The most significant force is the impact force of the car hitting an object (e.g., another car, a pole, a wall). This force is responsible for the rapid deceleration of the car. The force vector is directed opposite to the car's initial direction of motion.
2. The Inertial Force on the Occupants
This is not a "true" force in the sense of a physical interaction, but it's the fundamental principle explaining why unbelted occupants continue to move forward. According to Newton's First Law of Motion, an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Initial State: The car and the occupants are moving at the same speed.
During Collision: The car's motion is abruptly stopped by the collision force. However, the occupants continue to move forward at the car's original speed.
The "Force" of Inertia: The occupants' bodies have inertia, a resistance to change in motion. They will continue to move until a force acts on them to stop them.
3. Impact Forces on the Occupants
As the occupants continue to move forward due to inertia, they will collide with various parts of the car's interior. This is where multiple impact forces come into play:
Impact with the Dashboard/Windshield: The occupant's head, chest, and knees may strike the dashboard, windshield, or steering wheel. The force vectors here are directed opposite to the occupant's direction of travel. These forces are immense and happen over a very short period, leading to severe injuries.
Impact with Other Occupants: In a multi-occupant vehicle, unbelted passengers can collide with each other. The force vectors here depend on the relative motions of the colliding individuals.
4. Secondary Collisions
After the initial impacts within the car, the occupants might be ejected from the vehicle. This introduces a new set of forces:
Ejection Force: The force of the occupant's body breaking through the windshield or being thrown out of an open window. This is the force that propels them outside the vehicle.
Aerodynamic Drag: As the occupant flies through the air, they are subject to air resistance (drag). This is a force vector that opposes their direction of motion.
Gravity: Once ejected, gravity is a constant force vector pulling the occupant towards the ground.
Impact with the Ground/Other Objects: The final, often fatal, set of forces occurs when the occupant hits the ground, a tree, a signpost, or another object. The force vectors of these impacts are what bring the occupant's motion to a complete stop and cause the most significant injuries.
5. Rotational Forces (Torque)
In some accidents, the occupants may not just move linearly but also experience rotational motion (tumbling). This is due to torques applied to the body. For example, if a part of the body (like a shoulder) hits an object, it can cause the body to rotate.
Summary of Key Force Vectors
In a nutshell, the force vectors at play are:
Primary Collision Force: The external force on the car, causing its deceleration.
Inertial "Force" (not a true force): The tendency of the occupants to continue moving forward.
Internal Impact Forces: Forces from the occupant's body hitting parts of the car's interior.
Ejection Forces: The force that propels the occupant out of the vehicle.
Gravitational Force: The constant downward pull on the ejected occupant.
Aerodynamic Drag Force: The air resistance acting on the ejected occupant.
Secondary Impact Forces: The final forces when the occupant hits an external object (ground, pole, etc.).
All these forces are part of a cascading chain of events, starting with the primary collision and ending with the final resting position of the unbelted occupant. The key difference between a belted and unbelted occupant is that the seatbelt and airbag provide a force to decelerate the body over a longer period of time and a larger area, which significantly reduces the peak impact forces and prevents ejection