Accident Reconstruction Primer for Attorneys

 Underlying Science

 Accident Reconstruction (AR) is based on the laws of Newtonian physics.  Although many of us may have a tough time remembering these laws from our high school physics class, we live in a world that is governed by these principles so that even if we don’t remember the laws by name, we actually have an intuitive feel for them.

  Newton’s First Law states that a body in motion remains in motion unless acted upon by some outside force and a body at rest remains at rest unless acted upon by some outside force. 

Mathematically, the First Law states that if the sum of all forces on a body is equal to zero, its velocity is constant. 

In AR this means that if a driver maintains a constant speed while traveling on a level road in a given direction, the vehicle will continue at that speed and direction unless the driver brakes, accelerates, turns, crashes into another object, or the vehicle stops due to fuel consumption or mechanical failure. 

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Newton’s Second Law states that Force equals Mass times Acceleration (F=ma).  In AR, acceleration is a measure of the rate of change of speed of the car. 

If it speeds up, the acceleration is positive, if it slows down (decelerates) the acceleration is negative.  The force required to accelerate the car is equal to the mass of the car times the acceleration. 

This force, which is called the inertial force, is what you feel when your body is pressed into the seat back when you put the pedal to the metal; likewise it is the force that causes your briefcase to fly forward off of the seat and onto the floor when you slam on the brakes. 

When two vehicles collide, each vehicle experiences a sudden change in speed called acceleration (or deceleration). 

This change in speed is known as  Delta-V or ΔV.  The change of speed divided by the time of the collision is equal to the acceleration (ΔV/Δt=a).  And since Force is equal to Mass times Acceleration (F=ma), ΔV is related to the force of the collision by the following relation:

Or solving for the same equation for ΔV, one can write:

ΔV=(F Δt)/m

In auto collisions, the time the vehicles are in contact and transferring momentum (Δt) may vary, but typically it is about 1/10 second. 

Therefore, ΔV is generally a good measure of the acceleration (a) and ultimately the force (F) of the collision. 

Since force is what damages vehicles and injures occupants, ΔV is generally a good way to quantify the severity of a collision.

While this principal may also apply to human behavior, in physics it means that if I lean against the wall with a certain amount of force, the wall pushes.

In an auto collision the impacting car is referred to as the bullet vehicle, whereas the impacted car is called the target vehicle. 

In an auto collision, the force that the bullet car applies to the target car is equally opposed by the target car pushing back on the bullet car.

Application of Principles

Accident Reconstruction is based on the accurate science of Newtonian physics. Unfortunately a scientific answer can be no more precise than the worst input data, and input data is often assumed, guessed, estimated or crudely measured. 

One area that is confusing is units of measure.  Usually in America speed S is expressed in miles per hour (mph) and velocity V in feet per second (fps), but the terms may be used indiscriminately.

In physics V represents a vector, which defines both speed and direction.  However, in AR the term V is often used for speed only with no direction specified.

A vehicle can leave tire marks in at least three different ways: 

 1.      Acceleration Skid (“burn-out”)

2.      Braking Skid

3.      Centrifugal Skid (Yaw Mark)

Burn-outs tend to start out dark and then get lighter as the vehicle gains traction.  Generally, speed cannot be determined from burn-out marks. 

Burn-out skids can be curved or straight, and usually involve only one or two tires (the drive wheels). 

Braking skids on the other hand start out light and get darker as the skid progresses, since the tire is heating up during the skid and laying down progressively more rubber until the vehicle comes to rest. 

Braking skids can involve from one to four tires (on a 4 –wheeled vehicle) and they are generally linear.  Centrifugal skids occur during turning and are therefore curved. 

 The most basic technique for estimating vehicle speed is by measuring the length of braking skid marks.

The pre-skid speed (mph) is estimated by first multiplying: 30 times the distance the vehicle skidded before stopping without impact times the drag factor; and then taking the square root.

 30 is a derived mathematical constant. The distance a vehicle skidded should be straightforward.

It is nearly impossible to know the exact drag factor for a given accident because there are too many variables that determine it.

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