Friday, September 29, 2006

Tales from the California Driver's Manual

In preparing to take the laws exam as part of a misguided effort to drive in the State of California, I came across this particular gem on page 16.
Allow older pedestrians more time to cross the street. They are more likely to die as a result of a crash than younger pedestrians.
So, to be clear, if a younger pedestrian takes too long, don't feel bad about crashing into them... they can take it.

Some other qualities items include:
  • Never make a U-turn when other vehicles may hit you.
  • Do not shoot firearms on a highway or at traffic signs.
I am most dubious about this one.
The force of a 60 mph crash isn’t just twice as great as a 30 mph crash, it’s four times as great!
Now, I admit I'm no physicist, but I know that force = mass x acceleration. Now, the mass in this situation is constant... so did it really take four times as much acceleration?! Can anyone fill me in?


Anonymous said...

Well... force is the wrong term to be using, really, since the car isn't accelerating.

What you're really worried about in an accident is car's rate of deceleration and the object's rate of acceleration.

So what you want to figure out, really, is momentum, which is mass * velocity. Still, it's not 4 times as great, even if you're considering a car imparting momentum on a human. I'll think about this and get back to you.

Anonymous said...

So what they're talking about is work required to stop the car. Since work varies with velocity squared (W = 1/2*m*v^2), it takes a huge amount of work to stop a car going 60mph compared to stopping a car going only 30mph.

When hitting a pedestrian, this won't really matter: the car won't appreciably slow, and the pedestrian will be accelerated to the car's speed in short order.