A car traveling at a speed of 30.0 m/s encounters an emergency and comes to a complete stop. How much time will it take for the car to stop if it decelerates at -4.0 m/s^2?
-7.5?
v = Vi + a t
so 0 = 30 - 4 t
t = 7.5 seconds to stop
d = Vi t + (1/2) a t^2
d = 30 * 7.5 - 2 (7.5)^2