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Crime Scene Investigation Using Physics?!

Explore how physics is used in the world of crime scene investigation

Amy Liu

on 21 January 2013

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Transcript of Crime Scene Investigation Using Physics?!

We've all seen the CSI shows. Dead bodies, blood and murders. But have you ever thought how physics
can play a role in crime scene investigation? CSI: PHYSICS Where is Physics used in CSI? Car Crashes 1. Gun ballistics
2. Blood splatters
3. Car crashes Gun ballistics is the work of projectiles from the time of shooting to the time of impact with the target. Gun ballistics uses physics to help find the path of the bullet, height the gun was shot and speed of the bullet. This information can be used to find the guilty criminal. Gun ballistics is often broken down into the following four categories. Gun Ballistics Physics behind the car crash The shape of the blood spatter is mostly used to determine the size of the hole the blood is forced through. With a smaller hole, the stream will be more powerful, but the spatter size will be smaller. This helps determine the size of the wound.
Size of spatter is similarly very useful. For one, blood, like all objects, picks up speed as it falls. Unless it is from a fall above 26 feet, blood's terminal velocity, investigators can pinpoint from what height it fell. This is useful, for example, when trying to determine whether a person was standing or lying down during an attack.
Investigators can also judge the energy used to force the blood into flight. This can help tell how forceful the strike was. The higher the energy used, the smaller the dots of blood will be.
It is also possible to judge from which direction the strike came, by calculating the angle of the blood's impact using trigonometric formulas. Blood Splatters Car collisions happen everyday, and some times they are fatal. In some cases where the police don't have video tape of the car crashes or witnesses, they rely on physics to figure out who was responsible for the car crash 4 categories: Internal ballistics (sometimes called interior ballistics): treats of the motion of a projectile while it is still in the gun, for example the passage of a bullet through the barrel of a rifle.

Transition ballistics (sometimes called intermediate ballistics): the study of the projectile's behavior when it leaves the barrel and the pressure behind the projectile is equalized.

External ballistics (sometimes called exterior ballistics): the study of the passage of the projectile through a medium, most commonly earth's atmosphere. Considers the motion of the projectile from the time it emerges from the gun until it reaches the target

Terminal ballistics: the study of the interaction of a projectile with its target terminal. It deals with the effect of the projectile on the target. Newton's Third Law:
If the gun and shooter are at rest, then the force on the bullet is equal to the force on the gun-shooter. This is due to Newton's third law of motion (For every action, there is an equal and opposite reaction). Consider a system where the gun and shooter have a combined mass M and the bullet has a mass m. When the gun is fired, the two objects move away from one another with new velocities V and v respectively. But the law of conservation of momentum states that their momenta must be equal. Since force equals the rate of change in momentum and the initial momenta are zero, the force on the bullet must therefore be the same as the force on the gun/shooter.

Hollywood depictions of firearm victims being thrown through plate-glass windows are inaccurate. If this was the case, the shooter would also be thrown backwards with equal force. (Newton's third law) Gunshot victims frequently fall or collapse when shot; this is a result of the momentum of the bullet pushing them over.

Kinetic Energy
However, the smaller mass of the bullet, compared that of the gun-shooter, allows significantly more kinetic energy to be transfered to the bullet than to the shooter.
The ratio of the kinetic energies is the same as the ratio of the masses (and is independent of velocity). Since the mass of the bullet is much less than that of the shooter there is more kinetic energy transferred to the bullet than to the shooter. Once discharged from the weapon, the bullet's energy transfers its kinetic energy to the air molecules around it throughout its flight, until the remainder is dissipated by colliding with a target. Using the Laws Of Physics = M From Eq. 1 we can write for the velocity of the gun/shooter: V = mv/M. This shows that despite the high velocity of the bullet, the small bullet-mass to shooter-mass ratio results in a low recoil velocity (V) although the force and momentum are equal. =m Sound Interesting? Crime Scene Investigation
is a great field to get into, even if you enjoy
physics. Here is a clip of Raquel from University
Of Ontario Institute using physics in blood splatter
analysis. Passive Bloodstains Passive bloodstains are drops created or formed by the force of gravity acting alone. Passive bloodstains can be further subdivided to include drops, drip patterns, pools, and clots. Based on the size of the blood drip and shape, forensics can use the laws of gravity and motion to determine how and where the blood fell from. Passive blood stain Types of Blood Stain Patterns There are three types of blood stain patterns What blood splatters tells you -movement and direction of persons or objects as they are shedding blood
-position of persons or objects during bloodshed
-movement of persons or objects after bloodshed
-the mechanism or object used to on the person who is shedding blood
-direction the blood was traveling in before it stain
-area of origin of impact
-sequence of events -passive bloodstains: blood drops falling with only the force of gravity acting upon it

-projected bloodstains: when some form of energy is transferred to the blood splatters

-Transfer/ contact bloodstains: when an object with blood comes in contact with object without blood The velocity of the blood splatter effects the stain it creates: -Low velocity blood splatter: produced when the blood is moving less than 1.5 m/s
-Medium velocity blood splatter: produced when blood is traveling between 1.5m/s and 7.5 m/s (blunt forced trauma
-High velocity blood splatter: when blood is traveling more than 30 m/s Using the velocity of the blood and shape of the blood, physics and math can be used to determine the height, direction and magnitude of the blood. This type of collision usually results in the biggest momentum forwards for passengers in the car, due to the sudden stop in forward momentum of the cars. The upper bodies of passenger(s) are jerked forward while the lower body is anchored to the seat, because of the lower end of the seat belt, which is usually more secure. The force with which the body is thrown forwards depends on two factors: the combined speed at impact and the hardness of the car body. The greater the speed of the car on impact, the greater the force acting on the car passengers and hence the more they jerk forward. Also, if the car body is made of very hard metal, then it would crumple less and hence the time taken for the whole car to come to a complete stop would be very short. Since time is decreased, then the rate of deceleration of the car would be very fast and hence a greater force acting on the passengers, jerking them forward, because: Force = Mass X Acceleration; or in this case, Opposite Force (of the car) = Mass X Deceleration ( of the car). The bodies of the passengers will jerk forward because, with the car’s initial cruising speed, both the car and the passengers are moving at a constant speed forward. During collision, the car’s speed is slowed down very quickly, leaving the passengers to continue in the forward direction, hence their jerking motion. All the forces present in a head-on car collision are generally linear, and hence the physics behind it isn’t too hard. In such a scenario, passengers in a stationary car are first thrown towards the side of the collided vehicle because of the sudden displacement of the car (i.e. side-ways momentum). After this, passengers are usually thrown into the opposite side of the car because of the decrease in initial side-ways momentum their car has experienced. The car which started the accident, i.e. the one which collided into the stationary car, would experience the same forces as a head-on collision. Side airbags would be of the most use for the initially-stationary car, while front airbags would be of most use for the other car. Side Collisions The amount of force that causes an accident is largely dependent on Newton’s 2nd and 3rd laws, i.e. F=MA, and For every action, there is an equal and opposite reaction, respectively. Newton’s 1st law, Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it, explains why passengers in cars are thrown about in a collision, which usually results in a sudden acceleration or deceleration. Factors that affect the degree of damage to the car and injury to passengers are the hardness of the car body, the initial cruising speed of the car, angle of collision ( from where does the car impact on to), the design of the car, the weight of the car, and body mass and position of the passengers of the car.

1) The harder the car body, the greater the rate of acceleration or deceleration on the whole, for the car. A greater acceleration for the same mass results in greater force ( F=MA), which would mean passengers of the car would be subject to more violent jerking and might possible sustain more injury, even though the car would have less damage. Likewise, the softer the car body, the more damage the car would sustain. This would mean the car body would crumple up more and a slower rate of acceleration or deceleration on the whole for the car, resulting in less violent jerks of passengers, meaning less chance of injury. Of course, if the car were too soft, passengers would be crushed to death by the impact.

Case Scenario A car (A) of 1000 kg travelling at a constant speed of 100km/h collides with another car (B) also of 1000kg and travelling at a constant speed of 100km/h in the opposite direction of the first car. There is 1 passenger in both the cars and they both weigh 60kg each. After the collision, both cars were displaced 10 meters from the point of collision in 2 seconds
Given this, because of F=MA, the force that each car crashed with is [1060 (10/2) ] N, which is 5300N. The passenger of each car, would in return experience 5300N of forward momentum before being jerked back by the seatbelt. 5300N of force on any person is not a small amount, hence both persons might sustain brain hemorrhage on account of the sudden movement of the head. The head would move the most because it is furthest from the fulcrum which is the lower body strapped to the seat by the seatbelt. The airbag would probably cushion the head and slow its sudden acceleration forward and backward by some amount, but 5300N would probably still lead to, at least a broken nose from impact into the airbag. The ensuing snap back into the seat might be hard enough to injure the spinal chord of the passenger, and all this is assuming there are no sharp exposed metal pieces from the crumpled and twisted car metal to cause abrasions, lacerations and internal injury. CSI: Physics Solving crimes through ballistics, blood splatters and car collisions. 2) The faster the initial cruising speed of the car, the greater the impact felt. This is explained by 3rd law. If a car were to crash into a brick wall, the brick wall would exert an equal and opposite pressure on the car, which is what causes the car to crumple up and throw the passengers about. For a slower initial cruising speed, the less the force acting on the brick wall and since an equal and opposite reaction is acted on the car, the lesser the force crumpling up the car.

3)The greater the weight, or mass, of the car, the greater the forces dealt. This is because of F=MA, meaning if the car crashing into a stationary car were very heavy as compared to the stationary car, the stationary car could be sent flying. However if the roles were reversed and the car crashing into the stationary car were much lighter than the stationary car, the stationary car would not sustain much damage.
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