Energy physics question

Question

A driver carelessly ignores the posted speed limit of 40.0 km/h in a school zone and continues at 65 km/h. Assuming a good reaction time of 0.80 s, using the concept of energy how many more meters will it take him to stop than if he had reduced his speed? Assume a constant emergency braking acceleration of –7.8 m/s².

Answer 1

0 - mv²/2 = mas (change of Kinetic energy equel work of break force;

s = - v²/(2a) = - (65·1000/3600)²/(- 15.6) = 20.9 m