UNIT 3: FORENSIC PHYSICS

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UNIT 3 FORENSIC PHYSICS
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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.

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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.

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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.

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• 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.

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• 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.

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• 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.

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• Newtons 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.

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Newtons 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.

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Newtons second law of motion

• Force mass x acceleration

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Newtons third law of motion

• For every action, there is an equal and opposite
  reaction.
• Work A force acting through a distance.

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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
  Archimedes principle

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• 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.

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• 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
  system 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.

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• 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

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THE SKID FORMULA

• Often a forensic scientist is asked to
  reconstruct an automobile accident.
• One method frequently used is to measure skid
  marks left on the pavement.

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• 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)

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• 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.

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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

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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

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• 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.

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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?

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case IIsolution

• 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.

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• 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).

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• 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.

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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

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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.)

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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.

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• 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

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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.

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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

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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.

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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.

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• 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 available computer software
  that solves the equations automatically.

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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?

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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

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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

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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

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Stereo
polarizing light
Comparison

• Types of forensic
• microscopes

microspectrophotometer
Compound
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GLASS
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Characteristics of Glass

• Hard, amorphous solid
• Usually transparent
• Primarily composed of silica with various
  amounts of elemental oxides
• Brittle
• Exhibits conchoidal fracture

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• 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.

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• 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)

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Common Types

• Soda-limeused in plate and window glass, glass
  containers, and electric light bulbs
• Soda-leadfine table ware and art objects
• Borosilicateheat resistant, like Pyrex
• Silicaused in chemical ware
• Temperedused in side windows of cars
• Laminatedused in the windshield of most cars

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Physical Characteristics

• Densitymass 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
• Markingsstriations, dimples, etc

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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.

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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
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Determination ofRefractive Index

• Immersion methodlower fragments into liquids
  whose refractive index is different.
• Match pointwhen the refractive index of the
  glass is equal to that of the liquid
• Becke linea 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)

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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

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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.

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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
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• 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.

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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.

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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.

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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 ruleradial cracks form a right angle on the
  reverse side of the force.

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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.

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Individual or class evidence?
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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 mans story.

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• 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

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• 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

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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

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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 suspects house is the suspects 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.

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• 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.

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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.

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

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He was electrocuted on April 3, 1936, just over
four
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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.

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• 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.

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IMPRESSION MATERIAL

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

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Dental stone

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

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snow print wax

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

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

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• 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.

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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

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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?

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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

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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.

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• 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.

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• There is always a chance that it can be matched
  to a suspects 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)

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• 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

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Automobile paint chips viewed under the
stereomicroscope  
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cross section of multiple paint layers at 60x
magnification