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Trying to prove which car hit which with physics

  1. Jan 16, 2008 #1
    Say car A is stationary, and in front, car B is also stationary.

    Car B then, at a slow speed, begins to reverse, and hits car A, causing damage to the front-end of car A.
    There is no damage to the rear bumper of car B.

    Using momentun transfer, energy transfer?, or what have you, is there any way of proving that car B reversed into car A, while disproving car A rear-ending car B?

    Thanks for you thoughts.
  2. jcsd
  3. Jan 16, 2008 #2


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    Not directly, in practice the police uses extra techniques:
    The filament in lights which are smashed when they are on fails in a different way to lights that are off, this is used to tell for instance if someone was braking or not, so you could tell if the reversing lights were on.
    It might also be possible to tell from damage to the gearbox which gear it was in.
  4. Jan 16, 2008 #3


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    Welcome to PF, Sporticus. Even in a low-speed collision, there is also some possibility that the innocent vehicle is moved enough to leave tire marks. Of course, either the brake or the parking gear would have to be in effect for that to occur.
  5. Jan 16, 2008 #4


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    As well as the location of the cast off debris.
  6. Jan 18, 2008 #5
    About ten years ago, I went into New York early in the morning. As I turned onto Lexington, somewhere in the 40s, a taxi went flying past me, going the wrong way. But, the cute part was he was driving backwards, probably about 40 mph, so that he was headed the right way - just his velocity was wrong. I always wondered how that would have looked if he had hit me.
  7. Jan 19, 2008 #6


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    For insurance purposes up here, it is always the guy behind, regardless of circumstances. If I'm stopped at a red light behind you, and some guy slams into my car forcing it into yours, my insurance has to cover the damage to you, while the guilty guy's covers my damage and his own. It's considered a no-fault collision, though, so my rates couldn't be raised because of it. (Civil liability is a different matter; you could sue the other guy for injuries, but I'm safe.)
  8. Jan 20, 2008 #7
    If both cars had synchronised atomic clocks on board before the accident, then the car which moved backwards would show a minutely smaller elapsed time on his atomic clock than the car that didn't move.

    Time on car B < Time on car A
  9. Jan 30, 2008 #8
    Just to dispel a notion that's been swirling in my head,

    If Car A hits B, and B is at rest, and only car B sustains damage while A has not even a dent, is there not a way to tell if B was indeed hit?

    Becasue if B were to hit A at rest, would it not transfer some force to the body at rest (A), and since a car is a large mass, and wont move easily, instead, the force is tranformed into denting the car surface.

    Is that too oversimplified? I have an inkling there may be something to do with the strength of the composite matierials on each car, but if only one vehicle sustained damage, coundn't it be justified that it was b/c that vehcile was at rest, and had force/momentum transfered to it, that instead of moving the car, casued a structural damage?
  10. Jan 30, 2008 #9


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    In principle, it doesn't matter if one is at rest. The only factor is their relative velocity. A going 10, B going 0 is the same as A going 30 and B going 20. Think of it this way: the bumpers can't tell if the wheels are spinning or not.

    However, in practice a car at rest has different mechanisms keeping it that at a speed for 0 than those keeping it at a speed of 20.
    Last edited: Jan 30, 2008
  11. Jan 30, 2008 #10


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    If your car has any kind of data recording equipment on it, it might be possible to tell based on any sort of specific data. For example, correlating the speed you were traveling at when/if your airbag went off (if it did). Alternatively perhaps if the damage caused some sort of check engine light (or other warning light that could have been activated), it may have recorded when this CEL came on, and what was going on with the car at the time.
  12. Jan 31, 2008 #11
    Ah, yes, but they also should have had synchronized their clocks exactly before the accident! :smile:
  13. Jan 31, 2008 #12
    You could always check the phone records to see which driver was on his cell.
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