You are called as an expert witness in the trial of a traffic violation. The fac

Question

You are called as an expert witness in the trial of a traffic violation. The facts are these: A driver slammed on his brakes and came to a stop with constant acceleration. Measurements of his tires and the skid marks on the pavement indicate that he locked his car's wheels, the car traveled 186 ft before stopping, and the coefficient of kinetic friction between the road and his tires was 0.762. The charge is that he was speeding in a 45-mi/h zone. He pleads innocent.
What is your conclusion, guilty or innocent?

How fast was he going when he hit his brakes?
mi/h

Explanation / Answer

Let the mass be M and initial velocity be u

Then frictional force = ?mg

retardation due to friction = ?mg / m = ?g = 0.762*9.8 =7.4676 m/s2

1m/s^2 = 3.28ft/s^2

7.4676 m/s^2 = 7.4676 * .28 = = 24.49 ft/s2

Final velocity (v)= 0

distance traveled = 186 ft

Using the equation,

v2-u2=2aS

0 - u2=2*(-24.49)*186

u= 95.44 ft/sec

1ft/s = 0.6818 mi/hr

95.44 ft/s = 95.44 * 0.6818 =65.0.79 miles/hr

So the person is guilty .

He was moving at a speed of 65.079 miles/hour .

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