Many automobile passengers have suffered neck injuries when struck by cars from behind. how does newton's law of inertia apply here

Your inertia first tried to make you rest, but your seats resisted that phenomenon. And when you stared to travel with constant speed or velocity, then there is no net change in the acceleration; or in other words acceleration becomes zero.

F  you double the  force, you double the acceleration, but  if  you double the  mass, you velocity. all bodies accelerate at the  same  rate regardless of size or  mass.