an accident investigator measured the skid marks of one of the vehicles involved in an accident.

Question

an accident investigator measured the skid marks of one of the vehicles involved in an accident. The length of the skid marks was 175 feet. What was the speed of the vehicle before the breaks were applied?

Answer 1

There is not enough information to answer the question.  If we knew the rate of the constant acceleration "a" caused by application of the brakes (which in this problem is a negative number), we could get the initial speed from the kinematic relation

 

v² - v₀² = 2ad

 

v = final speed = 0 (came to a full stop)

v₀ = initial speed (what you want to solve for)

d = stopping distance = 175 feet

 

Then

 

v₀ = √(-2ad)  [Remember a is negative, so the argument of the square root is positive.]

 

But as you can see, we need the acceleration.

 

If we knew the time it took to come to a full stop, we could still answer the question.  Either way, we need more information than just the stopping distance.