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What Happens to the Body in a Crash? The Physics.

4 minute read

When travelling at any given speed, a vehicle and its occupants are moving at the same rate and generating kinetic energy – the energy something possesses as it moves. During regular travel, when a car brakes, friction between the brakes and the wheels converts kinetic energy into thermal energy. The heat produced then scatters into the atmosphere as the vehicle slows down.

This kinetic energy is constantly produced when a car accelerates and is continually transferred whenever a vehicle decelerates. Modern-day automobiles are built to absorb the energy from a sudden collision or deceleration. However, depending on what is hit, this same energy can also be transferred directly to the other object the car hits. This usually happens when the object in question is smaller than a car, for instance, a pedestrian, cyclist or an animal. In the case of single-vehicle crashes, or crashes between two cars, it is more common for the energy to transfer back onto the vehicle, where it will inevitably be absorbed by the bodies of the people inside the car.

Car wreck

To understand why this transfer of energy can have such a significant impact on the outcome of a crash, it’s important to note that three collisions occur in any given crash:

1. The first collision is between the vehicle and another external object. It occurs when a car hits something, whether another vehicle, motorist, tree or a wall. Here factors such as the other object, the angle, and speed will influence the outcome and degree of injury.

If a vehicle hits something big and static front-on, such as a wall, the force (determined by the speed and kinetic energy held by the car on impact) will be thrust back onto the vehicle in rapid motion. Here the car body crumples and bends. In the best-case scenario, the car’s crumple zone absorbs most of the shock impact.

If a vehicle hits another moving object front-on, for instance, another car, multiple factors such as vehicle size, mass and speed come into play. If the two cars are identical, travelling at the same speed and only in opposite directions, the force applied to one another will be the same, and the impact will be shared equally.

2. The second collision is between the occupant and the vehicle. It occurs inside the car and involves its occupants moving until they collide with either each other, the safety components or structures inside a car.

This is again due to kinetic energy, as explained by Newton’s 1st Law of Motion* which states that “an object will not change its motion unless a force acts on it”. In this scenario, the forces that stop a moving body include seatbelts, airbags, steering wheels, dashboards or windshields, with factors such as speed and where you’re sitting in a vehicle influencing the injuries sustained.

Such injuries can include fractures to the collar bone in frontal crashes, broken ribs in high-speed collisions, and damages known as a pneumothorax (or a collapsed lung) which occurs when air collects outside of the lung but within the chest cavity. During this type of collision, the misuse (whether deliberate or not) of protective measures like seatbelts can cause more damage. For instance, people shorter or taller could experience brute force and injury to the organs in their stomach due to incorrect seatbelt placement. This type of injury usually occurs when the seatbelt rests directly on the soft and delicate abdomen, rather than on the pelvis bone, which can more likely withstand impact.

3. The third and final collision occurs once the body comes to a halt. It involves organs smashing against each other or the inside of one’s body. Softer organs often move forward until halted, resulting in bruising or tearing. Other organs like the heart or the brain can also undergo so much trauma that they either rupture or stop functioning.

Taking preventive measures such as wearing your seatbelt and positioning your chair correctly or staying under the speed limit can seriously impact the extent of one’s injuries.

In fact, “the energy in a crash is proportional to the square of the speed”. If you travel at 100km/h in a 50km/h zone, you’re increasing the amount of energy that has to be dissipated (back onto the car and to you) by a factor of four.

Small speed increments can also have the same effect on your injuries. In a car crash where the occupant increases the speed by 10%, for instance (77km/h instead of 70km/h), the energy, and consequentially the extent of your injury, will go up by 20% per cent.

Conversely, the same happens when you decrease your speed – reducing your rate by 10 per cent means decreasing potential injury in a crash by 20 per cent.

While this is only a simplification of the physics that takes place during a crash, understanding these processes allows us to see why so many individuals can be seriously injured if not killed during road trauma.

NOTE: This article provides an oversimplified explanation of the physics involved in a road collision. Simplifications have been made in order to increase understanding of the effects of a crash.

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*Newton’s Law of Motion:

  1. An object will not change its motion unless a force acts on it.
  2. The force of an object is equal to its mass multiplied by its acceleration
  3. When two objects interact, they apply forces to each other of equal magnitude and opposite direction.

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