Properties of Physical Evidence

Physical Evidence

Physical evidence is tangible evidence (such as a weapon, document, or visible injury) that is in some way related to the incident that gave rise to the case; it is sometimes also called "real evidence". Many different items can be categorized as physical evidence.

Some common types of physical evidence include:

Paint chips
Glass shards
Traces or shrapnel from explosives
Bullets or cartridges
Dirt and soil samples
Fingerprints
Tire and shoe impressions
Questioned Documents
Biological evidence (saliva, blood, semen etc.)
Skeletal remains
Hair and fibers
Insects
Charred remains and objects from arson

Each of these types of physical evidence is collected, preserved and analyzed according to the type of material and type of analysis that will be performed. Because glass and soil are so common in crime scenes, this module will focus on those two types of physical evidence. When working with glass and soil evidence, it is important to recall the basic physical science concepts of mass, weight, density and refractive index.

Mass

Mass is a measure of how much matter is in an object. Mass does not account for the forces of gravity on an object, thus it is only a measure of the matter in an object. This means that it will not change in environments where acceleration due to gravity varies from that on Earth. For example, the weight of an object on Earth will be different than its weight on the moon, but the mass will always be the same in either place! The mass of an object is generally measured in grams or kilograms on lab equipment, such as a triple beam balance.

Weight

Weight is the force on an object due to gravity. This means that weight is a force that includes the mass of the object, as well as the acceleration of gravity in the place the object occupies. Essentially, weight is simply a measurement of gravity's effect on an object. The formula for weight then is Weight = Mass X Acceleration due to gravity, or W=mg. On Earth, the acceleration due to gravity is 9.8m/s². In physical science, because weight is considered a force exerted on an object, the unit for weight is Newtons. Let's see an example calculation using this information:

W= ?

m= 2.3kg

g= 9.8 m/s²

W= 2.3kg X 9.8m/s²

^{W= 22.54N}

Recall that the unit for force, the Newton, is equivalent to the units in the equation: $LaTeX: kg\times mls^2$

Density

Density is the mass per unit volume, or g/ml. Essentially, it is the relationship between the mass of an object and how much space it takes up. Sometimes density is thought of as the "thickness" of an object. Density is considered an intensive property. Intensive properties are properties of an object that do not change even when the amount of matter changes. For example, a piece of glass that shatters into 1,000 pieces retains the same density in all 1,000 glass shards! The volume changes, but the density of the glass does not change. This is especially helpful in an investigation because the density can then be matched and used to demonstrate that the glass pieces most likely came from the same glass object. Just as with weight, density can be easily calculated by using an equation relating the mass and volume of an object. Volume can be calculated using a formula for regularly shaped objects or determined by displacement in a graduated cylinder filled with water.

The formula for density is: Density= Mass/Volume. The following is an example of how to calculate density:

Using the equation, d=m/v, calculate the density of a shard of glass with a mass of 73 grams and a volume of 28 mL:

d= ?

m= 73 g

v= 28 mL

d= m/v

d=73g/28mL

d= 2.61g/mL

The easiest way to observe density is to see if a solid floats, sinks, or stays suspended in a liquid. If the object is less dense than the liquid, then it will float. If the object is more dense than the liquid, then it will sink. If the object remains suspended in the liquid, then the densities are approximately the same.

Interactivity: Density

Refraction

Refraction is the bending of a light wave as it passes from one medium to another. The Refractive Index, or RI, is the velocity of light in vacuum divided by the velocity of light in a transparent medium. Refractive Index varies by both the chemical composition of a substance, as well as its atomic structure. In glass, this differs by the material used to make the glass as well as the way in which it was cooled. Measuring the Refractive Index is useful in forensic investigations because it can easily be measured very precisely and can reflect many different types of glass based upon the RI value obtained. For example, in one method, a transparent solid is immersed in a liquid with a similar refractive index that can be varied until the precise RI value is matched at which time the solid seems to disappear. This property is valuable when analyzing glass evidence by helping match glass samples from more than one source to the corresponding matching source.

Birefringence

Another way in which glass can be examined based on refractive physical properties is by its birefringence. Birefringence is a property that results due to a difference in two indices of refraction, and it is typically exhibited by most crystalline materials. Given that glass does not have a crystalline structure, it displays birefringence differently than crystalline materials. Glass displays birefringence when it is strained, or bent.

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