how can seat belt lengthen the time of impact?

when a car is moving at a certain velocity, you also get the same velocity .

in a crash, the car will stop in a short time. but you still have a forward motion at the

time of crash. without a seat belt, you will hit the car front window instantaneously.

if a seat belt is present, the seat belt will exert a force to you to slow down your

forward motion . so the time of impact is lengthened .