Physics Help

You're driving at 39km/h , when the traffic light 25m away turns yellow.

I need to find the constant acceleration required to stop at the light, the stopping time, and weather the acceleration is reasonable.

I know how to find the average and instantaneous acceleration, but how to obtain the constant, I have no clue.

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asked by Eiban
  1. 39,000 m/3600 s = 10.83 m/s

    average speed during braking = 10.83/2 = 5.42

    25 = 5.42 t
    t = 25/5.42 = 4.62 seconds to stop

    v = Vi + a t
    0 = 10.83 + a (4.62)
    a = -2.34 m/s^2

    1/4 g is not an unreasonable deacceleration

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    posted by Damon
  2. 39 km/hr = 10.83 m/s

    s = Vi*t + 1/2 at^2
    25 = 10.83t + .5 at^2

    v = Vi + at
    10.83 + at = 0
    so,

    t = 2.35s
    a = -4.46 m/s^2

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    posted by Steve
  3. Go with Damon - I switched the a and the t.

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    posted by Steve

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