Acceleration is produced when a force acts on a mass. The greater the mass, the greater the
amount of force needed.
When the firing pin of a firearm strikes the primer cup of a cartridge, combustion occurs and the
rapid expansion of gases seek to escape the chamber. With the gas seeking a vent, the path of least
resistance is the pushing of the bullet down the bore and eventually out of the barrel. This concept
within internal ballistics demonstrates acceleration and is furthered when you take into account
the time and length that the gases act upon the bullet; the longer the barrel and subsequent time
behind the bullet, the faster the bullet will travel. This model is easily observed when comparing
the velocity of a 9mm Luger bullet fired from a semi-automatic pistol, versus the longer barrel of a
In a concept more closely related to Crime Scene Reconstruction, blood stain pattern analysis,
the reader can imagine varying volumes of free falling blood droplets. Droplets of equal volume
falling from differing heights onto a perpendicular surface exhibit an increasingly larger width to
a point. It is at this point where the free falling drop reaches its terminal velocity, and thus from
any height above that distance will impart similar stain sizes, however varying volumes is the key
consideration as well.
Newton’s Third Law
For every action there is an equal and opposite reaction.
Further applying Newton to the BPA aspect, if those blood droplets continue to fall within the same
location, it creates a stain pattern known as a drip pattern. A drip pattern is when blood falls within
blood, creating satellite stains with additional volume. With Newton, these patterns are created as
the accelerating energy of a drop is distributed into the pre-deposited blood causing the preexisting
blood to channel the energy out. This begins to introduce the Third Law of Motion, with each
interaction between matter equal forces act upon each of the interacting bodies, or simply stated for
every action there is an equal and opposite reaction.
Consider a shooter: when he discharges the firearm the force of the projectile must be equal to
the force exerted into the shooter. This is why Hollywood’s depiction of grossly knocking a person
back who is shot is inaccurate, as the same force would happen to the shooter. This seeming
unbalanced ratio of force is the result of the difference in the high velocity of a small projectile
vs. the mass of the shooter, resulting in a subjectively low recoil velocity. This principal is the
observable action of kinetic energy. Because the mass of the projectile is much smaller than that
of the shooter, more kinetic energy is concentrated in the projectile. Upon discharging the firearm, the opposite kinetic energy is displaced throughout the mass of the shooter; this is because
velocity is independent of mass. As the projectile travels in flight this kinetic energy is displaced
in to molecules of the medium in which it travels, until ultimately the remaining kinetic energy
is transferred into another mass.
Through the understanding and application of the Newtonian principals the crime scene reconstructionist can begin establishing the most probable flight path or origin of rest, asking the question of how an item got to the location in which it is and then determining its origin. The scrutiny
bore upon a coffee table knocked over or broken items at the foot of a wall; inquiring what caused
these objects to arrive at their current state of rest. Newton derived these principals from studying
the natural world around him, for those of us within the forensic disciplines crime and violence are
our natural world.
Zack Kowalske is a crime scene investigator and detective with the Roswell-Ga. Police Department, specializing in crime scene reconstruction and blood stain pattern analysis. firstname.lastname@example.org