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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.
     
  22. Jan 15, 2008 #21
    No, I think it has everything to do with velocity
    In fact the elevator lift could be in free-fall upwards if you want to be pedantic.
    The velociity of the elevator is then already upward. And all you then have to do is stand on tip-toes (or just sneeze) and you will jump from a free-falling elevator. Or even just step sideways.

    As an off-road motorcycling fan (?) you should know how easy it is for the biker to perform "amazing" mid-air aerobatics by launching himself from a free-falling motorbike - a bike that's just been launched from a ramp and has a small vertical velocity (either slightly upward or downward).
     
  23. Jan 15, 2008 #22

    Shooting Star

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    At least you can simply step off, and free fall next to the elevator. I am obviously considering a thought experiment type open elevator in vacuum. I will do it soon, if the OP does not ask his instructor the meaning of the statement.
     
  24. Jan 15, 2008 #23

    Shooting Star

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    At the risk of boring you with same question, would it be possible for you to ask your instructor the reason for his statement?
     
  25. Jan 16, 2008 #24
    Heard something like that too

    Long ago (perhaps in 1970-ies) I may have heard something like that too - perhaps it was a lorry-driver who had told it to a friend of mine. There also was an explanation to it, but I don't remember what it was. Maybe it was a biological explanation at very high relative speed assumed, where braking wouldn't help much - but could at collision moment strengthen body psychologically against injury.

    But personally I cannot understand anything at any circumstances is better than
    begin braking as early as possible.
     
  26. Sep 23, 2008 #25
    here's why the instructor gave the absolutely great advice:

    the original question does not say that someone stops in front of you. here's a true story of an accident i was involved in: i was in a situation where a driver approaching an intersection from the opposite direction as me failed to yield while making a left turn at a green light. i was doing 40mph in a 45 mph zone and traveling straight through the intersection with a green light. when i realized that the other driver was cutting me off, i had 2 reasonable options: swerve to the right to try and avoid the accident and possibly hit another (innocent) vehicle, or lightly apply brakes and steer straight ahead knowing full well that i would hit the other vehicle. my 3rd option was unreasonable as it was to swerve to the left and definately hit another vehicle and my 4th option was equally unreasonable as it would have been to slam on my (non-ABS) brakes and likely lose control of my vehicle.

    as a former car insurance adjuster here's my input: if speed/conditions make an accident unavoidable without complete knowlege that avoidance will not cause you to hit another car or stationary object, you always want to hit the guilty party and avoid at all costs the fatal (to your wallet) mistake of swerving to avoid the guilty party and hitting an innocent party or crashing your car without touching his. if the guilty party's car is unscathed (without even paint transfer to attest to the accident), he can get away free and clear and leave you with the headaches of triple insurance rates for the next 5 years if you hit another vehicle trying to avoid his.

    there is also the ethical dilemma in a situation like this: would you rather: potentially injure or kill the at-fault driver, OR potentially injure or kill a completely innocent party?

    me, i'll take the 100% chance of hitting the guilty party over even a 1% chance of hitting an innocent party.

    you wait until the last second to apply brakes because you don't inadvertently want to lose control and miss the guilty party and possibly strike an innocent party. the only reason to apply brakes at all is you want skid marks to kill his defense that you had the "last clear chance" to avoid the accident.

    no judge is going to fault you for an accident for failing to swerve. as long as you have proof (skidmarks) that you made any attempt (however small) to avoid the accident, you are free and clear.
    -------------------------------------------
    now on to the physics side of the dilemma: hit the side of a barn going 10 miles per hour and the car will suffer more damage than the barn. hit the side of a barn going 100 mph and the barn will suffer more damage than the car. OR, fire a .500 inch round 100% lead ball from a muzzle loading rifle with a max charge of approx 120 grains fff black powder (a typical deer hunting load). fire this into a .500 inch thick plate of 100% lead. the bullet will pass through the lead plate relatively intact while the plate will have a nice, round hole in it. bullet's momentum/inertia due to it's mass * velocity will cause it to "win" this contest - it has more inertia to keep going than the lead it displaces has to remain static.

    another way to look at it. say my 5000lb truck traveling at a rate of 30 mph hits the back of a fully loaded stationary 80000lb dumptruck. the dumptruck's inertia to stay at rest will cause it to move very slightly with most of the forces of the crash being absorbed by my truck and causing injury to myself and my truck. but what if i rear-end a 2000 lb stationary compact car at the same speed? my inertia to keep moving is a lot more than the car's inertia to stay at rest. the car will absorb most of the damage and my i will likely be able to drive my truck away from the accident.

    --------------------------------------------------------------------

    from a traffic law precidence/automotive accident claims standpoint, the instructor's advice is good and is exactly what i have recommended to friends/family if they find themself in the same situation.

    from a physics standpoint, the vehicle with the most momentum "wins" and will be less likely to be damaged and more likely to transfer the impact (damage) to the vehicle with the lesser momentum.

    it shouldn't take a history/philosophy major to figure this out, folks. put the calculators down and go experience some real, everyday, living. then read some plato. how smart people can be so single-minded totally blows my mind. the speed of one vehicle at the time of impact was all that was being considered on this thread. i can see ya'll missing the legalistic aspect of it, but the moral one? and, as to physics, an intact drinking straw imbedded 3 inches into a telephone pole during a hurricaine will testify that mass*velocity is the key to it all. the object with the most momentum whether by a high mass, a high velocity, or both, usually wins the collision game.
     
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