Angelo is driving at 15.0 m/s when he saw a stalled car 75 m ahead. At what rate must he slow down his car so that it will stop 0.5 m before hitting the stalled car? His reaction time is 1.2 s.

Answer 1

in 1.2 seconds, the car will travel 18m, so the stopping distance is only
75-18-0.5 = 56.5m
So now we need to find a and t such that
15+at = 0
15t + 1/2 at^2 = 56.5
15(15/a) - 1/2 a (15/a)^2 = 56.5
a = -2 m/s^2