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Crash Crash Question

  1. Jun 24, 2009 #1
    Hello everyone, my name is Jordan and I love Physics... Well my main question is, when two cars get into an accident, lets say one vehicle is traveling at a velocity of 40 miles per hour, the other vehicle at the same 40 miles per hour. When they get into a head on collision, you add the two velocities together right? So in fact the vehicles were subjected to a crash at the force of 80 mph?

    Is this correct?
  2. jcsd
  3. Jun 24, 2009 #2


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    Basically yes.
    In detail in a real crash it might make a difference - so accident investigators could probably tell if both cars where moving or not.
  4. Jun 24, 2009 #3


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    Quite right. One of the major components of the problem will be the relative weights and structural integrities of the vehicles. At the speeds that you mentioned, a Hummer would come out fairly unscathed in a crash with a Smart car. Might need a new bumper, and a repacked airbag, but the frame would probably be fine.
  5. Jun 24, 2009 #4
    What is the definition of a crash? Two cars that are the same mass and going equal opposite velocities that collide head on will be similar to one of the cars colliding with an immovable wall. Or one of the cars colliding at twice the velocity with the another car at rest.
  6. Jun 24, 2009 #5
    The above answers may not get to the more accurate underlying principle that it's relative , momentum that provides a better insight....mv =Mv where M is much bigger than m, "M" is the Hummer..

    In other words, if the two vehicles are identical, the velocities tell just about the whole story; but if one vehicle is twice as heavy as the other, it's momentum will be twice the other vehicle at any identical velocity......at the crash, the twice a heavy Hummer will still be moving in its original direction while the smaller vehicle will be forced backwards.....
  7. Jun 24, 2009 #6
    So this is what I am basing this on:

    (Please watch video before reading the rest)

    Ok so we figure, The family was in a 100kph zone which is like 62mph.

    The biotch that hit them was speeding so lets say he was doing 75mph. The neise family was probably being all slow and retarded and going like 60 mph. That means that the Mercedes endured a crash at the speed of 134mph if you add the forces together, and taking a look at the damage it was an offset crash, which is the standard today.

    Having said that, it did a damn good job holding up, especially the cabin which didn't collapse thank god and all the other crap that went on safety feature wise. And lets say Mr. Neise was like "O_O" and slammed on those brakes and went down to around 45 mph, the crash was still at a high force at 120mph on impact, again adding the forces together.

    The thing with the government testing and testing done around the world is that it's set to crash into a wall that has no speed. So yes, the car does in fact get in a crash at 45mph. But in real life where someone is coming at you with their own speed and then you have your speed, the crash is in fact the speeds of both cars put together. That's why that commercial by Mercedes was so awesome because it shows that it was was able to withstand the two velocities and keep the passengers alive, and in good shape.

    Having a debate with someone about this, lol What do you guys think?
    Last edited by a moderator: Sep 25, 2014
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