# Homework Help: Problem from Mastering Physics

1. Oct 19, 2009

### Raimuna

I really don't know how to find the acceleration at the intersection.
You are driving to the grocery store at 19 m/s. You are 140 m from an intersection when the traffic light turns red. Assume that your reaction time is 0.77 s and that your car brakes with constant acceleration.
I have found that d1=130m
I tried to find the acceleration and I got 25, but it's wrong.
Could someone help me with this problem?

2. Oct 19, 2009

### xcvxcvvc

you say d1 = 130m Is d1 supposed to be the total distance during the deacceleration(i.e. the initial distance subtracted by how much your car travels during your reaction time)?

if so, how does 140 - 19 * .77 = 130?

As for solving a, use the "big 5"

1/2 a t^2 = d

v=a*t +v(initial)

the above equation can be used to solve a & t. two equations, two unknowns. notice, in the second equation v is zero when your car is completely stopped, the point in time we're interested in.