**You are driving to the grocery store at 20 m/s. You are 150 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.**

**Required:**

**a. How far are you from the intersection when you begin to apply the brakes?**

**b. What acceleration will bring you to rest right at the intersection?**

**c. How long does it take you to stop?**

**Answer:**

a. Use the formula d = st to solve:

d = 20 × 0.5 = 10m

150 – 10 = 140m away when brakes are applied

b. Use the following kinematic equation to solve:

vf² = vi² + 2ad

Plug in known values:

0 = 20² + 2(150)(a)

Solve:

0 = 400 + 300a

-300a = 400

a = -4/3 (≈ -1.33) m/s² required

c. Use the following kinematic equation to solve:

vf = vi + at

0 = 20 – 4/3t

Solve:

4/3t = 20

Multiply both sides by 3/4 for ease of solving:

t = 15 sec