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Question regarding a car crash

  1. Jan 13, 2008 #1
    My driving instructor told me that if you know you are gonna crash into another car, then brake right before contact. Wouldn't it make more sense start to brake as early as possible because it would decrease velocity which would decrese the force of impact?
     
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  3. Jan 13, 2008 #2

    Ben Niehoff

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    I agree with you, it makes sense to brake earlier. The main thing to be careful of is that braking suddenly can cause you to lose control, or potentially cause you to be rear-ended by the guy behind you. So, brake as quickly as you can while maintaining control.

    Another tip: If you know that you are going to crash, and you are going to crash at a lower speed (say, 30 mph or less), then it is best to aim directly for the center of somebody's rear bumper. This way, the bumper can distribute the force evenly, and the cars should bounce off of each other like billiard balls; also, the direction of the bounce should be straight forward, and you avoid knocking their car into another lane.

    I once ran into somebody like this in the rain, because they braked suddenly at a yellow light. I slammed on my brakes, which locked (no ABS on my car), and so I skidded. Unable to stop, I aimed directly for the center of their bumper, and hit them at around 25-30 mph. My car stopped on a dime, and theirs slid forward about 30 feet. No damage to either car (not even scratches!), and no injuries.
     
  4. Jan 14, 2008 #3

    Danger

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    Why were you so close that their stopping took you by surprise? You should always be far enough behind that if that car stops instantly you still have plenty of time to slow down.
     
  5. Jan 14, 2008 #4
    did you even read my post? :P

    anyways, appreciate your replies :D
     
  6. Jan 15, 2008 #5

    Ben Niehoff

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    We were coming up on a long yellow light that everybody drives through. There would have been plenty of time for her, me, and three guys behind me to get through. She seemed to be committed to driving through herself, as she hadn't slowed down at all. Then she slammed on her brakes.
     
  7. Jan 15, 2008 #6
    I agree with Niehoff. We must always very careful but accidents do happen ..any time for any reason. In that case, the bumper to bumper hit is always the safest (or the least dangerous) because all the force is distributed evenly. That's why it is called bumper.
     
  8. Jan 15, 2008 #7

    Shooting Star

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    Is it possible to ask your instructor the reason for his statement? He may not give an impulse eqn, but we may be able to make something out of it.
     
  9. Jan 15, 2008 #8
    Theoretically the earlier you brake, the lower the velocity of the car. But your instructor may mean that, if you brake too early and suddenly, you will lose control and thus can not hit the other car by your car's bumper or even tumble before you crash.
     
  10. Jan 15, 2008 #9

    Shooting Star

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    Instead of theorising about what he may have meant, it is better to ask, if possible.
     
  11. Jan 15, 2008 #10
    I'm a certified OF, so I agree with you. In the several US states I've lived in, the fellow behind ALWAYS owns reponsibility for the accident. And, may in fact get a citation for following too close or failure to control.

    In any event, not braking as quickly as possible will also be seen as contributing to the accident.
     
  12. Jan 15, 2008 #11

    DaveC426913

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    This seems flawed. Imagine a new driver trying to not brake until just the right moment because that's what his instructor advised. He'll waste precious fractions of seconds that could be spent braking. Imagine them misjudging an inevitable crash for an avoidable one.

    The biggest factor in accidents - certainly the one that can be most easily mitigated in an emergency - is speed. Get you speed down.

    Which is worse, a 20km/h hit on the corner of a bumper or a 40km/h hit square on the bumper? I'll take the 20. What are you worried about? The bumper, or your neck?
     
  13. Jan 15, 2008 #12

    DaveC426913

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    Same in Canada. There's no defense for following too closely.

    Note that "following too closely" by definition is "not leaving enough distance, even if the driver in front slams on his brakes". (Forget the yellow light, what if there'd been a child in the crosswalk?)
     
  14. Jan 15, 2008 #13
    I also do not agree completely with the instructor. But there may be a situation that you brake early and suddenly that your car turns and you hit the other in your side, or another car coming to your side etc... In these cases, low velocity can be more dangerous.
    We d better wait to hear the explanations of the instructor or someone who knows.
     
  15. Jan 15, 2008 #14

    Shooting Star

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    None of us agree with the instructor, but the curiosity is beginning to get overwhelming. What did he mean, and why?
     
  16. Jan 15, 2008 #15

    berkeman

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    Agreed. I think you should ask your instructor if he believes that in a free-falling elevator, your best bet for survival is to jump up just as the elevator bottoms out.... :rolleyes:
     
  17. Jan 15, 2008 #16

    Shooting Star

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    Well, actually that would be better than jumping out of the elevator when it was twenty stories high...:biggrin:
     
  18. Jan 15, 2008 #17
    Is it theoretically possible to jump up from a free-falling elevator?
     
  19. Jan 15, 2008 #18
    yes.
    Actually very easy when the elevator has just started to freefall
    ie when the velocity is smallest. obvious really
     
    Last edited: Jan 15, 2008
  20. Jan 15, 2008 #19

    berkeman

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    It would have nothing to do with velocity, so not obvious. It would depend on how well you could grab the handrail and force yourself down into a spring loaded position to spring up. You would go up with respect to the car, and klonk your head on the ceiling, just like you would at zero net g in space.

    Still wouldn't help you survive, though.
     
  21. Jan 15, 2008 #20

    NoTime

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    Just a consideration.
    It can take a lot less time to steer a car 5' to the left or right then it does to stop it.
    Knowing how to do this saved myself, my passengers and the 10 people in the bar taxi that pulled out of the parking lot in front of me a whole world of grief last year as I would have broadsided them at about 30mph using brakes.

    There are professional driving schools that teach you how to do avoidance maneuvers safely.
    Expensive, but worth it in my opinion.
     
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