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Dumb car crash question. I think this is a conservation of momentum problem.

  • In one scenario, two 4000 lb cars (car A and car B) each traveling at exactly 35 MPH in opposite directions have a perfect head on collision with each other.

  • In the other scenario, one 4000 lb car (Car A) traveling at 35 MPH hits a steel reinforced concrete bridge abutment (i.e. a huge mass with zero speed.)

Which Car A sustains more 'damage'?

My gut feel is that both experience exactly the same crash damage, but I'm at a loss to describe why.

(Question inspired by a temporary stretch of freeway near my home, with just a painted double line between opposing directions of traffic.. It's quite scary...)

Qmechanic
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zipzit
  • 141

2 Answers2

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Your gut feel is correct. Both are exactly the same. Look at the acceleration in both scenarios. 35 mph to 0 in the time it takes for the cars to fold up and stop. Everybody gets this wrong. Good question.

Alex Tworkowski
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Remember that it is not the speed that kills you, it’s the sudden stop at the end of the road ;)

Translated into momentum conservation, this means that in your first scenario the two vehicles are approaching each other with a speed of 70 MPH whereas in the second the approach speed is only 35 MPH.

The details of the collisions are of course very different but considering approach speed alone, you would be worse of hitting something that is moving towards you than hitting something that is standing still.

Mikael Fremling
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