Stopping distance/time and constant deceleration

[gbedenba]

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?

Homework Equations

The Attempt at a Solution

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

 

[rl.bhat]

How did you get the answer? Show your calculations.

 

[gbedenba]

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]

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.

 

[gbedenba]

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]

Show your calculations. We have no idea how you are getting your answers if you don't show your work!