FORENSIC PHYSICS

ACCIDENT RECONSTRUCTION:
• Physics is the science that deals with natural
phenomena such as motion, force, work, energy,
momentum, light, sound, electricity, and
magnetism.
• A forensic physicist can use the evidence left
behind at an accident scene to determine what
happened and who was at fault. To do this the
scientist must understand kinetics (the study of
motion) and especially Newton's laws of motion
and how these quantities can be used to tell
what happened in a collision.

Let's start with some basic terms used in
physics and what they mean.
– Force: A push or a pull.
– Weight: The pull of the earth on an object.
A person who weighs 150 Ib has the earth
pulling on them with a force of 150 Ib.
Weight is a force.,
– weight = mass x acceleration of gravity.
– Mass: A measure of the amount of an
object that is present.

Friction: A special type of force that
causes an object to slow down.
– There are two types of friction, static and
kinetic.
– Static friction is the force that must be
overcome to start an object moving. The
force required to start a parked car moving
while the brakes are still on is static
friction.
– Kinetic friction is the force that slows
down a moving object and the force that
causes the skid marks left at an accident
scene.

– The coefficient of friction (u) is determined by
dividing the force it takes to move the object by
the weight of the object, friction = force/weight.
– Velocity: The speed and direction an object is
traveling.
– Velocity = distance/time.
– A positive or negative value is often associated
with the velocity to show in what direction an
object is moving.

– Acceleration: The increase or decrease in the
velocity of an object. Acceleration =
velocity/time.
– Momentum: The product of the mass of an
object and its velocity.
– Momentum = mass x velocity.
– Energy: The ability to do work.
– There are two types of energy, kinetic and
potential.

– Kinetic energy is the energy of motion. A
car driving down the highway at 65 mph
has kinetic energy. Kinetic energy = 1/2
mass x velocity2.
– Potential energy is the energy of position.
A car at the top of a hill has potential
energy relative to the bottom of the hill.
– Potential energy = mass x acceleration of
gravity X height.

• Newton’s three laws of motion explain
rest, constant motion, and accelerated
motion, as well as how balanced and
unbalanced orces act to cause these
states of motion.

Newton’s first law of motion:
• states that an object at
rest will remain at rest,
and an object in motion
will remain in motion
until acted upon by an
outside source. Newton
called this tendency of
objects to remain in
motion or stay at rest
Inertia.

Newton’s second law of motion:
• Force = mass x acceleration

Newton’s third law of motion:
• For every action, there is an equal and
opposite reaction.
• Work: A force acting through a
distance.

Have you ever wondered why
a car could sink in a lake then
float on the surface again?
– Fluid pressure is exerted in all directions: down,
up, and to the sides.
• The force of a fluid that pushes an object up
is called buoyancy ( with the upward buoyant
force of a fluid opposes the downward force
of gravity on the object. This relationship
between buoyant force and the weight of
fluid displaced is called Archimede’s
principle

– Density is the mass of an object divided by
its mass. Density can be used to identify
types of glass found at crime scenes and
to match to possible subjects.
– Work = force x distance.
– Power: The rate at which work is done.
Power = work/time.

• Some examples of these quantities in terms of
an average car would probably be useful.
• Consider the case of a 2000 Toyota Camry that
has a weight of 3600 Ib (112 Ibm) and is traveling
at a speed of 55 mph (81 ft/s).
• The calculations are normally done in the SI sys-
tem in the laboratory but are presented to the
jury in English units.
• For simplification all the calculations in this
section will be done in English units.
• In these units mass and weight are differentiated
by mass pounds (Ibm) and force pounds (Ibf).
• The units of speed in the English system are
normally ft/s.

– Mass: Mass = weight/g = 3600 lbf/32.2 ft/s2
= 112 Ibm
– Kinetic energy: Kinetic energy = mass X
velocity2 = 112 Ibm x (81 ft/s) 2 = 735,000
ft Ibf
– Momentum: Mass X speed =112 Ibm x 81
ft/s = 9100 ft Ibm/s

THE SKID FORMULA:
• Often a forensic scientist is asked
to reconstruct an automobile acci-
dent.
• One method frequently used is to
measure skid marks left on the
pavement.

• When a car skids to a stop, its kinetic energy is
dissipated by the frictional work of the tires on
the pavement. One can determine the speed at
which the car was moving using the skid
formula:
» Velocity = 5.5 x square root ((if x D)

• friction for the surface of the road and D is
the length of the skid mark. It is best to
determine the actual value of |4,f for the
accident scene. This can be done using
specialized sleds or other tools to get an
exact value.

Some typical values of |lf are given in Table 5.1.
• TABLE 5.1
• Friction Coeffiecent Surface
• 0.25 Grass
• 0.4 Gravel
• 0.7 Paved road

A graph can also be used to simplify the
calculation.
• In Figure 5.1 simply read up vertically from the length of the
skid mark to the line corresponding to the appropriate
coefficient of friction and read horizontally over to the speed
that the vehicle was going.
• 50 100 150 200 250
• Length skid mark in feet

• If the `vehicle comes to a complete rest,
then its initial speed can be read
directly from the chart.
• If it was not at a stop at the end of the
skid or hit another vehicle, the
additional speed must be accounted
for.

Here are two examples using a 2000 Toyota
Camry on a highway (juf =0.7).
• case I
• The vehicle left 150 ft of skid
marks on the pavement
before coming to a complete
stop.
• Read up from the 150 mark
on the jy-axis until you
intersect the 0.7 curve. At
the point of intersection read
over horizontally to the
speed in mph (-56). This
means the Camry was going
56 mph when it entered the
skid (bad news if the posted
speed limit was 30 mph).
• case II
• The vehicle left 100 ft of skid
marks before hitting a utility
pole. From the crush depth
of the Toyota it was
determined that the vehicle
was traveling 56 mph when it
hit the pole. What was the
initial speed of the Camry?

case II
solution
• Read over from the 56 mph vehicle speed on the y-
axis and note where it intersects the 0.7 curve.
• Read down to the length of the skid mark on the x-
axis and note the value (-150 ft).
• Add this value to the length of the skid mark on the
road to get a final value of 250 ft,
• Read up from the 250ft mark on the x-axis to where it
intersects the 0.7 curve and read over to the ^axis
from that point.
• This means the Camry was originally going about 72
mph before it went into a skid and then hit the utility
pole.

• Case II :required an estimate of the speed of the
vehicle from the amount of damaged caused
when it hit the utility pole.
• This is called the crush depth.
• When the crush depth is multiplied by the crush
stiffness, it gives an estimate of how fast the
car was traveling before impact.
• The crush stiffness is different for every vehicle
and even varies somewhat with speed. It can be
determined from crash test results from the
National Highway Traffic Safety Administration
(www.nhtsa.gov).

• In the case of a 2000 Toyota Camry the crush
stiffness is 1.6 mph/in.
• In Case II the Camry was crushed 35 in when
it hit the utility pole.
• The speed it was going before it hit the pole
can be calculated by the formula:
• speed = crush stiffness x crush depth.
• In this case,
• speed = 1.6 mph/in x 35 in = 56 mph.
• There are several commercial programs
available that contain all the crash stiffness
values and can be used to reconstruct the
most complicated scenarios.

THE SPEED FORMULA
– The initial speed of the vehicle in Case II
can also be calculated by determining the
speeds of the individual events (the skid
and the crush) and adding them together
using the speed formula
– In case II the car was going 56 mph when it
hit the pole, and its skid marks of 100ft
corresponded to a speed of 45mph.
– ( Stotal = SQRT ( 562 + 452) + SQRT (5161)
= 72 mph

THE SPEED FORMULA
– speed formula states that
the initial speed of a
vehicle is equal to the
square root (SQRT) of the
sum of the squares of the
speeds of the individual
events.
•
total
•
= SQRIXS,2 + S22
+ Ss2 + etc.)

CONSERVATION OF ENERGY
AND MOMENTUM IN ACCIDENTS
• In physics, collisions can be classified as
inelastic or elastic.
• Inelastic collisions occur when two objects
collide and stick together and then travel
together as one object in the same direction.
• Kinetic energy is not conserved in inelastic
collisions, so the law of conservation of
momentum is normally used.
• This law states that the total momentum before
a collision must equal the total momentum
after the collision.

• Elastic collisions occur when objects collide
and then travel off on their own.
• An example of an elastic collision is when
two billiard balls collide on a pool table and
then go off in different directions.
• In the case of elastic collisions both
momentum and kinetic energy are
conserved. Here are two examples of
collisions

Here are two examples
of collisions
» case III (inelastic collision)
• A 3596-lb Toyota Camry traveling at 30 mph
collides with a 3527-lb Geo Tracker LSI stopped at
a red light.
• What is the velocity of the two entangled vehicles
after the collision?
• In this case we can use force pounds since any
conversion to mass pounds would cancel out.
• The same holds true for using miles per hour
instead of feet per second.
• We also assume that the vehicles are moving
from left to right and make that the positive
direction for the velocities.

Solution
• Total momentum before collision
• Momentum of Camry + momentum of Geo
• 3596 Ib x 30 mph + 3527 Ib x 0 mph
•
= total momentum after collision
• = (mass of Camry + mass of Geo) x
velocity
• = (3596 Ib + 3527 Ib) x velocity
•
Final velocity = (3596 b x 30 mpb) /
(3596 Ib + 3527 Ib) 15 mph

case IV (inelastic collision,
different directions)
• 3596-lb Toyota Camry traveling at 30 mph (left to
right) collides head-on with a 3527-lb Geo Tracker
LSI traveling 15 mph (right to left).
• What is the velocity of the two entangled vehicles
after the collision?
• In this case we can use force pounds since any
conversion to mass pounds would cancel out.
• The same holds true for using miles per hour instead
of feet per second.
• We also assume that the positive direction for the
velocities is from left to right and that the velocity for
the Geo is therefore negative since it is right to left.

Solution
• Total mom before collision = total mom after
collision
• Mom of Camry + mom Geo = ( Mass of camry + mass
Geo) x
• Velocity
• 3596lb x 30 mph + 3527lb x (-15 mph) =3596lb +
3527) x veloc
• Final velocity = 54975lb/mph divided by (3596 + 3527)
• Answer = 8mph
• Since the final answer is positive, this means the
entangled mass will be traveling at 8 mph from left to
right.

• Elastic collisions require solving
equations for both the conservation of
momentum and the conservation of
kinetic energy.
• Since this can be complicated, most
investigators use commercially avail-
able computer software that solves the
equations automatically.

case V (conservation of energy)
• A 3596-lb Toyota Camry is parked at the
top of a hill.
• The driver forgets to set the brake, and
the car rolls down the hill and into a
lake.
• What speed was the car going at the
bottom of the hill if the change in
elevation was 100 ft?

Solution
• Potential energy at the top of the hill= kinetic
energy at the bottom of the hill
• Mass x gravity X height = ½ mass x velocity 2
• Gravity x Height t = 1/2 velocity 2
• Velocity = SQRT x (2 x gravity x height)
= SQRT x (2 x 32.2 ft/s2 x 100 ft)
= 80 ft/s= 55 mph

MICROSCOPES:
• Microscopes used in modern forensic laboratories
are compound, which means that they contain two or
more lenses.
• However, when the term compound microscope is
used in forensics, it refers to the normal microscope
used in the laboratory.
• The eyepiece contains the ocular lens, which is the
one closest to the viewer. The ocular lens normally
has a magnification factor of ten .
• The objective lens is the one closest to the object
being magnified.
• Total magnification = objective x occular

Five types of optical microscopes are
used in forensic laboratories:
1. Compound: microscope most commonly used in the
crime lab
2. Stereo: used to scan large carriers of trace evidence,
such as clothing, for fibers, gunpowder particles, specks
of blood
3. Comparison: can also be used to compare fibers,
hairs
4. polarizing light: observe glass samples
5. microspectro-photometer: used to check the
ink on questioned bills to determine if it is counterfeit

Chapter 14
Characteristics of
Glass
 Hard, amorphous solid
 Usually transparent
 Primarily composed of silica with various
amounts of elemental oxides
 Brittle
 Exhibits conchoidal fracture

• There are three main chemical types of
glass of interest to the forensic
scientist:
– fused silica,
– soda lime,
– borosilicate.
• The main component of glass is the chemical
silicon dioxide SiO2
• Glass made from pure sand is known as quartz
or fused silica.

• Fused silica is the strongest and most thermally stable
for of glass known. The windows for the space shuttle
are made of fused silica.
• Soda lime glass is relatively cheap to make and is used
in many applications such as windows, bottles, jars, and
most glass items that do not have to be heated thus not
very stable and tend to shatter when headed.
• Borosilicate glass can be heated and will not crack,
however, cracks if it is heated and Safety glass(
laminated glass), normally has 3 layers, 2 layers of soda
lime glass with a thin film of plastic sandwiched
between. Ex windshields then plunged into cold water.
For this reason, it is used for cooking and laboratory
glass ( pyrex, kimax)

Chapter 14
Common Types
 Soda-lime—used in plate and window glass, glass
containers, and electric light bulbs
 Soda-lead—fine table ware and art objects
 Borosilicate—heat resistant, like Pyrex
 Silica—used in chemical ware
 Tempered—used in side windows of cars
 Laminated—used in the windshield of most cars

Chapter 14
Physical
Characteristics
 Density—mass divided by volume
 Refractive index (RI)—the measure of light
bending due to a change in velocity when traveling
from one medium to another
 Fractures
 Color
 Thickness
 Fluorescence
 Markings—striations, dimples, etc

DENSITY AND REFRACTIVE
INDEX;
• The density of glass fragments can be
determined by the floatation method.
• A small shard of glass is put in a vial filled
with bromoform.
• Since the density of bromoform is greater
than that of glass, the shard floats.
• The formula for density is mass divided by
volume.
• The refractive index of glass is a measure of
how much it bends light.

Chapter 14
Density
Type of Glass Density
window 2.46-2.49
headlight 2.47-2.63
pyrex 2.23-2.36
lead glass 2.9-5.9
porcelain 2.3-2.5

Chapter 14
Determination of
Refractive Index
 Immersion method—lower fragments into liquids
whose refractive index is different.
 Match point—when the refractive index of the glass
is equal to that of the liquid
 Becke line—a halo-like shadow that appears around
an object immersed in a liquid. It disappears when the
refractive index of the liquid matches the refractive
index of the glass fragment (the match point)

Chapter 14
Determination of
Refractive Index
 The refractive index of a high boiling liquid, usually a
silicone oil, changes with temperature
 This occurs in an apparatus called a hot stage which
is attached to a microscope. Increasing the
temperature allows the disappearance of the Becke
line to be observed
 At match point, temperature is noted and refractive
index of the liquid is read from a calibration chart

Chapter 14
The Becke Line
The Becke line is a “halo” that can be seen on the inside
of the glass on the left, indicating that the glass has a
higher refractive index than the liquid medium. The
Becke line as seen on the right is outside of the glass,
indicating just the opposite.

Chapter 14
Refractive Index
Liquid RI Glass RI
Water 1.333 Vitreous silica 1.458
Olive oil 1.467 Headlight 1.47-1.49
Glycerin 1.473 Window 1.51-1.52
Castor oil 1.82 Bottle 1.51-1.52
Clove oil 1.543 Optical 1.52-1.53
Bromobenzene 1.560 Quartz 1.544-1.553
Bromoform 1.597 Lead 1.56-1.61
Cinnamon oil 1.619 Diamond 2.419

• Refractive index and density are both
listed as class evidence.
• Unless it is a jigsaw fit of larger glass
fragments fitting together than it would be
individual evidence.
• So glass evidence can be either individual
or class evidence depending on the
circumstances.

TYPES OF FRACTURES
• When a high speed projectile passes
through a glass window, it punctures
the glass rather than causing the
whole pane to shatter.
• The entrance side of the window shows
a smaller, more regular hole, and the
exit side of the window shows a larger,
more irregular hole.

In addition, 2 types of fracture
patterns are produced
1. Small concentric circles form around
the hole on the exit side.
2. Radial fractures begin at the hole and
radiate out like the spokes on a wheel.
Radial fractures ca be used t
determine the order in which multiple
gunshots have been fired through a
window.

Chapter 14
Fracture Patterns
 Radial fracture lines radiate out from the
origin of the impact; they begin on the
opposite side of the force
 Concentric fracture lines are circular lines
around the point of impact; they begin on
the same side as the force
 3R rule—radial cracks form a right angle
on the reverse side of the force.

Chapter 14
Sequencing
 A high velocity projectile
always leaves a hole wider at
the exit side of the glass.
 Cracks terminate at
intersections with others.
This can be used to
determine the order that the
fractures occurred.

Chapter 14
Individual or class evidence?

EXAMPLE:
• 2 men were drinking and watching a football game
on tv. They got into a heated argument, and one let
saying he as going to get his gun and come back.
When the police arrived at the scene they found one
man, with a gun, shot dead on the lawn outside the
house.
The other man was inside the house, also with a gun.
He told the police that he saw, through his living
room window, the other man waving a gun. He then
went and got his own gun. He said the man outside
fired his gun into his house and that he fired back in
defense and the shot killed the man on the lawn.
The police took the living room window to the crime
lab to see if the physical evidence could corroborate
the man’s story.

• There are 2 holes in the
window.
• Bullet hole A had a
larger hole on the
outside of the window,
and bullet hole
• B had a larger hole on
the inside.
• This meant that bullet A
was from the shot fired
by the man inside the
house and bullet B was
from the man outside
the

• Since the radial lines emanating from
bullet hole B end on radial fractures
from bullet A ,hole A was there first.
• This means that the man inside the
house fired first and the man outside
was already fatally wounded when he
fired a shot back into the house

Chapter 14
Glass as Evidence
 Class characteristics; physical and chemical
properties such as refractive index, density,
color, chemical composition
 Individual characteristics; if the fragments can
fit together like pieces of a puzzle, the source
can be considered unique

IMPRESSIONS AND TOOL
MARKS
• Tool marks are made when a harder object comes
in contact with a softer object, leaving marks on it.
• A tool such a s a screwdriver is made to certain
dimensions, and this process leaves unique
striation marks in the metal of the tool (these
microscopic imperfections in the blade make it
unique).
• One of the first things an investigator looks for at
a suspect’s house is the suspect’s tool box.
• Any tools used in the commission of a crime leave
unique scratch marks behind. These striation
marks can be used to match a tool to a n object it
came into contact with at crime scene.

• When a tool is sent to a crime lab, the
tool blade is scraped across a soft
metal brick such as lead.
• A cast is made of the scratch marks left
on the forced entry of the crime scene
as well.
• The cast and the lead brick are placed
under a comparison microscope to see
if the striation marks march up.

EXAMPLE
• 1932 Charles and Anne Lindbergh s infant son was
kidnapped from his nursery.
• A handmade wooden ladder was used to access to
the second floor nursery.
• A ransom note and some muddy footprints, and a
chisel were the only clues.
• The ransom was paid, but the infant was never
returned.
• His body was found in the woods near the Lindbergh
home.
•

• A suspect: Richard Hauptmann’s
toolbox was examined.
• In it was the hand plane used to
construct the homemade ladder.
• The imperfections in the plane’s blade
caused unique striation marks on any
wood it was used on and matched the
wooden ladder at the crime scene
proving Hauptmann’s guilt.

EXAMPLE
• A man was found dead in the early morning hours on the side
of a road in Binghamton, NY.
• There had been a rainstorm that night, so no tire tracks were
visible.
• In a search of the crime scene the police noticed a van parked
on the side of the road, and on closer inspection saw that there
was a man asleep behind the wheel.
• The police knocked on the car window and questioned the
drive.
• He explained that he was out driving in the early hours of the
morning and was too tired to make it home.
• The rain was also a factor in his decision to pull over and rest.
• He said he had almost fallen asleep and lost control of the van.
• It had fishtailed in the driving rain, and when he regained
control of the vehicle, he decided to pull over and get some
rest.

• When the police looked at the
passenger side of the van, they were
shocked to see the impression of the
pedestrian in the side of the van.

IMPRESSION MATERIAL
• There are three materials commonly
used in forensic science to make casts
of tool marks and other impressions:
1. Permlastic (polysulfide)
2. Polyvinylsiloxane

Dental stone
1. Dental stone: very
fine grade calcium
sulfate, and the
material of choice
when making a
cast of bite marks,
shoeprints, and tire
prints

snow print wax
1. In snow a waxy
substance called
snow print wax is
first sprayed over
the impression and
then the cast is
made.

• Regardless of the material, once the
print or impression has been taken, the
forensic scientist can develop a great
deal of class characteristic evidence.

• The pattern produced by the sole of the
shoe can be used to determine the
manufacturer.
• A footwear print about 11.5 in length
and 4.3 in width might indicate a size 8
½ D shoe.
• Many popular sneakers have the
manufacturers name in the tread
design.

tire tracks
• For tire tracks the width of the tread
impression gives the first number in the size
of the tire, ex tire size 235/60R16 stands for a
tire that has a 235 mm wide tread with an
aspect ration( ratio of height of the sidewall
for the tire to the width of the tread times
100) 60.
• It is also a radial and fits on a 16 inch
diameter wheel.
• Multiplying the decimal aspect ratio ( aspect
ratio divided by 100) by the width of the tire
gives the height of the sidewall of the tire

EXAMPLE
• A tire tread left at a crime scene was
about 9.3 in wide and showed a
repeating imperfection mark every 84.7
inches it traveled.
• Could this be consistent with the tire
mentioned above?

SOLUTION;
• Width (mm) = width(in) x 25.4 mm/in
• = 9.3 in x 25.4 mm/in
• = 236 mm ( consistent with tire size)
• Height of sidewall = width x aspect ratio/100
• = 9.3 x 60/100
• = 5.6 in
•
• Overall diameter = wheel diameter + (2 x sidewall height)
• = 16in + (2 x 5.6)
• = 27.2 in
• Overall circumference of the tire = 3.14 x diameter
• = 3.14 x 27.2 in
• = 85.4 in
• Any imperfections in the tire tread would be expected to repeat
every 85.4 inches, which is consistent with what was found at the
crime scene

PAINT:
• Paint is often transferred in hit and run
accidents and collisions. It is therefore
important that the forensic scientist
understand the automotive paint process.
• Cars surfaces normally receive four layers
of paint: electro coat primer, primer, base
coat, and a clear coat.
• The method of choice used to identify fibers,
• Pyrolysis GC is also used to identify the
binder in automobile paint chips.

• Automobile manufacturers often
change paint formulations every few
model years, which allows the forensic
scientist to narrow down the field of
suspect vehicles.
• Paint chips left behind at a crime scene
can be of great value. They should be
carefully packaged to prevent any
damage to the edges.

• There is always a chance that it can be
matched to a suspect’s vehicle and that
the random edges on the chip might
match the damaged section of the car.
• It is also important to always collect a
control (a paint sample taken from an
area away from the damaged section of
the car)

• A paint chip collected from a car should be
about ¼ inch by ¼ inch
• The paint chip collected should be scraped
down to the bare metal

Automobile paint chips viewed
under the stereomicroscope

cross section of multiple paint
layers at 60x magnification

FORENSIC PHYSICS

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