Question

use Newtons first law to explain how a seat belt helps prevent any injury?

Answer 1

Moving objects have momentum . Newton's First Law of Motion says that unless an outside force acts on an object, the object will continue to move at its present speed and direction.[1]
Automobiles consist of several objects, including the vehicle itself, the passengers inside and any other loose objects in the vehicle.[2]
Unless the objects inside the car are restrained they will continue moving at whatever speed the car is traveling even if the car is stopped by a crash.
Changing or stopping an object's momentum requires a force acting over a period of time. If momentum changes instantly, as in a car crash, the force is very, very great! If the momentum can be changed over a period of time, even a fraction of a second, much less force needs to be applied with less damage or injury.[3]
In a head-on collision, if a passenger flies into the dashboard of a car, their momentum is instantly stopped, and serious injury is often the result.[4]
If the passenger is restrained by a seat-belt, their momentum is reduced more gradually by the constant and smaller force of the belt acting over a longer period of time. Seat-belts can reduce the impact of a passenger to one-fifth of the impact suffered by the body of the car.

Answer 2

FYI that is all quoted ^^^^