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Dynamics of a Rear End Collision


Auto accident involving two cars on a city street

Rear-end accidents occur when a motor vehicle collides with the rear of the vehicle in front of it. These types of accidents happen frequently, and the National Highway Traffic Safety Administration (NHTSA) reports they are the most common type of collision, accounting for 29% of traffic accidents. Knowing what happens in rear-end collisions and why they cause common types of injuries can help you understand the reasons why these accidents can be serious.

Understanding Rear-End Accidents

Rear-end accidents can be dangerous to the passengers and driver in the front vehicle. These occupants frequently have little warning that they are about to be struck by another vehicle and have no time to take evasive action or brace themselves. This means that the front-vehicle occupants often absorb the physical forces released in the crash with their bodies, which can result in serious injuries.

When a rear vehicle strikes the one in front of it, an enormous amount of energy is transferred in the collision from the rear vehicle to the leading one. All moving objects have a type of energy called kinetic energy as a result of movement. The kinetic energy of a moving vehicle depends on how fast it is traveling and its mass. Because of this, vehicles that are traveling at high rates of speed and those that are heavier transfer more kinetic energy to the vehicles they strike from the rear than those that are traveling at slow speeds or that are lighter.

When the energy is transferred from the rear vehicle to the front one, it causes the struck vehicle to lurch forward before it comes to rest. This also causes a rapid deceleration of the rear vehicle. As the vehicles come to a stop, the occupants in the front vehicle will go through three phases of movement as the energy transfers in which they are thrown forward rapidly, decelerate backward, and then come to a rest.

Laws of Physics and Rear-End Collisions

According to Steven Sweat, a rear-end collision attorney in Los Angeles, it’s important to understand the basic laws of physics to comprehend the dynamics of a rear-end collision. These include the three laws of motion as described by Isaac Newton. These laws control how everything moves and include the following:

  • Newton’s First Law of Motion – All objects that are at rest or in a state of constant motion will stay at rest or in motion that proceeds in a straight line unless an external force is applied.
  • Newton’s Second Law of Motion – Force is equal to the mass of an object multiplied by its acceleration with both mass and acceleration having a direction and magnitude.
  • Newton’s Third Law of Motion – Every action has an equal, opposite reaction, which means that when one object exerts an external force on a second object, the second one will exert a force that is opposite to the first one and equal in its magnitude.

Applying these laws to a rear-end accident can help to understand the dynamics of this type of collision.

Before a rear-end accident, the occupants of both vehicles are traveling at the same speed as the vehicles in which they are traveling under Newton’s First Law of Motion. All of the objects in each vehicle, including passengers, drivers, and anything they are transporting, are moving in a straight line and with relatively uniform motion. They will continue moving this way until an external force causes them to come to a stop or alters their direction. In a rear-end accident, the external force from the rear car as it impacts the back of the front car causes the contents of the front vehicle to move. Under Newton’s Second Law of Motion, the occupants and objects inside of the vehicle will impact adjacent surfaces with a force that is related to the mass and acceleration of the rear vehicle. This means that loose objects in the back seat of the front car could fly forward and strike the occupants of the front seat in a collision.

When the impact happens, the occupants in both vehicles will move toward the impact point. This is because of Newton’s Third Law of Motion and the fact that the two vehicles will experience forces of equal magnitude that travel in opposite directions.
At the moment of impact, everything (and everyone) moves toward the point of impact. This is why the people in the rear vehicle will move forward toward the impact and those in the front will move back before going in the opposite direction.

Since rear-end accidents also involve vehicles that are traveling in the same direction, both cars will continue to move in that direction until they come to rest following the crash unless the rear vehicle strikes the front one off-center and knocks it off course.

Types of Injuries in Rear-End Accidents

Because of the dynamics of rear-end collisions, they tend to result in some predictable types of injuries. If the front vehicle is stopped, it will remain so until an external force is applied under Newton’s First Law. However, when it is struck from behind, the vehicle occupants will first be forced back into their seats as they move towards the impact, placing immense strain on their necks and backs. After their vehicle is thrust forward, Newton’s Second Law will cause them to move forward until they are stopped by any structure inside of the car, including airbags, seatbelts, dashboard, or steering wheel. This movement and striking of objects inside of the vehicle can cause other injuries to the head, chest, and neck.

The most common types of injuries that happen in rear-end accidents result from these physical forces and include the injuries discussed below.

Whiplash and Other Neck Injuries

Whiplash injuries are injuries to the soft tissues of the neck and shoulders. When a person’s body is thrown backward and forward, it can cause the neck muscles, tendons, and ligaments to strain as they attempt to prevent the head and neck from whipping around. As a result, these soft tissues can be damaged and lead to chronic pain, muscle stiffness, a loss of range of motion, and other symptoms.

When the head pulls on the neck, it can be compressed and stretched. This can cause some of the cervical discs in the cervical region of the spine to compress, slip out of their proper alignment, or rupture. Damage to the cervical discs can cause chronic pain. When the injuries are severe, the discs can compress the nerves that lead into the spine or the spine itself, potentially leading to lifelong spinal cord injuries and paralysis.

Head Injuries

When the front vehicle comes to a stop following a rear-end accident, a victim’s head can strike the dashboard, windshield, steering wheel, or airbag. This can result in several types of injuries to the head and face, including fractured facial bones, broken teeth, cuts, bruises, skull fractures, or traumatic brain injuries (TBIs).

Back Injuries

The muscles, vertebrae, and discs of the back can be injured in a rear-end collision. When the crash occurs, significant forces can be transferred to the body from the seat belt and seat. This can cause people to suffer herniated discs, slipped discs, ruptured discs, muscle tears, muscle strains, and other types of back injuries. The back can also be injured when the body is thrown back and forth during the crash.

Traumatic Brain Injuries

As briefly touched on above, brain injuries can happen in rear-end accidents. These injuries can either occur when a person’s head strikes an object such as the dashboard, windshield, or steering wheel, or when the person doesn’t strike anything at all. Traumatic brain injuries can occur without striking an object when the person’s body is thrown about, causing the brain to move inside of the skull and strike the bone. This can result in traumatic brain injuries that might range in severity.

Thoracic Injuries

Thoracic injuries can happen in rear-end accidents because of seat belts. Seat belts can bruise your chest or cause broken ribs or a fractured collarbone while they hold people in place. However, it’s important to always wear a seat belt because it could also save your life by preventing you from being thrown through the windshield.

Knee Injuries

Many people instinctively brace themselves right before car crashes, and their knees can strike the dashboard during a rear-end collision. Cartilage, tendons, and ligaments in and around the knees can be torn. These types of injuries might require surgery to repair.

The dynamics of a rear-end accident can cause serious injuries. The injuries can be even more severe when the front vehicle is at a complete stop while the rear vehicle is traveling at a high rate of speed. However, rear-end collisions at low speeds can still cause accidents because of the effects of the physical forces that are released. Whenever someone is involved in a rear-end accident, it’s a good idea to get a medical examination to identify any injuries and receive appropriate treatment.

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