Question

You are arguing over a cell phone while trailing an unmarked police car by 25.0 m; both your car and the police car are traveling at 120 km/h. Your argument diverts your attention from the police car for 2.0 s (long enough for you to look at the phone and yell,“I won't do that!”). At the beginning of that 2.0 s, the police officer begins braking suddenly at 6.00 m/s2. (a) What is the separation between the two cars when your attention finally returns? Suppose that you take another 0.400 s to realize your danger and begin braking. (b) If you too brake at 6.00 m/s2, what is your speed (in km/h) when you hit the police car?

Answer 1

Vo = 120km/h = 120000m/3600s=33.33 m/s.

d1 = Vo*t + 0.5a*t^2
d1 = 33.33*2 + (-3)*2^2 = 54.66 m Traveled by policeman in 2 s.

d2 = Vo * t = 33.33 * 2 = 66.66 m/s. =
Distance traveled by me in 2 s.

d3 = Do - (d2-d1)
d3=25 - (66.66-54.66)=13 m = Separation
between me and the policeman.

b. d1 = Vo * t = 33.33 * 0.40 = 13.33 m
Oops! I hit the cop.

Speed = 120 km/h Because I hit the car before I started breaking.