is from tyre marks on the road.

    Were there any (if he emergency-braked, there should be)?

    If there were, the police report should have recorded them … they should give you a copy.

    How fast were you going? Would he still have hit the front of your car if he'd been going at the speed limit?

    Since you were hit from the side, I assume you were at a cross-roads.

    If the other car had give-way lines, then its speed doesn't matter.

    If you had give-way lines, then the other car would have to be well over the speed limit for you not to be at least partly at fault.
  8. Jun 23, 2008 #7
    well unfortunately you couldn't see tire marks because it had been raining. which didn't contribute to the accident at all but the tire marks didn't show up. it's kind of complicated to explain what happened but it definitely seems like without tire marks or witnesses or anything i can't really do much. even though he almost ripped the back half of my car off and totaled it. but he wasn't speeding...
  9. Jun 23, 2008 #8


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    Although a Canuck, I don't understand all Brit terminology... but I interpret Tiny Tim's question of 'give-way lines' to mean who had the right-of-way. Regardless of whether he or you had it, the other guy's speed is irrelevant. If you had it, then all that his alleged speeding would do is compound his penalty. Insurance-wise, you still should get the same amount, which is a replacement vehicle if yours is totalled.
  10. Jun 23, 2008 #9


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    Hi Danger! :smile:

    In the UK, give-way lines are dotted double white lines (like stop lines, but dotted instead of whole) across the entrance to a junction, and most cross-roads are marked with them across the "minor" road.

    They mean that you have to give way, but you don't have to stop.

    I intended a distinction between the simple case of cross-roads where the right of way is dictated by the road, as opposed to the complicated case where the right of way is dictated by the circumstances (who gets there first, for example). :smile:
    Not quite … even if you have right of way, you still have to be careful, especially if you know that people tend to break the speed limit at that cross-roads.

    Your damages could be reduced, or even extinguished, if you're not careful enough.

    Unfortunately, you are not entitled to assume that everyone obeys the speed limit.
  11. Jun 23, 2008 #10


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    Agreed from the practical standpoint, Tim, but the laws on this side of the pond are different. For one thing, we don't have 'stop lines'; we have either 'stop', 'yield', or 'merge' signs. The only time that order of arrival matters is for 3- or 4-way stops. In either case, all combatants must come to a full stop, and the first has the right-of-way.
  12. Jun 23, 2008 #11
    Depending on the jurisdiction, a hit anywhere on the rear half of your car can automatically be the other driver's fault since he could have presumably avoided you. You might check with your insurance company.
    Last edited: Jun 23, 2008
  13. Jun 23, 2008 #12


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    True. From an insurance standpoint, anyone who hits someone else from behind is considered the offender. This also means that if someone rear-ends you and pushes you into another vehicle, you are considered at fault for the secondary collision. Stupid.
  14. Jun 24, 2008 #13


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    Perhaps the data you seek to calculate is already verified practically. Try looking up the safety testing from when your vehicle was being developed. They would have had to do crash-tests on all quarters of the automobile from many different directions and at many different speeds. Look for the hits on the rear quarterpanel at 90o, and see at what speed the vehicle does a 360.

    I've never done such a search before, but it maight be possible.
  15. Jun 24, 2008 #14
    id say one way to calculate it is to check what the energy is at 35 mph and figure out, based on a normal roads kinetic friction, how much your car should spin around your cars center of gravity... if you end up finding that you rotate less than 300 degrees you are golden because there is no way that all the energy was directly transfered from his car to yours due to deformation of yours and his frame at impact. therefore he had to be going faster than 35 mph!
  16. Jun 28, 2008 #15


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    How serious is this situation? Someone posted something similar a few months ago, and the consensus was that a professional accident reconstructionist had to be hired. If there's a fair bit of court action to be gone through, that's the only proper approach. None of us here are able to give absolute answers based upon the given information, although I'm sure that some could do it if all pertinent data are provided, including photos.
  17. Jun 30, 2008 #16


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    I agree with Danger. If you need this information for a court of law, get a professional crash site ivnestigator. Even if we could give you answers with a high degree of certainty, it wouldn't help you in court, because our word carries no weight, there.
  18. Jun 30, 2008 #17


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    This would have allowed your car to skid much easier. Trying to determine energy consumed by deformation is virtually useless. If there were no friction, then momentum would be conserved, even though it's an inelastic collision. However, the tires do have friction with the ground, but the friction varies with the load, so how much traction there is in a high "jerk" (sudden acceleration) situation also makes looking at post colliison skid marks not too useful, especially in the rain.

    Crash site investigators normally specify a range of potential speeds, instead a single estimated speed, and even so, it's more "art" than science. Generally what the investigators look at is who had the last chance to avoid the accident, and which, if any laws were violated.
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