# Stopping distance/time and constant deceleration

## Homework Statement

You are driving to the grocery store at 18 m/s. You are 130 m from an intersection when the traffic light turns red. Assume that your reaction time is 0.50 s and that your car brakes with constant acceleration. What acceleration (magnitude) will bring you to rest right at the intersection? How long (total) does it take you to stop?

## The Attempt at a Solution

I got how far from intersection til brakes are applied... = 119.775 m

rl.bhat
Homework Helper

vi=18 m/s t = 0.5 secs a= 9.8 m/s/s

0= 18 + a(.5)

= 10.225

130-10.225= 119.775m

rl.bhat
Homework Helper

0.5 s is the reaction time, not the total time.
During that time car moves 18*0.5 = 9 m. Then the brakes are applied.
Now the initial velocity is 18 m/s, distance is (130 - 9)m and final velocity is zero. Find acceleration and then time.

I am having a hard time finding acceleration and time...and this is due in ten mins...I keep getting -36m/s/s for acceleration...

hage567
Homework Helper