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To stop a car, first you require a certain reaction time to begin braking. Then the car slows down at a constant rate. Suppose that the total distance moved by your car during these two phases is 56.7 m when its initial speed is 85.0 km/h, and 24.4 m when the initial speed is 52.8 km/h.
(a) What is your reaction time?

(b) What is the magnitude of the deceleration?

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Lance

  1. 85.0 km/h = 23.61 m/s
    52.8 km/h = 14.67 m/s
    Total distance = reaction time distance + braking distance
    = V*t + V^2/(2a)
    Write two equations in two unknowns and solve for the two unknowns, a and t.
    56.7 = 23.61 t - (23.61)^2/(2a)
    24.4 = 14.67 t - (14.67)^2/(2a)

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    drwls

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