Inertia is the tendency of an object to resist a change in velocity.
When a car stops suddenly, your body is still going at the same speed the car was before it stopped (say 65 MPH). Without the belt strapping you in from behind, you would fly forward at nearly that same speed (65).
But thanks to the seatbelt, it stops your body's inertia.
Seat belts are in cars to the effects of inertia. ... When the car hits the wall, you would keep moving in a straight line and at a constant speed until the dashboard applies a force. Seat belts hold you and passengers down, protecting them from their own inertia.
