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Q & A: front and back seat in car crashes

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Most recent answer: 09/19/2013
I was talking with a friend about the physics of car crashes and we came to a disagreement. I was making the argument that the back seat is more safe for kids because they will feel less of a force from the collision (because of the distance from the front of the car and the time it takes for that force to reach the back of the car). He was saying that the front and back seat experience the same momentum so it doesn't matter where you are. I also know that there are crumple zones in cars to help mitigate the amount of force on people. My question is, is conservation of momentum more important that the forces acting on a body in a car? Which causes damage to the person?
- Christina (age 24)
Corvallis, OR, USA
A person does have the same momentum regardless of whether they're riding in the front or back seat. However, momentum isn't something you "experience". What you experience is force, the rate of change of momentum, or impact, the net change of momentum. Since after the crash the momentum (as viewed in an earth frame) falls to zero, giving the same impact, what we're concerned with here is just the force. To make it small, we need a way of stretching that impact out over as much time as possible.

The crumple zones you mention help. When the front of the car stops abruptly in a collision, the passenger compartment can move forward for a bit as the front crumples. For a passenger in the back seat, the front seat serves as an extra crumple zone, offering some more protection. Air bags are often used to try to get a similar effect for the front seat.

Mike W.

(published on 03/29/2013)

Follow-Up #1: Use the brake pedal !

This question will seem cruel, but I am curious. I drive a heavy vehicle, approximately 4200 lbs. If a smaller car was to end up in my lane, about to strike me head on,given the superior mass of my vehicle, would it be in my best interest to slam on the brake pedal or slam the gas pedal to the floor. The object being to reduce the force applied to me by the impact. Thanks
- Bob (age 59)

The impact force on both you and the smaller car is proportional to the relative velocity between the two.  Anything you can do to reduce this relative velocity will help both you and the smaller car. As you suspect, the smaller vehicle will bear the brunt of the impact but braking will help you too.



(published on 09/19/2013)

Follow-up on this answer.