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A Moment of Science

Gimme a Brake

As you are driving along, some part of your mind is probably constantly estimating how much space you would need to brake if the car in front of you suddenly stopped. All too often, however, drivers don’t realize the difference between stopping time and stopping distance. Let’s figure it out.

First, let’s assume the road is a uniform surface. Even though pumping your brakes is a better idea, let’s also assume the stopping car is skidding to a halt, so its speed decreases uniformly. Now suppose, a car going 30 takes four seconds to skid to a halt. How long will it take at 60?

Did you say eight seconds? Correct. As you would expect, twice the speed equals twice the stopping time. Now here’s the catch. Say that same car going 30 required a 100 feet of skidding distance before stopping. How much distance does it need at 60?

Did you say 200 hundred feet? It’s sensible to guess, that twice the speed equals twice the stopping distance as well, but it’s wrong.

Stopping distance is the average speed of the car times the average stopping time. Since both these quantities have doubled, the stopping distance is four times as great as before, not just twice as great. The car moving at sixty requires four hundred feet of skidding room.

Realizing that stopping distance goes up twice as fast as stopping time might remind you to leave more space in front of your car than you think you need. It may also save you from a close encounter of the most unpleasant kind.

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