Real-Life Math
An automotive forensic investigator pieces together the puzzle when
parts are scattered about. It is sometimes possible to estimate precisely
how fast a car was going at impact from the skid marks and road conditions.
Most
of this math work is done on a computer. Still, some basic principles of physics
definitely come in handy in accident investigations.
You have been
called to defend a plaintiff in a recent case where the plaintiff was hit
by a truck. You look at the evidence before you:
The accident happened
on a rainy day, according to both the plaintiff and defendant. At least there
is agreement there, you think. Of course, you could always check the sheriff's
weather report for that area if there was any doubt.
The truck driver,
the defendant, estimates he was traveling at about 30 miles per hour when
he saw the plaintiff on his side of the road. The truck driver says the plaintiff
was about 200 feet in the distance. He braked immediately and came to a stop
about when the impact happened, at about 200 feet after he began braking.
Unfortunately,
this was too late. You can tell the distance from the skid marks (200 feet)
on the asphalt road. However, you doubt from looking at this evidence that
the truck driver was traveling at 30 miles per hour. You can use a basic formula
to estimate the speed the truck was traveling. If it indicates the truck was
going faster than what the truck driver said, you can do some more in-depth
calculations to confirm your results.
Here is the minimum speed formula.
X represents the minimum speed.
D = distance
F = drag
factor
The drag factor (also called the coefficient
of friction) is a number that represents how the surface of the road interacts
with the truck's tires. If the road is wet, it's a different factor
than when the road is dry. The type of surface also changes the factor. The
drag factor for asphalt is different than for gravel.
You have a range
of tested values that you can choose from to put into your formula. If your
calculations indicate a higher speed, then you'll go out to the accident
site and take some measurements so you can confirm your findings. You'll
have to pick the correct factor for your formula.
Range of Values
for Drag Factor
Dry Asphalt = 0.5
Dry Gravel = 0.8
Wet
Asphalt = 0.6
Wet Gravel = 0.7