For the sake of safety, the necessary distance should be kept between the vehicles driving on the highway. It is known that the maximum speed limit of a certain expressway is v = 120km / h. suppose that the vehicle in front stops suddenly due to fault, and the driver in the rear stops suddenly after discovering this situation, The time (reaction time) t = 0.50s. When braking, the resistance F of the car is 0.40 times of the gravity of the car. What is the safe distance between cars on the highway? (take the acceleration of gravity g = 10m / S ^ 2, the calculation result is in M unit and kept to an integer

For the sake of safety, the necessary distance should be kept between the vehicles driving on the highway. It is known that the maximum speed limit of a certain expressway is v = 120km / h. suppose that the vehicle in front stops suddenly due to fault, and the driver in the rear stops suddenly after discovering this situation, The time (reaction time) t = 0.50s. When braking, the resistance F of the car is 0.40 times of the gravity of the car. What is the safe distance between cars on the highway? (take the acceleration of gravity g = 10m / S ^ 2, the calculation result is in M unit and kept to an integer

V = 120km / h = 33.3m/st = 0.5s vehicle displacement S1 = VT = 50 / 3 = 16.67m,
The solution of V ^ 2 = 2as2 = V1 ^ 2 / 2a is S2 = 138.89m
S=S1+S2=156m

The speed of the car on the highway is 72km / h, suddenly found that there is an accident 60m ahead, immediately emergency brake, if the car stops after 4 seconds with constant acceleration, is there any safety problem
Process, thank you~

v=20m/s
a=v/t=-5m/s^2
From 2As = V ^ 2-V ^ 2
V = 10 root sign 2 > 0
There's a security problem