Solve Retardation Problem: 25m from Traffic Light

[takando12]

Homework Statement

A driver applies brakes on seeing a traffic signal 400 m ahead. At the time of applying brakes the vehicle was moving with 15m/s and retarding with 0.3m/s². The distance of the vehicle after 1 min from the traffic light is?
A) 25m B) 375m C) 360m D) 40m

Homework Equations

The Attempt at a Solution

Using v=u+at I found the time that the car takes to come v=0 is 50 sec. And using s=ut+1/2at² I found the distance it traveled in 50 sec is 375m . Hence distance from the traffic light is 25m.
The same problem , this time using s=ut+1/2at² and t=60 secs (this is what i did first) , i got s= 360m and distance from traffic light =40m. Why exactly is this not working? Is the reason that the equation gives me this answer is that the equation assumes that the car is going to stop at t=50 sec and then continue speeding up in the opposite direction? Please help.

 

[Nathanael]

Is the reason that the equation gives me this answer is that the equation assumes that the car is going to stop at t=50 sec and then continue speeding up in the opposite direction? Please help.

Yes, exactly.

You are expected to model the situation of a person driving up to a stop light. (People don't reverse at stop lights).

Your model was: acceleration = -0.3 m/s/s

But the better model is: For time 0<t<T acceleration = -0.3 m/s/s; for time t>T acceleration = 0 (where T is the time to come to a stop).

 

[takando12]

Yes, exactly.

You are expected to model the situation of a person driving up to a stop light. (People don't reverse at stop lights).

Your model was: acceleration = -0.3 m/s/s

But the better model is: For time 0<t<T acceleration = -0.3 m/s/s; for time t>T acceleration = 0 (where T is the time to come to a stop).

thank you sir,