Final answer:
The least likely mechanism of injury in a collision involving modern safety features like airbags and crumple zones is one involving an instantaneous high force as these features extend the collision time and spread out the force.
Step-by-step explanation:
The mechanism of injury least likely to occur in an 'up and over' or 'down and under' collision is the injury resulting from the excessive force acting over a very short period. In these scenarios, the car's safety features like airbags and crumple zones are designed to extend the duration of the collision, thereby reducing the force.
When considering impulse, which is the product of force and time, extending the collision time reduces the force experienced by the occupants. The airbags deploy upon impact, cushioning the passengers and increasing the time over which they come to a stop. Similarly, the crumple zones in cars allow for a longer deformation time, lowering the net force on the vehicle during a collision. Given the design of modern safety features in vehicles, injuries involving instantaneous force are least likely compared to injuries where force is spread out over time.
Bones in a body will fracture if the force on them is too high. Safety features like airbags and crumple zones help extend the time over which a collision takes place, thereby decreasing the peak force and reducing the likelihood of bone fractures.