I found this problem in the text "Elementary Physics Classical and Modern" by Weidner and Sells. Two automobiles are both moving at 90 km/h in the same direction, one directly behind the other. The driver of the lead car suddenly applies his brakes, decelerating at 7.5 m/s^2. The other driver applies his own brakes after a delay of 0.40 s, and slows down at a rate of 6.0 m/s^2. (His tires are worn.) If there is to be no collision, what is the minimum separation between the cars at the instant the lead car's brakes are applied? The answer given in the back of the book is "greater than 30 5/6 m or about 30.9 m." I'm wondering if this anwer is correct. Let the lead car be car A: vo=25m/s , v=0 , a=-7.5m/s^2 using the formula v^2=vo^2+2a(x-xo) , car A stops after 41.7m, car B after 52.8m+10m(for the delay). I can get a close anwer by using a different approach. Why doen't the first method give the correct answer???