A ranger in a national park is driving at 15.0 m/s when a deer jumps into the road 60 m ahead of the vehicle. After a reaction time, t, the ranger applies the brakes to produce an acceleration of a = -3.00 m/s2. What is the maximum reaction time allowed if she is to avoid hitting the deer?


  1. Answer:

    t = 5 s


    In uniform rectilinear movement, the equation for final speed is:

    vf = v₀ + a*t

    In this case we need that the car stops just before 60 m after applied the brakes, then

    vf = v₀ – a*t

    vf = final speed = 0

    v₀ = initial speed =  15 m/s

    And negative acceleration is 3 m/s²

    0 = 15 (m/s) – 3 ( m/s²)*t

    t = 15 / 3   (m/s /m/s²)

    t = 5 s

    The point is that with that value ranger will hit the deer so in order to not to hit the deer that time should be smaller than 5 seconds

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