A motorist suddenly notices a stalled car and slams on the brakes, negatively ac

Question

A motorist suddenly notices a stalled car and slams on the brakes, negatively accelerating at 6.4 . Unfortunately this isn't enough, and a collision ensues. From the damage sustained, police estimate that the car was going 18 at the time of the collision. They also measure skid marks 31 long.How fast was the motorist going when the brakes were first applied?How much time elapsed from the initial braking to the collision?

Explanation / Answer

a) The initial speed of the car was : v² = v0² + 2a?x Solve for initial velocity, v0: v0² = v² - 2a?x = (5.0m/s)² - 2(-6.4m/s²)(31m)--->20km/h = 5.0m/s = 20.54 m/s (b) Now, the time from braking to collision is : v = v0 + at t = (v - v0) / a = (5m/s - 20.54m/s) / -6.4m/s²) = 2.43s

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