unit 3: forensic physics. accident reconstruction: physics is the science that deals with natural...
TRANSCRIPT
UNIT 3: 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 divid ing 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 com ing 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 vehi cle 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 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 Administra tion (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 conser vation 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 col lision?
• 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 conserva tion 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 for gets 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 nor mal 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 mag nified.
• 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
GLASS
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Characteristics of Characteristics of GlassGlass
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)
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Common TypesCommon 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
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Physical Physical CharacteristicsCharacteristics
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
DensityDensityType 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 ofDetermination ofRefractive IndexRefractive 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)
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Determination of Determination of Refractive IndexRefractive 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 LineThe 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 IndexRefractive Index
Liquid RI Glass RIWater 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.
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Fracture PatternsFracture 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.
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SequencingSequencing 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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Chapter 14
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Individual or class evidence?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
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Glass asGlass as EvidenceEvidence
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