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Help reduce the severity of accidents

  1. Jul 20, 2011 #1
    To help reduce the severity of accidents, an engineering company designs large plastic barrels filled with antifreeze that can be placed in front of bridge supports. in a simple test, a 1200 kg car moving at 20 m/s (W) crashes into several barrels. the car slows down to 8.0 m/s (W) in 0.40 s.

    i found the average net force to be -36000 N (W)

    b: what would happen if the car hit the bridge supprts instead of the barrels.

    c: why do they use antifreeze instead of water.

    i'm confused but i think that for both b and c, it's about the car's ability to bounce back?
  2. jcsd
  3. Jul 20, 2011 #2
    Re: collisions

    If the author of the problem has a tacit assumption that the bridge supports cannot move at all and that the car would not have any ricocheting velocity (which would seem to need to be specified, since this can easily be assumed not to be an elastic collision), then all the kinetic energy of the car must get converted into heat, making the wreckage and the portion of the bridge struck a much hotter mess.
    I would suspect that a given volume of antifreeze has the capacity to absorb more heat energy than the same amount of water. The more energy is absorbed by the liquid in the barrels, the lower the temperature of the car wreckage(?)
  4. Jul 20, 2011 #3
    Re: collisions

    does that mean the net force would increase?
  5. Jul 20, 2011 #4
    Re: collisions

    I was thinking the water would freeze when it gets cold outside and cause a lot more damage.
  6. Jul 20, 2011 #5
    Re: collisions

    I'd say yes, it takes more force to push something to a stop than it does to push something down to 8.0 m/s if you get to assume that the time duration of 0.40 s is the same in both cases.
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