Question

A driver takes 0.2 s to apply brakes. If he is driving at a speed 54km/hr and the brakes cause a deceleration of 6m/s^2 find the dist travelled by the car. I've solved this ques by adding the dist travelled before and after brakes were applied and it is correct. My doubt is, before brakes were applied dist=ut+1/2at^2 so 1.5x2=3 m which is correct. But when I use v^2-u^2=2as for the same then s is coming out 0 not 3 but how. Pls tell me how can it be wrong?

Answer 1

you are taking accelerating as zero in equation that has given you this 3 m result. if there is no acceleration then how can there be a change in velocity .use the second equation whenever there is some acceleration or retardation so that there will be an change in velocity