# Physics - proving the stopping distance of a car

## Homework Statement

an accident occurs up ahead on the highway. a driver travelling at 120km/h [e] reacts in 0.20s and applies the brakes causing an acceleration of 8.0m/s2 [w]. show that the stopping distance is 76 m.
what am i doing wrong? i changed the acceleration to match the directions, so i made it negative. but i am not getting the right answer.

## Homework Equations

d = vi * t + 0.5(a * t^2)

## The Attempt at a Solution

i attempted this by using:

d = (120km/h / 3.6[e])(0.20s) + 0.5(-8.0m/s[e] * 0.20s^2)

d = 6.5m?

SteamKing
Staff Emeritus
Homework Helper

## Homework Statement

an accident occurs up ahead on the highway. a driver travelling at 120km/h [e] reacts in 0.20s and applies the brakes causing an acceleration of 8.0m/s2 [w]. show that the stopping distance is 76 m.
what am i doing wrong? i changed the acceleration to match the directions, so i made it negative. but i am not getting the right answer.

## Homework Equations

d = vi * t + 0.5(a * t^2)

## The Attempt at a Solution

i attempted this by using:

d = (120km/h / 3.6[e])(0.20s) + 0.5(-8.0m/s[e] * 0.20s^2)

d = 6.5m?
The problem is you have assumed that the car goes from 120 kph to 0 kph in 0.2 s, which is not what the problem states. The driver takes 0.2 s to press the brake pedal after he sees the accident ahead of him.

You should pick another SUVAT equation which relates distance, acceleration,and initial and final velocity.

totomyl
The problem is you have assumed that the car goes from 120 kph to 0 kph in 0.2 s, which is not what the problem states. The driver takes 0.2 s to press the brake pedal after he sees the accident ahead of him.

You should pick another SUVAT equation which relates distance, acceleration,and initial and final velocity.
So, I just have another question, in this question the final velocity would be at a stop, so 0 m/s correct? and also would i have to find the distance traveled before pressing on the brakes and add it to the distance it took while slowing down?

SteamKing
Staff Emeritus