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  • Physics is the science that deals with natural
    phenomena such as motion, force, work, energy,
    momentum, light, sound, electricity, and
  • 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
  • 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
  • Velocity The speed and direction an object is
  • 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
  • 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

  • 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.

  • 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.

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.

Newtons second law of motion
  • Force mass x acceleration

Newtons third law of motion
  • For every action, there is an equal and opposite
  • 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
    Archimedes 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

  • 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
  • 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

  • Often a forensic scientist is asked to
    reconstruct an automobile accident.
  • 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
  • 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
  • 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

  • If the vehicle comes to a complete rest, then
    its initial speed can be read directly from the
  • 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 IIsolution
  • Read over from the 56 mph vehicle speed on the
    y-axis and note where it intersects the 0.7
  • 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

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  • 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

  • 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

  • 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

  • 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.

  • 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
  • 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.

  • Total mom before collision total mom after
  • 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
  • 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 available computer software
    that solves the equations automatically.

case V (conservation of energy)
  • A 3596-lb Toyota Camry is parked at the top of a
  • 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?

  • 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 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,
  4. polarizing light observe glass samples
  5. microspectro-photometer used to check the ink on
    questioned bills to determine if it is

polarizing light
  • Types of forensic
  • microscopes

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,

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

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

  • 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
  • The refractive index of glass is a measure of
    how much it bends light.

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
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)

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

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.

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.

  • 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
  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

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.

  • 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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  • 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.

  • 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

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

  • Tool marks are made when a harder object comes in
    contact with a softer object, leaving marks on
  • 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
  • One of the first things an investigator looks
    for at a suspects house is the suspects tool
  • 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.

  • 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
  • His body was found in the woods near the
    Lindbergh home.

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  • A suspect Richard Hauptmanns toolbox was
  • 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.


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

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

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

snow print wax
  • 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

  • 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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  • 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?

  • 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
  • 16in (2 x 5.6)
  • 27.2 in
  • Overall circumference of the tire 3.14 x

  • 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 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 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)

  • 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
cross section of multiple paint layers at 60x
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