newtons first law
newtons first law cleary states that an bject at rest tends to stay rest stays that way and the same thing applies for an oobject in motion. until an unbalanced force acts upon the object.
how does this tie in
the way this ties in with a seatbelt is because without a seatbelt you would keep going because there would be no unbalanced force acting upon you.
if you have your seatbelt then inertia wich is an objects abilty to resist a change in motion keeps you from flying out wich is an unbalanced force acting upon so you stay there.