Car Crash
Car crash (traffic collision) causes injury, damage or even death. It occurs when a moving
motor vehicle collides with either another moving object or a stationary object. The risk of a
traffic collision occurring is determined by the following factors which also influence the physics
of a car crash: vehicle design, road design, driving skills of the driver, speed of the motor vehicle
before impact, road environment, level of psychological impairment (caused by alcohol
consumption or substance abuse) and behavioral disposition like street racing and speeding
(Bartley, 2008).
Physics of a Car Crash
The physics of collision describe the process and outcome of a car crash. As stated
earlier, the motor vehicle before crashing was in motion. Newton’s laws of motion do aid in
understanding 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 involved and also inversely
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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 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 have different levels of
friction, thus, each material has its coefficient 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 coefficient of friction in 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 coefficient of
friction. That 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
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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
where;
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 coefficient 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 coefficient of friction, the shorter the braking distances.
Thus, when driving on a dry road where the tire is not sliding on the surface, then the static
coefficient of friction is dominant (it always has a higher value than the kinetic value). That
implies that the braking distance would be shorter as compared to a car driving on a wet road
where it is sliding (which means that the kinetic coefficient 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 its
straight course and, thus, drive off the road and collide onto whatever object is on its path, thus
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causing a traffic collision (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. That 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 essential types of crashes; the elastic and the 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 after an elastic
collision, is conserved. That 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. If a traffic collision occurred 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.
During an inelastic collision, the objects do collide with each other, and none of them
bounces back. In the process, the total momentum is conserved, but the kinetic energy is
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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 with 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. Therefore, it is quite
evident that most traffic collisions are inelastic collisions.
Conclusion
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 frictions also influence the course of driving and thus bear a direct relationship with traffic
collision. Car crashes can be categorized as either an elastic collision or an inelastic collision. In
an elastic collisions, the total kinetic energy as well as the total momentum are conserved. In
elastic collision, the total momentum is conserved.