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# Who's At Fault

Physics and Automobile Accident Reconstruction
by

## Kelsey Mitchell

on 8 January 2015

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#### Transcript of Who's At Fault

WHO'S AT FAULT?
On August 7, 2012, an automobile accident occurred at the intersection of Elwood and Awesome Streets in Joule, NJ between a compact car, driver by Mr. C. McInerney, and a tractor-trailer, driver by Ms. D. Lang. At the intersection of the two streets, the truck driver had a flashing yellow light, while the car driver had a flashing red light.

Neither driver claims responsibility for the accident.
Friction is the force exerted by a surface as an object move across or against a given surface, and always acts opposite the direction of motion, unless otherwise specified. The force of friction enabled the tires to grip the surface of the road, allowing them to propel the car in any given direction, and also lessened the likelihood that the car would flip over. The level of friction that different materials experience is measured by the coefficient of friction. However, the are also two forms of friction, those being kinetic and static.
SETTING THE SCENE
FRICTION
In Physics and in everyday life, objects around us are constantly in motion, from the dust particles to the cars on the expressway, to the squirrels in the trees. The motion of all objects can be described using on simple term- kinematics. In a collision, kinematics and kinematic equations help us to make calculations using given variables and values. Using kinematics we can calculate the velocity of the automobiles post-collision, and even the acceleration.

Momentum, on the other hand, is a derived quantity that acquired after mass and velocity have been multiplied together in order to find a final value. Qualitatively, momentum helps us to analyze the correlation between a vehicle mass, and its velocity. The Law of Conservation of Momentum states that the total momentum of a system remains the same, as long as there are no new momentum-carrying objects.
KINEMATICS AND CONSERVATION OF MOMENTUM
3. Find velocity of vehicles immediately following collision
Physics and Automobile Accident Reconstruction
BASIC THEORY
In any collision, like the theoretical collision between Mr. McInerney and Ms. Lang, there are physics principles and theorems that apply to the situation. Forces, conservation of momentum, and kinematics are some of the few principles that are used to perform calculations.
NEWTONIAN MECHANICS
A force is defined as a push or pull on an object in relation to an interaction between one object and another.
Newton's First Law and Second Law apply directly to car crashes, as Newton's First Law says that an object in motion will stay in motion at a constant speed, and in a straight line, unless compelled to change due the presence of a net force.
Newton's Second Law, on the other hand, says that when a net force F, acts on an object with a mass m, the acceleration a, that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass.
In short, the force that acts upon an object, is equal to the mass of the object, multiplied by the acceleration, which can be determined using kinematics.
Static friction is the frictional force that is exerted between two stationary objects, and the surface on which they are resting. Once the static frictional force barrier is 'broken,' the static friction becomes kinetic friction.
Kinetic friction, also known as moving friction, is the force that is exerted as two different object move against each other.Kinetic friction may cause changes to velocity, specifically decreases.
WORK AND ENERGY
Work is defined as the change in mechanic energy, which is represented by the equation, force times distance equals work.
Doing work helps to change the mechanical energy of a system, as a change in force or distance may cause a change in the velocity of your system.
There are two primary types of mechanical energy, those being kinetic energy and potential energy. Similarly to static and kinetic friction, kinetic friction exists when an object is in motion, while potential energy exists when an object is slowing down, or becoming more stationary. Both are essential topic to automobile collision.
EQUATIONS USED
AUTOMOBILE COLLISION
CALCULATIONS
1. Find the coefficient of kinetic friction for both vehicles
Car - 0.77
Truck - 0.54
2. Find acceleration of vehicles post-collision.
Car- 7.55 m/s^2
Truck- 5.29 m/s^2
Car- 11.13 m/s
Truck- (-) 10.79 m/s
4. Find Initial Velocity Pre- Collision
Initial Velocity of Car- 12.80 m/s (+y)
Initial Velocity of Truck- 12.53 m/s (-x)
WARNING
CAR
WHO'S AT FAULT ?
JUSTIFICATION OF VELOCITY WITH
WORK ENERGY THEOREM
Car- 11.12 m/s
Truck- 10.79 m/s
DISPROVING CAR DRIVER'S CLAIM
ASSUMING FINAL VELOCITY IS THE SAME
According to Ford Motor Corporation, the maximum acceleration of a comparably loaded Ford Escort is about 3.0 m/s^2. Although the acceleration of the vehicle is greater than this value after the collision, this is ideal as the car experienced an impulse force, which was exerted by the truck. However, before the collision, the acclerartion should not be equal more than the specified value.
FINAL VERDICT
THE VERDICT?

Based on the information researched and calculated concerning the theoretical collision, we found that both Mr. McInerney and Ms. Lang are both at fault in the eyes of the law. However, Mr. McInerney did not obey traffic laws, as he did not have the right-off-way, and therefore is the primary individual at fault.
Ms. Lang was facing a flashing yellow light, which is not indicative of stopping, but merely informs the driver to proceed with caution. However, Ms. Lang was not braking before the collision as she claimed, or else, she would not have achieved the final velocity that was calculated, and it would have impossible for her to have exhibited such a massive impulse force, without changing the mass of the truck. Although Ms. Lang was wrong like Mr. McInenery, she only disobeyed court laws, as her claim did not match the calculations made. However, Ms. Lang should have been sure to brake in order to reduce impulse force exerted, and decrease the severity of the collision.
MS. LANG'S FAULTS
MR. MCINERNEY'S FAULTS
Mr. McInerney was facing a flashing red light at the intersection and did not stop, as he claimed. This itself is a violation of both traffic and court laws, specifically perjury. In terms of traffic laws, a flashing red light is the same thing as a stop sign. However, based on the calculations concerning acceleration, it is impossible that Mr. McInerney made a complete stop at the flashing red light. Although Ms. Lang also made mistakes, Mr. McInerney did not come to a stop as the traffic law indicates, and therefore, is the main culprit.
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