# Constant acceleration and brakes

1. May 27, 2006

### elizabethR

how would i do this problem...

You are driving to the grocery store at 20 m/s. You are 110 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.

1)How far are you from the intersection when you begin to apply the brakes? I know this answer: 100m.
...i can't figure out the next two. please help me figure out the formula for this!
2)What acceleration will bring you to rest right at the intersection?
3)How long does it take you to stop?

2. May 27, 2006

### al_201314

Since your acceleration is constant, you can use this equation:

v^2 = u^2 + 2as

Where v is your final velocity which is 0, u intial velocity, a is acceleration and s displacement.

For the 3rd part, use v = u + at where t is time taken. The value for a is the one you've found for the 2nd part.

Should give you all the answers. :)

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