# Answer on Mechanics | Relativity Question for marnie

Question #12914

a driver traveling at 12m/s sees a stalled vehicle 17m ahead of his car. he steps on the brakes to slowdown the car. At what rate must the car slow down so that it stops just before hitting the stalled vehicle? Assume that the drivers reaction time is 0.25s. Reaction time is the time for a person to notice , think, and act in response to a given situation.

Expert's answer

The driver steps the brakes after 0.25s after he sees a stalled vehicle. During that time his car goes 0.25s*12m/s = 3m so the distance between the car and the vehicle becoms 17m-3m = 14m. So, the maximum stopping distance is 14m. To stop the car that moves at 12m/s within 14m the deceleration time must be

t = 14m/((12m/s)/2) = 2.(3)s,

and so deceleration needed is

a = (12m/s) / 2.(3)s ≈ 5.1429 m/s².

t = 14m/((12m/s)/2) = 2.(3)s,

and so deceleration needed is

a = (12m/s) / 2.(3)s ≈ 5.1429 m/s².

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## Comments

Assignment Expert17.07.2014 09:45Dear Victoria,

the distance S = 14 m equals time (unknown) multiplied by average velocity. In our case average velocity is (0 + 12 m/s)/2 = 6 m/s. Thus division by two appears here.

Victoria16.07.2014 08:59why do you have to divide it with two?

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