Two cars are travelling down a highway at 100 km/h in opposite directions. Both drivers are tired from
driving all day and cross over the yellow line and hit head on. Crash! The driver of car A has remained
inside the car and has broken ribs due to hitting the steering wheel. The driver of car B however is on the
hood of car A and is pronounced dead at the scene, cause of death, a severe case of disobeying the laws of
Although both cars were heading at the same velocity one driver ended up dead while another
survived. This seems like a complicated and hard thing to explain and to the untrained person it may seem
that driver A just had plain luck on his side however this is untrue. Using the knowledge of basic physics I
hope to explain why the outcome for each driver was different and could have been avoided.
Sir Isaac Newton was the first man to explain what happens in a collision even before automobiles
were invented. He proposed the idea that an object in motion will continue in motion with the same speed
and direction unless acted upon by an outside, unbalanced force. His theory is better known as the Law of
Inertia. The driver of car B was not wearing a seatbelt and as a result was not connected to the body of the
car. According to Newtons first law and object, the car, and its occupants, was moving until it hit and
external force, car A. When the collision occurred the car and its cargo changed its motion and direction.
However because driver B wasnt attached to his car by his seatbelt he continued with the same speed and in
the same direction as the car before the collision. Driver B flew through the windshield and onto the hood
of car A. The windshield and car A acted as the necessary external force needed to bring driver B to rest.
Since the driver of car A was wearing his seatbelt he experienced the same state of motion and deceleration
as the car and avoided major injury.
By increasing the mass of an object you increase the inertia that the object has therefore increasing
the amount of force due to an increase in acceleration. Many parents often allow their children to ride in the
front seat of the car unbuckled. They say that if a collision occurs that theyll just reach over and hold the
child back. If you really look at the physics behind their theory you will see that these parents are dead
wrong. What is the force required to stop a 27 kg child travelling at 25 m/s? Momentum is equal to mass
times velocity and impulse is equal to the change of momentum. Using the derived formula from the
previous statement F * t = m * Vf – Vi we see that the force exerted on the child by the seatbelt is 1687.5
Newtons which is the equivalent of 172.4 kg. This force is only exerted on the body for the time it takes to
bring the car to rest which is approximately 0.4 seconds. Using Newtons second law that states that
acceleration is directly proportional to the force and indirectly proportional to the mass we can see how the
force on an adult travelling at the same speed will experience a different force than that of the child. From
the formula derived from Newtons second law, F=m*a we see that a 67.5 kg person in the same accident
will experience a force of 949.95 kg for 0.4 seconds by their seatbelt. This is a lot of force for one person.
So I wonder how those parents with the unbuckled children in the front seat expect to hold back a child
exerting 172.4 kg of force while they are trying to hold themselves from crashing into the windshield with
949.5 kg of force.
Newtons third law states that for every action there is an equal and opposite reaction force. This
law help explain why a collision at low speeds is less serious than one at high speeds. If a car exerts a small
force on a brick wall because its speed was low then the car and its passengers will experience a force of
equal magnitude but it will be in the opposite direction.
If cars of unequal mass collide the more massive car will force the smaller vehicle backwards and
the smaller car will experience more force. This is due to the conservation of momentum. The Law of
Conservation of Momentum states that in an isolated system the momentum before a collision is equal to the
momentum after the collision, if we disregard friction. So in the example above we can see that the
momentum lost by the larger car was gained by the smaller car. If two cars of equal mass collide and all the
potential energy is converted back into kinetic energy after the collision then the collision is said to be
elastic. However if the two bodies collide and stick together and continue with the same final speed then we
call it an inelastic collision. Most collisions are neither completely elastic or inelastic. The conservation of
momentum is often very obvious in a collision on a slippery road where friction is very small. The Law of
Conservation of Momentum holds true for collisions in two dimensions also. The total momentum of a
system is equal to the vector sum of the momentum of all parts of the system.
For many decades people have used the laws of physic to explain the event of car crashes.
Accident reconstructionists are people who devoted their careers to using their knowledge of physics and
motion along with remains from accident scenes to determine the cause of the accident and how it could
have been avoided. They also work with manufacturers to come up with ways to reduce injuries that occur
from car accidents. Reconsturctionists use formulas and what they know to determine masses of the vehicle,
impact location, rest position, postimpact direction which can be found by studying tire marks and gouges in
the road. The deceleration from impact to rest is found by looking at postimpact braking, rotation, terrain,
and contact with roadside brush. A reconstructionists conclusions are very useful for car engineers,
manufacturers and consumers. By studying accidents they have been able to come up with new safety
features to help keep a person safe in the event of a collision. As we have already seen the seatbelt allows a
person to be connected to the vehicle so they can decelerate at the same speed as the car and are the single
most effective safety feature in reducing fatalities. Seatbelts allow the passenger to take advantage of the
cars energy absorbing design. The shoulder strap of a seatbelt stretches to allow more time for a person to
slow down but they also place a tremendous force on the chest. While the seatbelt helps in most cases
sometimes it isnt good enough and the driver is injured by the steering wheel. After careful research
engineers came up with airbags that in combination with a seatbelt can increase a passengers safety in a
frontal collision. However airbags do not deploy in low impact collisions and roll rollovers because they
would be of no help and may make matters worse. Sensors on the car detect a high rate of deceleration
inside the passenger compartment and cause the airbag to deploy and within fractions of a second of being
inflated it deflates. While inflated it prevents the head and chest of front seat occupants from striking the
steering column, dashboard, and windshield. So airbags greatly reduce a person who is wearing a seatbelt
and is involved in a head on collision from fatal injury.
Another safety feature that is less obvious to many car buyers is crumple zones. In specific areas of
an automobile engineers integrate variable grades of steel and fiberglass in the front and rear end of the car
that allow the frame to buckle under stress and keep the passenger compartment intact. This redirects the
energy in the collision and reduces injury. Without a crumple zone the vehicle rebounds in an elastic manner
regaining nearly all it kinetic energy. The occupants of the vehicle in turn experience a force of equal and
opposite direction which is very large. With a crumple zone much of the kinetic energy is transferred into
heat and sound energy resulting in a much smaller force being applied to the car and its passengers. With a
better crumple zone comes an increase in the time of collision which results in a large decrease in force.
Other safety devices being devised are break away poles, collapsing steering wheel columns, and
side airbags. Each of theses new items is being created to improve the safety of passengers by looking
carefully at the physics of collisions and using Newtons three laws of motion. I hope that this information
has provided you with a better understanding of what happens in a car crash and has provided you with
enough reason to obey speed limits and to wear a seatbelt at all times while moving in a vehicle because
remember its not only the law, its the law of inertia!