Speeding problem with Coefficient of Friction

Question

A traffic officer arrested John Doe for speeding in a 50-ml/hr zone. During the trial, John Doe defended himself by claiming that he brakes the car with constant acceleration and eventually come to a stop at 223ft from where he started braking. The skid marks on the road shows that his wheels were locked when he started braking. If the coefficient between the car tires and the road is 0.750, how fast was he traveling at when he slammed his car's brakes? And did the officer make a genuine arrest?

Answer 1

-fmg = ma; so a = - fg = -0.75·9.81 m/s² = - 7.36 m/s²; 0 - v₀² = 2as; v₀ = √- 2as = √2·7.36m/s²·223m =

57.3 m/s = 128.2 mi/h > 50 mi/h

So, officer did right arrest