A motorist suddenly notices a stalled car and slams on the brakes, decelerating at the rate of 6.3 meters/second^2. Unfortunately this isn't good enough, and a collision ensues. From the damage sustained, police estimate that the car was moving at 13 kilometers/hour at the time of the collision. They also measured skid marks 33 meters long. How was was the motorist going when the brakes were first applied? How much time elapsed from the inital brraking to the collision?
Vf^2=Vi^2 + 2ad
Vf=Vi + at
The Attempt at a Solution
Alright so to answer the first question, I used the first equation above.
0=Vi^2 + 2(-6.3m/s^2)(33m)
Vi= 20.4 m/s
To answer the 2nd question, I just used Vf= Vi + at
0=20.4m/s + 2(-6.3m/s^2)(t)
This all seems right, but the fact that the car is moving at 13 km/hour is bothering me. Is it useless data? or is it meant to be used in such a way? Please help.