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Is it better to accelerate into a head on collision?

  1. Oct 8, 2012 #1
    Naturally, one would think, the faster your going the more energy you will create and the harder you will hit. But take a deeper look.

    Under the basis that in a car accident, it isn't the "Crash" that kills you, but the stop (inertia). If you were to be involved in a head on collision with an equally similar car, would it be best to slow down and brake, or accelerate into the car. This plays off the concept of two cars traveling 50 mph would have a combined force of 100 mph. This was dis-proven on mythbusters. So the question now is, if you were to travel faster than the opposing car would the inertia spread out over a longer time provide any additional protection. Would there even be a difference in inertia?

    To start off, If Newton's 3rd law states each reaction has an equal and opposite reaction, and a 50 mph car hits another 50 mph car of identical weight and symmetry, they will both decelerate equally from a 50 mph collision. What if one car was going 60 mph and the other was doing 40? which driver is going to experience the larger force of deceleration? The driver going 60 isn't going to meet an identical force of 60 mph deceleration because the object this car is hitting is not carrying identical momentum. SO my question remains.

    To be clear i am not interested in the energy and damage to the vessel, however i am interested in the effects is has on the person inside. To eliminate a variable, lets assume the vessel does not get damaged at all.
    Last edited: Oct 8, 2012
  2. jcsd
  3. Oct 8, 2012 #2
    Reason two vehicles traveling at 50mph don't add up to 1 vehicle at 100 mph is because energy is related to velocity SQUARED. So 1 vehicle with 100 mph will have more KE than 2 equally sized ones with 50 mph. What stops you in a car crash isn't your brakes it's the energy of the two cars breaking. To break a piece of material or deform it requires energy. Once the energy is completely dissipated you stop moving.

    Hitting the other car faster will only matter if you get enough energy to rip through the other car completely (like if you punch a wall your hand hurts more if the wall doesn't break then if it does). So if you car was big enough to not take damage from the equivalent of 1 50 mph or so crashes (probably a bit more and is a bit more complicated than that) you'd be fine and could just rip through the puny car that does take damage only being slowed down by the energy of the other car ripping apart.

    However, that isn't how it works because your car can't take that much energy. So when you hit both cars are going down because the material / design isn't suited for those conditions. So accelerating will result in more damage to your car because you are adding more energy to the mix.
  4. Oct 8, 2012 #3
    Slow down and brake. Consider a crash in the vacuum of space. Two vessels collide and stick together inelastically. This forces both vessels to change their velocities to the center-of-mass velocity over some timescale. Speeding up will only cause a greater differential between an individual ship's velocity and the center-of-mass velocity.

    If one car is moving 60 mph to the left and the other 40 mph to the right, the center of mass is moving 10 mph to the left. Because both cars have the same mass, both cars experience the same change in velocity. This is true regardless of the cars' velocities for an equal mass collision.

    If one car is moving 60 mph to the left at 4 tons and the other is moving 60 mph right at 2 tons, the center of mass is moving left at 20 mph. The lighter car experiences the change in velocity, and speeding up only increases the change (because it's harder for that car to affect the center of mass velocity).

    Now, if it is only the change in velocity over the timescale of the crash that causes injuries to the occupants, then that's all there is to it, but these are inelastic collisions, and lost energy has to go somewhere--deforming objects or bodies or something.
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