Motion, Finding stopping distance

[TheRedDevil18]

I just want to confirm if what I am doing is right, please correct me because my textbook gives me a different answer.

Homework Statement

The mass of a minibus with the driver is 1.5*10^3 kg. The driver is moving at a speed of 30m.s when he notices that the traffic lights ahead have turned red. He applies a force of 9*10^3 N on the brake pedal.

A) How far must he have been from the traffic lights if he were able to stop in time?

Homework Equations

a=Fnet/m
s=d/t

The Attempt at a Solution

a=9*10^3/1.5*10^3
= 6 m.s^-2

using the equation a=v/t i got a time of 5 seconds

so
d = s*t
= 30*5
= 150 m but my textbook says it is 75 m

 

[doppelganger]

The equation you are using for distance only works for a constant speed.

Try other equations like:

d=d0 + v0t +1/2at^2

that are suited to constant acceleration.

 

[TheRedDevil18]

The equation you are using for distance only works for a constant speed.

Try other equations like:

d=d0 + v0t +1/2at^2

that are suited to constant acceleration.

What is that 0 in the equation. Is it initial distance and final velocity?

 

[SammyS]

What is that 0 in the equation. Is it initial distance and final velocity?

d₀ is initial position.

v₀ is initial velocity.

 

[TheRedDevil18]

His initial velocity would be 30m.s but what is his initial position?

 

[doppelganger]

If you find out how much distance the driver covers as he stops, you will know the minimum distance that he must be from the light to stop in time.

 

[TheRedDevil18]

Ok thanks, i will try it out