A Revelation

If our (braking) acceleration is 20 fps, then the distance traveled in coming to a complete stop will be ½ multiplied by 20 multiplied by 4.4 squared, or about two hundred feet.

We learned that with a braking acceleration of 20 fps and an initial speed of 88 fps, we travel 193.6 feet in coming to a complete stop.

Satisfy yourself that this is true by plugging the given values into the simple formula s= ½ at2

0.5 times 20 times 4.4 times 4.4

Now assume that your speed is doubled to 176 fps.

If the braking acceleration remains at 20 fps, it will take not 4.4, but 8.8 seconds to reduce our initial velocity to zero.

The distance traveled will be:

0.5 times 20 times 8.8 times 8.8

774 feet, give or take.

Twice the speed, FOUR TIMES THE DISTANCE.

I put that in capitals because that is the revelation.

Formally, the distance traveled in coming to a complete stop is proportional to the SQUARE of the velocity.

Triple the speed, NINE TIMES THE DISTANCE.

Ouch!

And, of course:

Half the speed, ONE QUARTER OF THE DISTANCE.

We will see later on that speed doesn’t kill, but it is idiot drivers traveling at speed who do not realize the consequences of that speed who kill.

If you get my drift.

Safe Stopping Distance

Search