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?

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