(Original post by oo00oo)
It wasn't true though... the part about 'speed' itself not being the culprit directly is true, but the part about 'sudden stopping' being the culprit was at least only part of the truth... sudden stopping or sudden starting is what hurts - acceleration or deceleration.
The quip is misleading because it intends to tell us that going fast in your car isn't the problem, it's slowing down in your car too quickly that's the problem. From the point of view of the driver having a crash into a wall, then yes, that's the case... flying at 100mph, you're safe in your car. Stopping very suddenly from 100mph, and you're flying through the windscreen.
But from the point of view of one car being smashed into by another with a much higher speed, or from the point of view of a pedestrian being hit by a car, then the extent of the accident is always going to be directly related to the speed of the vehicle.
The sudden acceleration of a pedestrian is always dictated by the original speed of the car, so in an indirect way, speed is the culprit.
Whether you're hit from the front or the back doesn't make a massive difference where car accidents are concerned.
Force and acceleration are directly proportional, so to say that one is the cause rather than the other is really a moot point... Force is mass times acceleration, by Newton's 2nd law, and the mass of the pedestrian is, for the purposes of a car accident, not variable. So it's the acceleration and the acceleration alone which dictates the extent of their injury and the likelihood or their demise. And the acceleration is, of course, dictated by the velocity of the car that impacts them.
Oh, and a pedestrian will only accelerate to a very high speed if the car was travelling at a very high speed - the car is only accelerating them whilst it is in contact with them, and by the time the pedestrian is going the same speed as the car they are no longer being accelerated.
What ARE you talking about?
The pedestrian's velocity changes from almost 0 to a very high speed in a very short period - hence, they experience a very high acceleration, which is what is responsible for their injury and/or death.
The period of time that they're in contact with the car is always very small, but the smaller the velocity of the car, the smaller the acceleration of the pedestrian, and the less their chances of injury and/or death are.
Don't make me bring out the equations.