UNIT 3: FORENSIC PHYSICS - PowerPoint PPT Presentation

PPT – UNIT 3: FORENSIC PHYSICS PowerPoint presentation | free to download - id: 6c907e-NTFiN

The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
Title:

UNIT 3: FORENSIC PHYSICS

Description:

unit 3: forensic physics – PowerPoint PPT presentation

Number of Views:37
Avg rating:3.0/5.0
Slides: 94
Provided by: owenkl
Category:
Tags:
Transcript and Presenter's Notes

Title: UNIT 3: FORENSIC PHYSICS

1
UNIT 3 FORENSIC PHYSICS
2
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.

3
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.

4
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.

5
• 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.

6
• 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.

7
• 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.

8
• 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.

9
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.

10
Newtons second law of motion
• Force mass x acceleration

11
Newtons third law of motion
• For every action, there is an equal and opposite
reaction.
• Work A force acting through a distance.

12
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

13
• 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.

14
• 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.

15
(No Transcript)
16
• 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

17
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.

18
• 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)

19
• 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.

20
Some typical values of lf are given in Table
5.1.
• TABLE 5.1
• Friction Coeffiecent Surface
• 0.25 Grass
• 0.4 Gravel

21
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

22
• 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.

23
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?

24
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.

25
• 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

26
• 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.

27
(No Transcript)
28
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

29
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.)

30
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.

31
• 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

32
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
• We also assume that the vehicles are moving from
left to right and make that the positive
direction for the velocities.

33
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

34
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
• 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.

35
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)
• Since the final answer is positive, this means
the entangled mass will be traveling at 8 mph
from left to right.

36
• 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.

37
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?

38
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

39
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

40
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

41
Stereo
polarizing light
Comparison
• Types of forensic
• microscopes

microspectrophotometer
Compound
42
GLASS
43
Characteristics of Glass
• Hard, amorphous solid
• Usually transparent
• Primarily composed of silica with various
amounts of elemental oxides
• Brittle
• Exhibits conchoidal fracture

44
• 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.

45
• 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
• 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)

46
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

47
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

48
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.

49
Density
Type of Glass Density
window 2.46-2.49
pyrex 2.23-2.36
porcelain 2.3-2.5
50
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)

51
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

52
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.

53
Refractive Index
Liquid RI Glass RI
Water 1.333 Vitreous silica 1.458
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
Cinnamon oil 1.619 Diamond 2.419
54
• 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.

55
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.

56
In addition, 2 types of fracture patterns are
produced
1. Small concentric circles form around the hole on
the exit side.
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.

57
Fracture Patterns
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.

58
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.

59
(No Transcript)
60
(No Transcript)
61
Individual or class evidence?
62
(No Transcript)
63
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.

64
• 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

65
• 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

66
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

67
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.

68
• When a tool is sent to a crime lab, the tool
blade is scraped across a soft metal brick such
• 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.

69
EXAMPLE
• 1932 Charles and Anne Lindbergh s infant son was
kidnapped from his nursery.
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.

70
(No Transcript)
71
(No Transcript)
72
• A suspect Richard Hauptmanns toolbox was
examined.
• In it was the hand plane used to construct the
• 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.

73

74
He was electrocuted on April 3, 1936, just over
four
75
(No Transcript)
76
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.

77
• 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.

78
IMPRESSION MATERIAL
• There are three materials commonly used in
forensic science to make casts of tool marks and
other impressions
• Permlastic (polysulfide)
• Polyvinylsiloxane

79
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

80
snow print wax
• In snow a waxy substance called snow print wax is
first sprayed over the impression and then the

81
• Regardless of the material, once the print or
impression has been taken, the forensic scientist
can develop a great deal of class characteristic
evidence.

82
• 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

83
(No Transcript)
84
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

85
(No Transcript)
86
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?

87
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

88
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.

89
• 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.

90
• 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)

91
• 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

92
Automobile paint chips viewed under the
stereomicroscope
93
cross section of multiple paint layers at 60x
magnification