Saturday, 13 August 2016

Mechanics of a Traffic Collision

Car crash (traffic collision) causes injury, damage or even death. It occurs when a moving motor vehicle collides with either a moving object or a stationary object. The chances of a traffic collision occuring is determined by the following factors which also influence the physics of car crash: vehicle design, road design, driving skills of the driver, speed of the motor vehicle before impact, road environment, level of psychological impairment (usually caused by alcohol consumption or substance abuse) and behavioral disposition like street racing and speeding (Bartley, 2008).
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Physics of a Car Crash
The physics/mechanics of collision describe the process and outcome of a car crash. As stated earlier, the motor vehicle before crashing has usually gained momentum by virtue of its motion. Thus, Newton’s laws of motion can help explain the physics of traffic collision. 
Laws of Motion
Newton’s first law of motion states that a stationary object remains stationary if no external force is applied to the object. It also states that a moving object would remain in motion at a constant velocity if no external force is applied to it (Field, 2015). Therefore a stationary object on the road would remain stationary if no external force is applied. Also, a moving car would travel in constant velocity if no external force is applied.
Newton’s second law of motion deals with objects in motion. It states that the rate of acceleration is directly proportional to the combined net force and is also inversely proportional to the mass of the object (Field, 2015). Thus, it can be deduced that force is the product of mass and the rate of acceleration.
Newton’s third law of motion deals with collision. It states that when two objects, A and B, collide; the the force that is exerted by object A onto object B is of equal magnitude but in opposite direction to the force exerted by Object B onto Object A (Field, 2015). Thus, an action has an equal reaction albeit in opposite direction.
Kinetic and Static Friction
Another important concept in traffic collision is friction. Friction is the force which resists motion when two objects contact each other. Different materials exert different levels of friction, and thus each material has its own co-efficient of friction. For motion to occur when Object A is resting on the ground, the applied force must be greater than the normal force of Object A and the force of friction. Static friction keeps two objects from moving away from each other, and force greater in magnitude than the static friction must be applied to cause the two objects to slide on each other, and this force must be applied constantly. This force is defined as kinetic friction (Popov, 2010).
The concept of static and kinetic friction applies to automobiles when they are driving on wet roads. The values of static and kinetic co-efficient of friction of a dry road are 1.00 and 0.80 respectively. Similar values for a wet road are 0.60 for static and 0.40 for kinetic co-efficient of friction. This implies that wet roads are more slippery than dry roads. Also, considering Newton’s second and third laws of motion, the driver must drive at a slower speed as compared to dry roads so as to be able to decelerate safely when negotiating corners as well as ensure that the vehicle can come to a stop at a distance which is safe for other road users. The equation which is used to calculate the braking distance is shown below (Popov, 2010).
d = V2 / 2gµ
d is the braking distance
V is the initial speed of the vehicle (in meters per second)
g is the acceleration caused by gravity, and its affixed value is 9.80 meters per second squared
µ is the co-efficient of (kinetic or static) friction between the road and the tyre.
From the above equation, it can be noted that if the velocity of the vehicle is doubled; then the braking distance will be more than doubled (that is, the braking distance will increase exponentially). Also, the higher the co-efficient of friction, the shorter the braking distance. Thus, when driving in a dry road where the tire is not sliding on the surface, then the static co-efficient of friction is dominant (it always has a higher value than the kinetic value); and this implies that the braking distance would be shorter as compared to a car driving in a wet road where it is sliding (which means that the kinetic co-efficient of friction is dominant).
When a moving vehicle is negotiating a corner, then Newton’s first law of motion applies. The curve forces the car to stop traveling on a straight path. Normally, the car would want to continue to travel in a straight course and at a constant velocity as explained by Newton’s first law of motion. However, the vehicle must change its lateral velocity so that it can negotiate the curve. The change in lateral velocity is accomplished by applying adequate frictional force on the tires. If the frictional force is inadequate, the vehicle would continue on its straight course and thus drive off the road and collide onto whatever object is on its path, thus causing a vehicle crash (Popov, 2010). To avoid this, the driver must reduce the speed of the vehicle so as to ensure that the tire maintains static friction with the road. This reduces the frictional force required to change the lateral velocity, thereby reducing the chances of a car crash.
Elastic and Inelastic Collisions
In physics, collision is defined as the event which occurs when a moving body, which has momentum, impacts onto another body; thus causing a transfer of kinetic energy. Momentum is defined as a product of velocity and mass of the object. Thus, the momentum of a vehicle is the product of its mass and its velocity. Kinetic energy is defined as the energy of motion, and it is directly proportional to the mass of the object as defined by the following equation (Katz, 2015);
Kinetic energy = ½ x mass (in kilograms) x velocity (meters per second)2
There are two basic types of collisions; elastic and inelastic collisions. An elastic collision defines an encounter between two (or more) different bodies where they bounce from each other after collision. The total kinetic energy as well as the total momentum in an elastic collision is conserved. This implies that the total kinetic energy and total momentum before the collision is the same as the total kinetic energy and total momentum after the collision (Katz, 2015). This implies that there is no energy transduction from kinetic energy into heat or sound energy in a perfect elastic collision. If a traffic collision occurs when the vehicle was moving slowly, the amounts of kinetic energy and total momentum are low, thus permitting the bumper to be deformed upon impact and thereafter regain its shape, and in the process transfer all the energy back into motion. Thus, elastic collision usually occurs when a car travelling at a low speed collides with another object.
During an inelastic collision, the objects do collide onto each other, and none of them bounces back. In the process, the total momentum is conserved but the kinetic energy is converted into other energy forms. Energy is neither created nor destroyed but converted from one form to another in a process called transduction (Katz, 2015). Car crashes are usually inelastic collision due to the speeds involved. When the vehicle collides into another object, its kinetic energy is transformed into heat, sound and mechanical energy. It is the mechanical energy which causes deformation of the vehicle involved in the crash. The heat energy can cause the crashed car to burst into flames. Therefore, it is quite evident that most traffic collisions are inelastic collisions.
A car crash occurs when a moving motor vehicle collides with either another moving object or a stationary object. The physics of collision describe the process and outcome of a car crash. Newton’s laws of motion aid in understanding the physics of traffic collision. Kinetic and static friction also influences the course of driving and thus bears a direct relationship to traffic collision. Depending on the speed of the motion, car crashes can be categorized as either an elastic collision or an inelastic collision. In elastic collisions, the total kinetic energy as well as the total momentum is conserved. In elastic collision, the total momentum is conserved.
Bartley, G. P. (2008). Traffic Accidents: Causes and Outcomes. Nova Publishers.
Field, J. H. (2015). Differential equations, Newton’s laws of motion and relativity.
Katz, D. (2015). Physics for Scientists and Engineers: Foundations and Connections. Nelson
Popov, V. (2010). Contact Mechanics and Friction: Physical Principles and Applications.
Springer Science & Business Media.

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