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22 June, 06:43

A police officer finds 60 m of skid marks at the scene of a car crash. Assuming a uniform deceleration of 7.5 m/s to a stop, what was the velocity of the car when it started skidding?

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  1. 22 June, 09:18
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    The equation to be used is for the rectilinear motion at constant acceleration:

    x = v₀t + 0.5at²

    a = (v-v₀) / t

    where

    x is distance

    v and v₀ is the final and initial velocity

    t is time

    a is acceleration

    Because the acceleration is decelerating, that would be reported as - 7.5 m/s². Substituting,

    -7.5 = (0 - v₀) / t

    v₀ = 7.5 t - - > eqn 1

    x = v₀t + 0.5at²

    60 = (7.5t) (t) + 0.5 (-7.5) (t²)

    Solving for t,

    t = 4s

    Thus,

    v₀ = 7.5 m/s² * 4s

    v₀ = 30 m/s
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