Motion, Finding stopping distance

  • #1
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
 

Answers and Replies

  • #2
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.
 
  • #3
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?
 
  • #4
SammyS
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What is that 0 in the equation. Is it initial distance and final velocity?
d0 is initial position.

v0 is initial velocity.
 
  • #5
His initial velocity would be 30m.s but what is his initial position?
 
  • #6
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.
 
  • #7
Ok thanks, i will try it out
 

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