Acceleration velocity question?

You are driving to the grocery store at 20m/s. You are 110m from an intersection when the traffic light turns red. Assume that your reaction time is 0.50s and that your car brakes with constant acceleration.



What acceleration will bring you to rest right at the intersection?



How long does it take you to stop after the light changes?





I really dont know what equation to use...Acceleration velocity question?Use v^2 = u^2 - 2as, where v is the final velocity, zero in this case, u the initial velocity, a the acceleration, and s the distance.



Using distance travelled = speed x time, then



d = 20 X 0.5,



d = 10 m (this is the distance travelled during the reaction time, so the remaining distance, s, is 100m), so



v^2 = u^2 -2as, so



0 = (20)^2 - 2 x a x 100, so



0 = 400 - 200a, so



200a = 400, so



a = 2, m/sec/sec.



However since the vehicle is slowing down, it`s a negative acceleration, so the answer is -2 m/sec/sec.



Hope this helps, Twiggy.