Homework Help: Physics

Posted by Dave on Saturday, January 28, 2012 at 3:31pm.

You are driving to the grocery store at 12.7 m/s. You are 108.0 m from an intersection when the traffic light turns red. Assume that your reaction time is 0.470 s and that your car brakes with constant acceleration. How far are you from the intersection when you begin to apply the brakes?

Physics - Damon, Saturday, January 28, 2012 at 3:55pm
Well, I suppose I am to assume that you stopped in 108 meters.

first you went 12.7*.47 = 5.97 m before your foot hit the brake.

So the brakes must stop you in (108 -6) = 102 meters

That is the answer you asked for.
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The rest of the problem:
102 = 12.7 t - (1/2) a t^2
and
0 = 12.7 - a t
t = 12.7/a
so
102 = 12.7^2/a - (1/2) (12.7^2)/a
204 a = 12.7^2
a = .8 m/s^2 about a tenth of g

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