Newtons first law proves why you need to wear a seatbelt
Newtons first law
An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force. I guess your wondering how this relates to seatbelts, well it does. When your in in a moving vehicle you are going the same speed of the vehicle. So if the vehicle stops and your not wearing a seatbelt you will still be going the speed of the vehicle because there wasnt a force to stop you. But if you are wearing a seatbelt, when the vehicle stops then you will stop also.