Question

To a police investigator, skid marks evidence is extremely important in determining how vehicles moved in a collision. Skid marks are the equivalent of fingerprints in a criminal investigation, and just as important. The speed a car was traveling can be calculated using the skid marks made as the car came to a stop. The formula for finding the speed of the vehicle is:

S equals square root of 30 times D times f times n end root

Where:

S = speed in miles per hour
30 = a constant value used in the equation
D = length of the skid marks in feet
f = drag factor for the road surface
n = braking efficiency as a percent

A car skids to a stop, leaving four skid marks with an average length (D) of 60 feet. The road is asphalt. Skid tests reveal a drag factor(f) of 0.75. Since all four wheels were braking, the braking efficiency (n) is 100% or 1.00. To the nearest mile per hour, what was the car’s speed at the time of the accident.


40


34


37


30

Answer 1

It is given to us that the formula to determine the speed of the car is:

$S=\sqrt{30\times D\times f\times n}$

where: S = speed in miles per hour

30 = a constant value used in the equation

D = length of the skid marks in feet

f = drag factor for the road surface

n = braking efficiency as a percent

It is also given to us that average length of skid is 60 feet so D=60.

Drag Factor, f=0.75

And Brake efficiency, n=100%=1

Thus, using the given data in the Formula for calculating the speed we get:

$S=\sqrt{30\times 60\times 0.75\times 1} =\sqrt{1350}=\sqrt{36.74} \approx37$ miles per hour.

Thus, from the given set of options, the third option which has 37 is the correct option.