# Motion, Finding stopping distance

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?

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

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.

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
Staff Emeritus
Homework Helper
Gold Member
What is that 0 in the equation. Is it initial distance and final velocity?
d0 is initial position.

v0 is initial velocity.

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

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.

Ok thanks, i will try it out