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Q & A: car braking distance vs. speed

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Most recent answer: 09/27/2013
when the speed of a car doubles, why is the breaking distance more than double?
- valerie lau (age 15)

The maximum friction force between the tires and the road doesn't depend much on speed. So using Newton's second law, F=ma, where m is the car's mass, gives about a constant acceleration a=F/m.  That's the rate at which the car is losing speed. If the initial speed is v, it takes time v/a to come to a stop. In that time, the average speed will just be v/2. So the distance travelled will be v2/2a. The distance goes about as the square of the initial speed.

Mike W.

(published on 09/27/2013)

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