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## anonymous 4 years ago If a car is speeding down a road at 40 miles/hour (\rm mph), how long is the stopping distance D_40 compared to the stopping distance D_25 if the driver were going at the posted speed limit of 25 \rm mph?

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1. JamesJ

Let us suppose in both cases the braking force applied is the same; call it F. Then by the Work-Energy theorem, the amount of work done, Work = Fd, where d is distance, must be equal to the change in energy, which here is all kinetic energy, KE. Hence Work = Change in KE Fd = (1/2)mv^2 Now find $$d_{25}$$ and $$d_{40}$$ and compare them.

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