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I have a question about what's the "best" thing to do if you're about to be hit by another car.

Imagine you're in your car, and you're at rest. There's another car coming at you, and they're going to hit you (no matter what). S'pose your choices are only to (1) leave your foot on the brake or maybe apply the parking brake, or (2) take your foot off the brake and maybe put your car in neutral. Maybe you're stuck in traffic and are boxed in by other cars. I don't know; it's a hypothetical. Just roll with it ...

Which do you pick to minimize the forces you experience in the impact?

Since that may well depend on the design of the car, assume a spherical cow. (Errr, "car".) Assume it dissipates the same energy in crumpling from any direction. Does your choice depend on the impact angle? (From head-on to t-bone to rear-ender and anything in between.) Does your choice depend on the impact offset? (From glancing to the right to center-of-mass to center-of-mass to glancing to the left.)

What other assumptions come into play? How well your tires grip the road (from perfect slip to glued to the spot)? How rigid or crumply your car is? What other effects might dominate your choice?

This question is similar, but only considers a rear impact with no offset. I'm also interested in what other effects are first order.

Armadillo Jim
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1 Answers1

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If we assume two things:

  1. you are attached securely to the centre of mass of the car;
  2. the car about to hit you reaches rest with respect to your car before it has got through the crumple zone.

Then you want to minimize the acceleration of the centre of mass of your car. And to do this you want to transfer as much momentum as possible to the ground, and you therefore want the brakes on hard and the car in gear.

However in real life you may be worried about the enemy car penetrating the crumple zone (I think it's really game over if it does), and you are not securely attached to your own car but may worry about whiplash etc.

Additionally I have assumed that you don't care about the car: leaving the brakes off might reduce damage to the car in a very low speed collision, while the acceleration you experience would be survivable still. I just don't care about that: that's why you have insurance!