Final answer:
A child is much safer when secured in a seat belt than held in an adult's arms because without restraints, the child is exposed to higher impact forces due to inertia during a sudden stop, which can cause serious injury or worse.
Step-by-step explanation:
When a car suddenly stops, a child held in an adult's arms rather than being secured by a seat belt is in significantly more danger. This scenario relates to the concept of impulse in physics, which is the change in momentum of an object. In the case of a car accident, the seat belt and airbags serve to increase the time over which this change in momentum occurs, thereby reducing the force exerted on the passenger.
Without a seat belt, both the child and the adult are subject to Newton's laws of motion, specifically the law of inertia which states that an object in motion will stay in motion unless acted upon by an external force.
Thus, if the car stops suddenly, the child will continue moving forward at the same speed at which the car was traveling, leading to a potential impact with parts of the car's interior or even ejection from the vehicle. The adult, no matter how strong, is unlikely to be able to hold onto the child due to the extreme forces at play during a sudden stop.
Government regulations requiring the use of seat belts are in place to protect individuals in vehicles and to reduce the societal impact of accidents. Impulse and the crumpling of the car's body work together to extend the time of impact and lower the force that occupants experience, thereby increasing their chances of survival and reducing the likelihood of severe injury.